Re: tire size and speedometer accuracy

"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=blue]
>
> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
> news:eggn1l$16rd$1@agate.berkeley.edu...[color=green]
>> When I was hot rodding in the late '60s, I spent a lot of time
>> researching how tire size and different differential gear ratios affected
>> speedometer accuracy. I bought a lot of different speedo drive gears for
>> the tranny (this was on a GM TH400) to compensate for the changes I made.
>> All this just to keep my speedo as close accurate as possible as I didn't
>> want to speed and collect tickets. Now someone tells me that after
>> putting in larger diameter tires on his car (Honda Element), the tire
>> dealer said that his speedo would still be accurate. I find this hard to
>> believe. Are today's electronics intelligent enough to automatically
>> adjust for different diameter tires? I would think at least something
>> needs to be manually re-calibrated? This is a good group to get
>> information even though this is not a Toyota specific issues. Thank you.[/color]
>
> If the circumference of the replacement tire's tread is the same as the
> circumference of the original tires, then the speedometer and odometer
> will still be accurate. 2 tires can have different diameters and have the
> same circumference if the aspect ratio is proportionally different. A
> tire's diameter is measured from the center of the "hole" to the bead, not
> the tread.
> --
>
> Ray O
> (correct punctuation to reply)[/color]

Oops, brain fade... Correction: A tire's radius is measured from the
center of the hole to the bead...
Diameter is 2 times the radius.
--

Ray O
(correct punctuation to reply)

Okay, then my question now is if the circumference of tires are changed,
will speedo recalibration be necessary?.

"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=green]
>>
>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>> news:eggn1l$16rd$1@agate.berkeley.edu...[color=darkred]
>>> When I was hot rodding in the late '60s, I spent a lot of time
>>> researching how tire size and different differential gear ratios
>>> affected speedometer accuracy. I bought a lot of different speedo drive
>>> gears for the tranny (this was on a GM TH400) to compensate for the
>>> changes I made. All this just to keep my speedo as close accurate as
>>> possible as I didn't want to speed and collect tickets. Now someone
>>> tells me that after putting in larger diameter tires on his car (Honda
>>> Element), the tire dealer said that his speedo would still be accurate.
>>> I find this hard to believe. Are today's electronics intelligent enough
>>> to automatically adjust for different diameter tires? I would think at
>>> least something needs to be manually re-calibrated? This is a good
>>> group to get information even though this is not a Toyota specific
>>> issues. Thank you.[/color]
>>
>> If the circumference of the replacement tire's tread is the same as the
>> circumference of the original tires, then the speedometer and odometer
>> will still be accurate. 2 tires can have different diameters and have
>> the same circumference if the aspect ratio is proportionally different.
>> A tire's diameter is measured from the center of the "hole" to the bead,
>> not the tread.
>> --
>>
>> Ray O
>> (correct punctuation to reply)[/color]
>
> Oops, brain fade... Correction: A tire's radius is measured from the
> center of the hole to the bead...
> Diameter is 2 times the radius.
> --
>
> Ray O
> (correct punctuation to reply)
>[/color]

"Dennis Leong" <spgdxl@berkeley.edu> wrote in message
news:egh3el$1gk8$1@agate.berkeley.edu...[color=blue]
> Okay, then my question now is if the circumference of tires are changed,
> will speedo recalibration be necessary?.[/color]

If the circumference of the tires are changed, then speedo recalibration
would be necessary for the speedometer and odometer to be accurate. Modern
speedometers use speed sensors to send a signal to to ECM and instrument
clusters, and to be honest, I do not know what it would take to recalibrate
the speedometer. My guess is that the sensor is magnetic and is sensing the
teeth on a geared hub, much like the speed sensors for ABS. I doubt if
different hubs are available and the ECM is not designed to be re-programmed
so the owner has to calculate the speed difference and keep it in mind when
looking at the speedometer.

Besides speedometer accuracy, vehicle shift points will be affected by the
error caused by different size tires.

I've seen aftermarket trip computers that use driveshaft-mounted speed
sensors and that can "learn" the characteristics of a vehicle, and that or a
GPS system may be the best way to get accurate speed readings.

