Tire rollout, mpg, speed, odo confusion HELP

[Hillingdon]

Can't get my head around this thing. Every time I think I've got it confusion sets in. Also, lack of info probably too, lol. Not really looking for a "they are all close enough" or "the slight difference will not make any significant..." type answers. Wanting to know the correct way of logicing this thing out.

First, not going to massive wheels and tires and needing to know the max that will fit etc. The dilemma is replacing stock size with stock size and the varying specs.

05 Camry xle. 215/60/16 tires. Came with Bridgstone RE92 that show a roll out of 798 revs per mile.

Have Sumitomo Touring LSV on there now (came with the car when I bought it) that need replacing due to a camber problem on the driver rear tire causing the inner tread band to wear (whole other question as not found the issue causing it yet). Can't find exact specs on these, but close as I can tell from specs I've looked at, these tires have 811 revs per mile roll out. Now this could be an error and I guess I need to go out and actually try to measure, but basic logic questions below are the same either way I believe.

On trips with gps our speed on the speedometer is showing a little over 1 mph faster than we are actually going.

So, here is the question/confusion area.

Tires I've been looking at on Tire Rack have roll outs listed anywhere from 792 to 800.

I don't have data on how accurate the Bridgestones would have been on speed showing and actual odo miles covered. My thinking is that if the Sumitomos are 811 rpmile then the GPS should be showing that I am moving faster than the speedo indicates. Am I thinking the right direction on this?

Also, the odo would be showing less miles covered than actual. Am I in error?

One wonders if a closer mph speedo error could be dialed using the different roll out numbers, but then which way to go? (know it will probably be very little difference, but my question remains, as enquiring old minds want to know. Well at least mine does)

I'm also assuming that if the roll out on the tire can bring the speed in mph showing on the speedo closer then the actual odometer distance covered should be calculated closer to true. Correct or am I way off on this?

My guess logic right now is saying that at 811 my odo miles is showing less than it should be. Thus my mpg should be higher as I'm actually covering more ground than shown(not really supported by the gps numbers though that show 1mph slower than speedo indication)?

If I go to a smaller tire roll out then what happens...................? My speed slows down even more than what is indicated when compared to the GPS? What happens to the odometer reading. Does it end up showing more miles than actually covered and so I then get an inflated mpg?

Someone please edumacate me.

[tengiday]

You can try this:
http://www.miata.net/garage/tirecalc.html

I'm using 225/45/18 tires. My speed is about 0.5 mph lower than the actual speed when I'm travelling at 60 mph. I'm happy about this since I'm sure I'm not overspeed.

PS: Don't rely on the speed meter on the road as an exact measuring tool.

[Hillingdon]

Thanks for the post and calculator. I'll keep it bookmarked. Only calculates the numeric on a tire size or compares different size tires though, but still handy.

Looking at the same size tire 215/60/16.

Various manufacturers and models within their lines have different specs listed though for revs per mile. That is what I'm zeroing in on.

Effect of a same designated sized tire, but with say one listed by the manufacturer supplied spec at 790 revs compared to one listed at 800 say.

What does going from one number to a lower number do to the speedo reading and odometer reading (effects how fast you are actually going and also effects mpg calculations).

Any formulas/calculations for determining this that anyone knows of?

Thanks again for your post!

Cheers

[Robert1820]

Lets say you're going FROM a tire that rolls 811 times per mile TO a tire that rolls 800 times per mile.

In this example, simply divide the difference in revolutions (11) by the smaller number. 11 divided by 800= .01375 or 1.375% The speedo will now be reading faster by 1.375%. Example: 65mph X .01375= .89375mph. Now your speedo will be reading 65.89375. Not a big difference. The odometer will also be reading 1.375% more miles. A 300 mile trip now would be 300 X .01375= 4.125 or 304.125 Your gas mileage would "improve" unless you factor in the small odometer error.

[Hillingdon]

Thanks for the great info! That gets me thinking the right direction!