I know this has been beaten to death in every thread about bigger tires, but this weekend i put my Am. Racing rims with 215's (snow tires seeing as my a/s's didn't handle well in a milwaukee winter, let alone a houghton one) and i noticed on my drive back up to school, my trip read 360 miles, whereas google maps insists the trip (from the exact same gas station) is only 336 miles. now i know there's gotta be a pretty big tolerance in the google measurements, but could a swap from 235's to 215's cause a 30 mile difference?

 

It's reading 8% faster now (assuming google is correct, and your speedo was correct before). What are the actual measurement for the 215s and 235s?

 

u better believe that changing tire size has an effect on speedo reading. mathematically, you can easily figure out tire curcumference. (tire width x aspect ratio x 2 + rim diameter) x pi dont forget to make units of measure same: tire width is in mm and rim size is in inches
((235x 0.75 x 2 / 25.4) + 15) x 3.1415927 = 90.7" cir
((215x.075 x 2 / 25.4) + 15) x 3.1415927 = 87" cir

for every revolution, the smaller tire travels almost 4" less.

there is 63360 inches per mile. tire 1 rotates 698.5 times per mile tire 2 rotates 728.3 times per mile

 

The problem with that math equation is that tires are almost never the size they claim to be. If you use his formula with your actual measurements it should work out just fine. The stated tire size is often off by an inch or more.

 

The easiest way to know is get a GPS. Even one of the cheepy's with no maps, just for hiking. Compare you speed at 10mph increments. Then find out what the average MPH it is off by per 10mph increments. Then increase your speedo gear teeth by that percentage.

For a good example Going from stock to 31's tires and from stock rear end gear ratio of 3.55 to 3.90 it was off by 10%. So at 10mph off by 1, at 20mph off by 2 and so on. Now I went from a 26 tooth gear to a 29 tooth. (Not quite sure on the gear numbers.)

Now to get the speedo gear you have to go to dealership. Some can be cheep like $10. Some can be as high as $100.

Also when I went from 31's to 33's using my GPS it showed it was off by 2MPH at 75. That is the most it was off. So for me it was not worth buying another gear.

 

i know that i was just using that math as a arbitrary point of reference as to how tire size effects speedometer readings. i personally am fixing a 97jeep grand cherokee for a guy that blew up the transfer case. he has 2 different brand tires of the same size on the jeep and i suspect that's what blew the case. the front tires are goodyear and have a circumference of 1¼" greater than the rear which is some off brand/ so the rear tires were turning more per mile than the fronts.