--

Ray O
(correct punctuation to reply)

[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
> news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=darkred]
>>>
>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>>> news:eggn1l$16rd$1@agate.berkeley.edu...
>>>> When I was hot rodding in the late '60s, I spent a lot of time
>>>> researching how tire size and different differential gear ratios
>>>> affected speedometer accuracy. I bought a lot of different speedo
>>>> drive gears for the tranny (this was on a GM TH400) to compensate for
>>>> the changes I made. All this just to keep my speedo as close accurate
>>>> as possible as I didn't want to speed and collect tickets. Now
>>>> someone tells me that after putting in larger diameter tires on his car
>>>> (Honda Element), the tire dealer said that his speedo would still be
>>>> accurate. I find this hard to believe. Are today's electronics
>>>> intelligent enough to automatically adjust for different diameter
>>>> tires? I would think at least something needs to be manually
>>>> re-calibrated? This is a good group to get information even though
>>>> this is not a Toyota specific issues. Thank you.
>>>
>>> If the circumference of the replacement tire's tread is the same as the
>>> circumference of the original tires, then the speedometer and odometer
>>> will still be accurate. 2 tires can have different diameters and have
>>> the same circumference if the aspect ratio is proportionally different.
>>> A tire's diameter is measured from the center of the "hole" to the bead,
>>> not the tread.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)[/color]
>>
>> Oops, brain fade... Correction: A tire's radius is measured from the
>> center of the hole to the bead...
>> Diameter is 2 times the radius.
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>>[/color]
>
>[/color]

On Tue, 10 Oct 2006 14:30:57 -0700, Dennis Leong wrote:
[color=blue]
> Okay, then my question now is if the circumference of tires are changed,
> will speedo recalibration be necessary?.[/color]

How much over- or undersized are you going?

I had a Toyota Tercel I put 185/60-13's on, when the standard was
155/80-13. I did a spreadsheet to calculate the difference. It was about 2
MPH at 65 MPH. I put 185/60-14's on a Tercel wagon that came with
175/70-13's, but with the lower profile the speedometer was the same.

Google Tire Sized Calculator Dodge Ram; there is an excellent one there,
there is also a very good one if you change Dodge Ram to Miata, and there
is another at 1010tires.com.

You enter the stock size for your vehicle, and the size you want to
replace it with, and it will give you the difference as a percentage and
in MPH.




[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
> news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=darkred]
>>>
>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>>> news:eggn1l$16rd$1@agate.berkeley.edu...
>>>> When I was hot rodding in the late '60s, I spent a lot of time
>>>> researching how tire size and different differential gear ratios
>>>> affected speedometer accuracy. I bought a lot of different speedo drive
>>>> gears for the tranny (this was on a GM TH400) to compensate for the
>>>> changes I made. All this just to keep my speedo as close accurate as
>>>> possible as I didn't want to speed and collect tickets. Now someone
>>>> tells me that after putting in larger diameter tires on his car (Honda
>>>> Element), the tire dealer said that his speedo would still be accurate.
>>>> I find this hard to believe. Are today's electronics intelligent enough
>>>> to automatically adjust for different diameter tires? I would think at
>>>> least something needs to be manually re-calibrated? This is a good
>>>> group to get information even though this is not a Toyota specific
>>>> issues. Thank you.
>>>
>>> If the circumference of the replacement tire's tread is the same as the
>>> circumference of the original tires, then the speedometer and odometer
>>> will still be accurate. 2 tires can have different diameters and have
>>> the same circumference if the aspect ratio is proportionally different.
>>> A tire's diameter is measured from the center of the "hole" to the bead,
>>> not the tread.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)[/color]
>>
>> Oops, brain fade... Correction: A tire's radius is measured from the
>> center of the hole to the bead...
>> Diameter is 2 times the radius.
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>>[/color][/color]

I was not my car. This is a general question. If the circumference of
tire changes (to any significant degree), speedometer accuracy is affected.
Are today's cars endowed with intelligence to automatically compensate for
the tire size change?

