Tires and how they can Effect your Speedo..

I just had to explain this to another guy on another site so figured itd make a good write up for here...

Now everyone knows that when you change your rim size and tire dimensions you need to re-calibrate your speedometers... now we all know that the markings/measurements on the tires go Width/Height/Rim diameter.. and if you know how to "play" with those numbers when changing rim sizes you can keep it close to what your cars/trucks have to read on stock ones...

Everyone knows that the sidewall height will effect the speedometer.. but what very few people realize is so will the tire width... for instance my truck originally came with 195/75R14... now it has 205/75R14 and the speedo is off by 3 mph.. and as some guy tried to point out on another site and failed in the attempt that cuz i have the same sidewall dimension that my "tire diameter" is the same... WRONG!!!!

the tire diameter will change too because the side walls will be stronger from when you widen them.. the side wall thickens making it more the tire more rigid.. which will change its overall diameter because the tire wont squat with the weight of the vehicle as much as it did with the thinner walls...


going with the example i used of my truck heres how much of a difference it makes...

(rounded it off to the thousandths place..)
-----195/75/14---------------.................... ---------205/75/14----------
Stock tire width is 7.678 inches.................... New tire width is 8.071 inches.
Stock tire thickness is 5.758 inches............... New tire thickness is 6.053 inches.
Stock tire diameter is 25.516 inches.............. New tire diameter is 26.106 inches.

which makes that a little over 1/2 an inch you just added to your tires by widening them... doesnt look like much but all of it adds up to it effecting the speedometer..

thank you.... and yeah were trying to be informative here lol...