I recently put 235/55/18 Goodyear Assurance Comfortreds on my Mustang and that tire is a little bit taller than the stock 235/50/18. I am planning on buying the Fuego tuner from Granatelli when the 08' upgraded tuner becomes available to match my CAI. Since I don't have a tune now, it seems the odometer is slightly off. Maybe 10%. The reason I can tell is I was driving along Skyline Drive in the scenic mountains here in Virginia and they have each mile marked with mile markers. I noticed each mile further made my odometer fall behind about .1 mile. So, my question is; do taller tires make the odometer read less than actual mileage and does the taller tire make my speed on the speedometer show slower than it actually is? Say my speedometer is reading 60. Am I actually going about 62?

 

Yes you nailed it.

 

well when your speedo reads 60 you are actually going 63.94 mph (google tire calculator and it'll show you the diff speeds)

 

Which site were you looking at for those figures? I looked at a few different ones. The one at Discount Tire says when my speedometer says 60 I am actually going 62.04.

http://www.discounttire.com/dtcs/infoTireMath.dos

 

THats all seems to work out fine but what if you change gears? from a stock 355 to a 410 how does this effect your speedometer? does it go up or down. I know you can plug gear changes into your handheld to correct some of this but ????
could you plug in a higher number so your speed is actually slower than your running?

 

With 4.10s you have to turn more RPMs to go the same speed
as 3.55s. So you are going slower than the speedo reads
with 4.10s than 3.55s without tuner correction.

 

I recently tried changing settings from a 373 (which my speedo was off because of gear and tire sizes) to a 273 using my handheld , The Speedometer was indicating I was doing about 80 when really going 60, Then I cranked it up to like I had a 430 gear and my speedometer was under by 8 mph at 60..
This might be of some use for anyone trying to reset there speedometer to a gear size or getting an improper equipment ticket instead of a speeding ticket.