"Hachiroku" <Trueno@ae86.gts> wrote in message
news:pf6Xg.3224$HP.927@trndny08...[color=blue]
> On Tue, 10 Oct 2006 14:30:57 -0700, Dennis Leong wrote:
>[color=green]
>> Okay, then my question now is if the circumference of tires are changed,
>> will speedo recalibration be necessary?.[/color]
>
> How much over- or undersized are you going?
>
> I had a Toyota Tercel I put 185/60-13's on, when the standard was
> 155/80-13. I did a spreadsheet to calculate the difference. It was about 2
> MPH at 65 MPH. I put 185/60-14's on a Tercel wagon that came with
> 175/70-13's, but with the lower profile the speedometer was the same.
>
> Google Tire Sized Calculator Dodge Ram; there is an excellent one there,
> there is also a very good one if you change Dodge Ram to Miata, and there
> is another at 1010tires.com.
>
> You enter the stock size for your vehicle, and the size you want to
> replace it with, and it will give you the difference as a percentage and
> in MPH.
>
>
>
>
>[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
>> news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=darkred]
>>>
>>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...
>>>>
>>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>>>> news:eggn1l$16rd$1@agate.berkeley.edu...
>>>>> When I was hot rodding in the late '60s, I spent a lot of time
>>>>> researching how tire size and different differential gear ratios
>>>>> affected speedometer accuracy. I bought a lot of different speedo
>>>>> drive
>>>>> gears for the tranny (this was on a GM TH400) to compensate for the
>>>>> changes I made. All this just to keep my speedo as close accurate as
>>>>> possible as I didn't want to speed and collect tickets. Now someone
>>>>> tells me that after putting in larger diameter tires on his car (Honda
>>>>> Element), the tire dealer said that his speedo would still be
>>>>> accurate.
>>>>> I find this hard to believe. Are today's electronics intelligent
>>>>> enough
>>>>> to automatically adjust for different diameter tires? I would think
>>>>> at
>>>>> least something needs to be manually re-calibrated? This is a good
>>>>> group to get information even though this is not a Toyota specific
>>>>> issues. Thank you.
>>>>
>>>> If the circumference of the replacement tire's tread is the same as the
>>>> circumference of the original tires, then the speedometer and odometer
>>>> will still be accurate. 2 tires can have different diameters and have
>>>> the same circumference if the aspect ratio is proportionally different.
>>>> A tire's diameter is measured from the center of the "hole" to the
>>>> bead,
>>>> not the tread.
>>>> --
>>>>
>>>> Ray O
>>>> (correct punctuation to reply)
>>>
>>> Oops, brain fade... Correction: A tire's radius is measured from the
>>> center of the hole to the bead...
>>> Diameter is 2 times the radius.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)
>>>[/color][/color]
>[/color]

"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=green]
>>
>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message news:eggn1l$16rd$1@agate.berkeley.edu...[color=darkred]
>>> When I was hot rodding in the late '60s, I spent a lot of time researching how tire size and different differential gear ratios
>>> affected speedometer accuracy. I bought a lot of different speedo drive gears for the tranny (this was on a GM TH400) to
>>> compensate for the changes I made. All this just to keep my speedo as close accurate as possible as I didn't want to speed and
>>> collect tickets. Now someone tells me that after putting in larger diameter tires on his car (Honda Element), the tire dealer
>>> said that his speedo would still be accurate. I find this hard to believe. Are today's electronics intelligent enough to
>>> automatically adjust for different diameter tires? I would think at least something needs to be manually re-calibrated? This
>>> is a good group to get information even though this is not a Toyota specific issues. Thank you.[/color]
>>
>> If the circumference of the replacement tire's tread is the same as the circumference of the original tires, then the speedometer
>> and odometer will still be accurate. 2 tires can have different diameters and have the same circumference if the aspect ratio is
>> proportionally different. A tire's diameter is measured from the center of the "hole" to the bead, not the tread.
>> --
>>
>> Ray O
>> (correct punctuation to reply)[/color]
>
> Oops, brain fade... Correction: A tire's radius is measured from the center of the hole to the bead...
> Diameter is 2 times the radius.
> --
>
> Ray O
> (correct punctuation to reply)
>[/color]


I thought say a 225 / 50 series tire would mean the tread width was 225 and the sidewall was
50% of the tread. That also make a 225 / 50 - 15" tire shorter than a 225 / 50 - 16" tire.

