265/65/18 vs 285/65/18 tires

[ealopez]

I no my speedo is off by a couple miles an hour but what does it do to my odometer? Am I going a further distance per mile or less?What is the formula?

[i1dery]

lets see if this helps
the dia of your 265 tire is 31.56 x 3.14 (pie) gives you a circumfrance of 99in
and the 285 is 32.58 x 3.14 gives you 102in when you multiply the revs per mil
639 for the 265 and 659 for the 285 anyhow the 265 tire will have to travel an
extra 5 ft per mile so if you travel 100mi it will be an extra 500 ft and if i did this right
you will have to travel 1056 miles on the 265 to equal the same on the 285.
so for any math wiz's out there to confirm?

[Xyzzy]

P265/65R18

265 / 25.4 = 10.43 inches wide
x * 0.65 * 2 = 13.56 inches total height of the sidewalls
13.56 + 18 = 31.56 inches tire diameter

P285/65R18

285 / 25.4 = 11.22 inches wide
11.22 * 0.65 * 2 = 14.59 inches total height of the sidewalls
14.59 + 18 = 32.59 inches tire diameter

There is roughly a 3.3% difference.

Both of the situations, small to big or big to small, is going to cause your speedometer and odometer to read wrong. For example, if you drive a real 100 miles, the odometer may read ~103 miles or ~97 miles. I don't know which one is which but I bet someone here does.

(This assumes my math is correct, which is assuming a lot.)

[ealopez]

That is my mystery 3 miles or 3 miles less???

[Xyzzy]

That is my mystery 3 miles or 3 miles less???

My gut tells me the larger tires are going to rotate slower at a given speed. The slower rotation will cause the digital sensor (I assume on the driveline or transmission) that drives the speedometer and odometer to read less pulses per given unit of distance.

Thus, you get "free miles" - a speedometer that reads lower than the actual speed and an odometer that counts fewer miles than actually traveled.

But I've been wrong before.

[johnjohn2784]

if you get a tech at the dealer to re-program your ecu with the new tire size, will this correct this issue?