Hey all.

Going up a bit in size on the Silverado.
But, since I'm taking a trip for work that involves accurate mileage records (and its last minute, so no time to re-program the cluster), how do I go about getting an accurate calculation?

Going from a Firestone (? type) P265-75-16
To BFG All Terrain LT 285-75-16.
Stock gear set is 3.73'* on a 2001 Silverado 1500 4x4.

Since its 2 weeks of driving, everywhere, and I have to keep track of every bit of it, what can I use for a formula to give them an accurate mileage record? I know Miata.net has a good calculator, but I can't seem to come up with the conversion factor.

Help a poor working slob out, please?

I could be wrong, but I thought MPH only changed if you changed the actual rim size. IE, going from a 16" to 20" rims would show almost a 7MPH difference. But a 1" change would be a 1.875MPH difference.

EDIT:
Found this nifty conversion calc...

http://www.4lo.com/4LoCalc.htm

Actually, it does make a difference.
A 265/75-16 has 637 revolutions per mile.
A 285/75-16 has 614 revolutions per mile.
Overall diameter is going to affect both the speedometer and odometer by a factor of 3.7%.

But I can't grasp the math today for some reason... say I travel an indicated 100 miles. Is the true mileage 3.7% more, or less? Am I driving 103.7 miles, or 96.3 miles...
I honestly wouldn't care about the differences under normal conditions, but when they are paying me $.55 per mile, and I'm going to be doing well over 1000 miles, its gonna add up.

I think you have to add in the extra 3.7 miles for every 100, because with less revs per mile on the bigger tires, it will indicate traveling less than a mile, when you are traveling a mile. So, for 100 miles, you multiply by 1.037. If I travel 1000 miles, I should add an extra 37 miles into the mileage.

...I think.

I think your math may be a little off...

265/75/16
265/24.5*.50 = a 5.40" sidewall height from rim to top of tread.

285/75/16
285/24.5*.50 = 5.81" sidewall height from rim to top of tread.

24.5 is the used to convert MM to inches.

This is a 4/10th of an inch difference height wise.

To determine overall tire circumference...
Sidewall height X2+rim diameter
5.40+5.401+16=26.80 inches.
D*pi=84.1922 inches

63,360 inches per mile divided by tire circumference
63,360/84.1922=752.5637766919026 revolutions per mile.

Do the same for the 285 tire...
730.9518447958482 revolutions per mile.

General rule of thumb is, the engine has to work more(higher RPM) to turn taller tires=less MPG.
Hope I didn't confuse anyone. Cause, I'm sure as heck confused.

With the taller tires your speedo reading 60MPH is actually a SLOW reading.

So, with that in mind if your mileage states 100 miles traveled, you actually traveled farther.

Conversion calculator showing the math.

gm gauges arent that good. i would gps it anyway. my truck has bigger tires and the speedo still reads 2mph faster than gps speed. all the gps'* ive used have a trip counter

the smaller the tire, the more rpm the engine will turn. the taller tire the rpm will get lower. i just got thru with this on my gto. it came factory with 245/45/17'*. i put on 245/45/18'* on it and the rpm went from 1900rmp at 70 with the 17'*, to a litttle less than 1800 rpm with the 18'* on. i now have the 17'* back on and i am now getting better mileage again. it is all in the tune and my tune and power will get me better with 17'*.
and there is about a 1" of height between the two sizes.

Yep. Went through the deal with my dad last night. (He'* a retired math teacher). And came to that same conclusion.

My GPS doesn't have a trip counter. Its gonna be paper, pen, and the trip odometer.

Rim size has nothing to do with it. Its just the hole in the center of the donut. The diameter of the donut is going to be the same, no matter how big the hole in the center is. If you go to places like Tire Rack, and do a plus1 or plus2 rim size, the sidewall height shrinks down for an equal tire to keep it the same overall diameter.

And actually, this trip is looking to be about 1600 miles over the 2 weeks. That error rate is gonna make a big difference when that mileage check comes.
And they are very nit picking so far about every expense on this trip, despite how stupid and illogical they set the whole deal up.
If they made this a logical trip, it would be closer to 700 miles for the whole 2 weeks.

your gps will tell you if your speedo is close or off at least then you can use a correction factor calculation to get real mileage

no it isn't alway'* going to be like that. i took the same size tire on my 17'* and 18'* and there is a height difference between the two.