Am I mistaken?
Dan

gearing calculator program I use shows that if you were actually going 57 mph with 24" tall tires
and the only change is the diameter of the tire.

a 25" tall tire is 59 mph.
a 30" tall tire is 71 mph.

Dan

"Dennis Leong" <spgdxl@berkeley.edu> wrote in message
news:egjgav$8dm$1@agate.berkeley.edu...[color=blue]
>I was not my car. This is a general question. If the circumference of
>tire changes (to any significant degree), speedometer accuracy is affected.
>Are today's cars endowed with intelligence to automatically compensate for
>the tire size change?
>[/color]

No, because there is no way for the ECM to know that the tire size is
different.
--

Ray O
(correct punctuation to reply)

"Danny G." <dandog@pacbell.net> wrote in message
news:ZheXg.5205$NE6.447@newssvr11.news.prodigy.com...[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
> news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=darkred]
>>>
>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>>> news:eggn1l$16rd$1@agate.berkeley.edu...
>>>> When I was hot rodding in the late '60s, I spent a lot of time
>>>> researching how tire size and different differential gear ratios
>>>> affected speedometer accuracy. I bought a lot of different speedo
>>>> drive gears for the tranny (this was on a GM TH400) to compensate for
>>>> the changes I made. All this just to keep my speedo as close accurate
>>>> as possible as I didn't want to speed and collect tickets. Now
>>>> someone tells me that after putting in larger diameter tires on his car
>>>> (Honda Element), the tire dealer said that his speedo would still be
>>>> accurate. I find this hard to believe. Are today's electronics
>>>> intelligent enough to automatically adjust for different diameter
>>>> tires? I would think at least something needs to be manually
>>>> re-calibrated? This is a good group to get information even though
>>>> this is not a Toyota specific issues. Thank you.
>>>
>>> If the circumference of the replacement tire's tread is the same as the
>>> circumference of the original tires, then the speedometer and odometer
>>> will still be accurate. 2 tires can have different diameters and have
>>> the same circumference if the aspect ratio is proportionally different.
>>> A tire's diameter is measured from the center of the "hole" to the bead,
>>> not the tread.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)[/color]
>>
>> Oops, brain fade... Correction: A tire's radius is measured from the
>> center of the hole to the bead...
>> Diameter is 2 times the radius.
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>>[/color]
>
>
> I thought say a 225 / 50 series tire would mean the tread width was 225
> and the sidewall was
> 50% of the tread. That also make a 225 / 50 - 15" tire shorter than a 225
> / 50 - 16" tire.
>
> Am I mistaken?
> Dan
>
>[/color]
No.
--

Ray O
(correct punctuation to reply)

"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:75de8$452d792a$44a4a10d$32581@msgid.meganewsservers.com...[color=blue]
>
> "Danny G." <dandog@pacbell.net> wrote in message news:ZheXg.5205$NE6.447@newssvr11.news.prodigy.com...[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=darkred]
>>>
>>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...
>>>>
>>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message news:eggn1l$16rd$1@agate.berkeley.edu...
>>>>> When I was hot rodding in the late '60s, I spent a lot of time researching how tire size and different differential gear
>>>>> ratios affected speedometer accuracy. I bought a lot of different speedo drive gears for the tranny (this was on a GM TH400)
>>>>> to compensate for the changes I made. All this just to keep my speedo as close accurate as possible as I didn't want to speed
>>>>> and collect tickets. Now someone tells me that after putting in larger diameter tires on his car (Honda Element), the tire
>>>>> dealer said that his speedo would still be accurate. I find this hard to believe. Are today's electronics intelligent enough
>>>>> to automatically adjust for different diameter tires? I would think at least something needs to be manually re-calibrated?
>>>>> This is a good group to get information even though this is not a Toyota specific issues. Thank you.
>>>>
>>>> If the circumference of the replacement tire's tread is the same as the circumference of the original tires, then the
>>>> speedometer and odometer will still be accurate. 2 tires can have different diameters and have the same circumference if the
>>>> aspect ratio is proportionally different. A tire's diameter is measured from the center of the "hole" to the bead, not the
>>>> tread.
>>>> --
>>>>
>>>> Ray O
>>>> (correct punctuation to reply)
>>>
>>> Oops, brain fade... Correction: A tire's radius is measured from the center of the hole to the bead...
>>> Diameter is 2 times the radius.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)
>>>[/color]
>>
>>
>> I thought say a 225 / 50 series tire would mean the tread width was 225 and the sidewall was
>> 50% of the tread. That also make a 225 / 50 - 15" tire shorter than a 225 / 50 - 16" tire.
>>
>> Am I mistaken?
>> Dan
>>
>>[/color]
> No.
> --
>
> Ray O
> (correct punctuation to reply)[/color]

