how much do tires affect the speedo? i went from stock 255/70/16
to 285/75/16 and was wondering how much this would affect the reading and how this is figured.
thanks terry

 

Try this link;

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


You can enter your original size of tire and wheel, then the new size. It will give the diferences in dimensions as well as the revs per mile and percetage difference in the speedo. I use it for all my vehicles. Check it out.

 

it came up with a 9.2% difference but is that going to be + or- ?

 

The tire would be about 2.7 inches bigger (diameter) and your speedo would say 60mph when you're realy going 65 mph. Be careful!

 

Something else to think about, if you don't have it recalibrated you are putting less miles on your truck than you are actually driving. I went to Ford and they told me they could not adjust...I ended up buying the Hypertech Programmer to adjust for larger wheels and tires. Good Luck.