2nd Gen Non-Engine/Transmission tires vs. speedo

[DieselTorque]

how much would the speedo be off if i would install 285 over the stock 245 rear tires, i have the 3. 54 rear end. thanx



dt

 

[Ereynolds72]

This method worked for me and it checks against GPS and milemarkers on the highway. Example: I went from 265/75/16 to 305/70/16. (265mm/25. 4) x . 75 x 2 + 16 = 31. 65". Actual rolling diameter will be less but the calculated diameter works for this purpose.

The 305s dia. = (305/25. 4) x . 70 x 2 + 16 = 32. 81"

32. 81/31. 65 = 1. 037 or 3. 7% I use this multiplier each time I check my mileage. Hope this helps

 

[Jschafer]

I run 305s and have come up with 8% under for the odometer, but for some reason I don't know why but my speedo seems to be perfect, every time there is on of those radar signs along the road that say how fast you are going I carefully check it and I am always within 1 mph, over the past 4 or 5 yrs I have done this 50 times anyway always within 1mph

 

[Ereynolds72]

I've never had 245s before but I assume the aspect ratio is 75%. That calculation would jive with what you are finding... .



(245mm/25. 4mm/inch) x . 75 x 2 +16 = 30. 47"

new tire size: 32. 81" / OE tire size: 30. 47" = 1. 077 or 7. 7%

 

[Tcolesanti]

Way back when, my truck had 285's on it and my speedo was dead on. Now that I have 315's, the truck at 2000rpm, in 6th gear is indicating 72mph, Gps says I am doing 78mph. That is roughly 10% off.

 

[master1578]

Try this handy little tool.



http://www.miata.net/garage/tirecalc.html

 

[dieselshu]

Tire speed differences

Try this one... . Tire speed computation