Thanks

"Danny G." <dandog@pacbell.net> wrote in message
news:UGsXg.14363$6S3.1276@newssvr25.news.prodigy.net...[color=blue]
>[/color]
<snipped>[color=blue][color=green][color=darkred]
>>>
>>>
>>> I thought say a 225 / 50 series tire would mean the tread width was 225
>>> and the sidewall was
>>> 50% of the tread. That also make a 225 / 50 - 15" tire shorter than a
>>> 225 / 50 - 16" tire.
>>>
>>> Am I mistaken?
>>> Dan
>>>
>>>[/color]
>> No.
>> --
>>
>> Ray O
>> (correct punctuation to reply)[/color]
>
> Thanks
>[/color]
You're welcome!
--

Ray O
(correct punctuation to reply)

"Danny G." <dandog@pacbell.net> wrote in message
news:ZheXg.5205$NE6.447@newssvr11.news.prodigy.com...[color=blue]
>
> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
> news:327f7$452be1f6$44a4a10d$22193@msgid.meganewsservers.com...[color=green]
>>
>> "Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message news:...[color=darkred]
>>>
>>> "Dennis Leong" <spgdxl@berkeley.edu> wrote in message
>>> news:eggn1l$16rd$1@agate.berkeley.edu...
>>>> When I was hot rodding in the late '60s, I spent a lot of time
>>>> researching how tire size and different differential gear ratios
>>>> affected speedometer accuracy. I bought a lot of different speedo
>>>> drive gears for the tranny (this was on a GM TH400) to compensate for
>>>> the changes I made. All this just to keep my speedo as close accurate
>>>> as possible as I didn't want to speed and collect tickets. Now
>>>> someone tells me that after putting in larger diameter tires on his car
>>>> (Honda Element), the tire dealer said that his speedo would still be
>>>> accurate. I find this hard to believe. Are today's electronics
>>>> intelligent enough to automatically adjust for different diameter
>>>> tires? I would think at least something needs to be manually
>>>> re-calibrated? This is a good group to get information even though
>>>> this is not a Toyota specific issues. Thank you.
>>>
>>> If the circumference of the replacement tire's tread is the same as the
>>> circumference of the original tires, then the speedometer and odometer
>>> will still be accurate. 2 tires can have different diameters and have
>>> the same circumference if the aspect ratio is proportionally different.
>>> A tire's diameter is measured from the center of the "hole" to the bead,
>>> not the tread.
>>> --
>>>
>>> Ray O
>>> (correct punctuation to reply)[/color]
>>
>> Oops, brain fade... Correction: A tire's radius is measured from the
>> center of the hole to the bead...
>> Diameter is 2 times the radius.
>> --
>>
>> Ray O
>> (correct punctuation to reply)
>>[/color]
>
>
> I thought say a 225 / 50 series tire would mean the tread width was 225
> and the sidewall was
> 50% of the tread. That also make a 225 / 50 - 15" tire shorter than a 225
> / 50 - 16" tire.
>
> Am I mistaken?[/color]


Not completely.

The 225/50 would have the same tread width and sidewall, but the rim would
cause the tire to be 1 inch different.

225 is the width, 50 is the aspect ratio (ratio of sidewall to width). If
one has a 22d/50x15, and decided to get 16 inch rims, then the new tires
would be inch bigger around if the same 225/50 was selected. One would
require a 225/45x16 to get the same diameter -- actually the diameter would
be larger by 0.12 inches.