Toyota Nation Forum banner

1 - 11 of 11 Posts

·
Registered
2006 Corolla S
Joined
·
185 Posts
Discussion Starter #1 (Edited)
So I currently have 205/50R16 tires on my Corolla from the factory 195/65R15. After driving through a speed detector that displayed my driving speed through a neighborhood it appears that my speedometer reads 2-3mph faster than my actual speed while going 30mph or less. Let me explain.

So after seeing this, when my speedometer said 25mph the speed detector on the street said 22. I thought to myself, this cant be right. So I pulled out my GPS and sure enough when my speedometer reads 25mph the GPS and the speed detector read 22mph. At first I thought, well it must be the tires I am running, the overall tire diameter is slightly smaller than the factory so that should make sense right?

I used a speedo calculator online to see what the change in my speedometer reading from my tires should be and overall it basically comes to my speedometer reading ~2.5mph faster. Okay, that makes sense for what I was seeing on my GPS and the detector when the speedometer says 25mph.

But then I then began noticing that once I hit around 30mph the speedometer seems to read more accurately, reading only 1mph faster than what the GPS says, which is a margin of error which seams reasonable. Going 75 on my speedometer the GPS says 74, going 40 says 39, etc. etc. (I checked it on my commute today) but according to calculators online it should read ~2.5mph faster all around (at all speeds roughly)

I guess the only real way to verify the issue is to see if when I put my factory tires on the car (which I do during the winter) the speedometer reading matches the GPS under 30mph. Could there possibly be some kind of nanny within the ECU that makes the speedometer read faster under 30mph so that people are not speeding through residential areas? I would think it would read speed linearly and not just suddenly read accurately once I hit 30mph (which it seems to do when comparing to multiple GPS')

***I know it really isn't a big deal and it seems ridiculous that I would investigate something like this, but being an engineer I like to investigate problems and fix things, or at least know why something isn't functioning as it should***
 

·
Premium Member
Joined
·
4,248 Posts
Well, radar signs are typically 1-3 MPH inaccurate only because they aren't calibrated everyday. Also, GPS could also have your speed off as well due to the fact that there is a sure lag on information communication. It's also possible, however unlikely that your speedometer itself is off one or two mph. The real way to tell is to see what the ecu is really getting
 

·
Registered
2006 Corolla S
Joined
·
185 Posts
Discussion Starter #3
so putting a diagnostic tool or OBD reader in it? I used cruise control to maintain a constant speed to help with the lag on the gps. I find it unlikely that the speed detector and the GPS to read exactly the same
 

·
2002 Ford Focus SE
Joined
·
5,928 Posts
First off - I think you will find that the speedometer errs on the high side so the tire error is not that great.

(i.e. you may find the 25 indicated on the Speedometer is 23 actual with the OEM tires and 22 actual with the wider tires - so the tire difference may be 1 mph, not 3 mph like you are thinking).

Secondly - I think for whatever reason the speedometers have to be tested and calibrated at 60 mph, so I think the error might be greater the further away from that you are (high or low), but I think it always is higher than actual. (i.e. I don't think you will ever get where the speedo shows 100 but you are actually doing 105 mph).

I don't think there is a "nanny ECU for residential areas" >:D
 

·
2002 Ford Focus SE
Joined
·
5,928 Posts
GPS is also only accurate if traveling in a straight line.

I think the speedometer and the ECU typically agree so an OBD reader woudln't help and they both typically show higher indicated than actual speed.
 

·
Registered
2006 Corolla S
Joined
·
185 Posts
Discussion Starter #6
I won't worry about it then, but when I put my factory tires on it, I'll just check it out of curiosity now. Thanks
 

·
Registered
2006 Matrix
Joined
·
626 Posts
Speedometers are not precision instruments. Not perfectly linear through their range.
Car makers intentionally set them high enough to avoid ever reading low even with unit to unit variations.
Years ago they made them (odometers too) even more optimistic.
This made the vehicle appear to have higher performance and better gas mileage.
This was one of many issues Ralph Nader pestered the car makers about.
 

·
2002 Ford Focus SE
Joined
·
5,928 Posts
On new cars the odometer and speedometer are separate and both are conservative - i.e indicated speedometer is 1-2 MPH above actual speed, indicated odometer is 1-2 miles under actual distance travelled.

In the old days, odometer was the opposite (mainly b/c it was geared off the speedo), but it was intentional also - not so much for MPG, but b/c if the car had a 36K mile warranty, that would expire after you went 34K actual miles.
 

·
2002 Ford Focus SE
Joined
·
5,928 Posts
I don't have an actual source, but it's fairly common knowledge and I've read it lots of places.

I'm pretty sure there were class-action lawsuits about it.

Yep - Honda in 2008 ...

http://lmgtfy.com/?q=odometer+warranty+lawsuit

http://www.autoblog.com/2007/02/19/odo-uh-oh-honda-extending-warranties-on-6-million-cars/

Interestingly:

The lawsuit also prompted lawyers to test the odometer accuracy of other vehicles. It was found that on average domestic vehicles were nearly perfect in their odo accuracy, while Toyotas actually racked up fewer miles on the clock than they did in reality. Nissans, however, didn't fare as well, and a new class-action suit has been filed on behalf of Altima owners who say their odometers are counting the miles 2.5% to 3% faster than they should.
 
1 - 11 of 11 Posts
Top