I've finally got a set of 305's on my truck. Now I'm trying to figure out my actual MPH. I've found a conversion chart @ http://www.miata.net/garage/tirecalc.html

Says my speedo reads 7.7% too slow. Anybody here know the math to get my actual reading?

 

Just multiply your speed on the speedometer by 1.077 to get actual "ground speed."

 

But where did you get 1.077? 7.7% is .077

Sorry, math wasn't my best subject in school

Thanks for your help

 

I have always used a GPS to give me my true speed

 

new tire size/old tire size=answer x speedometer reading=actual speed

 

That step eliminates the (actual speed X 0.077) + actual speed.

Thats some big rubber..

 

with 285s and 3.54s, 146mph is when the speedo hit the reset peg, i let off there. i was afraid it might break something.

 

Like the tires??? I am sure those are Z rated...

 

I put on Q78-16 buckshoot mudders and using a GPS(4.10 gears)at 65 it is realy 72 mph