Stock tire size

[Morris]

Hey guys, I am trying to figure out the odometer error ratio for my car. Finding calculators to do this for me is no problem. But, I can't, for the life of me, figure out the diameter of the stock tire.

Can someone help me out?

 

[ecurie_ecosse]

Morris,the Midget/Sprite Club web site has a chart on the ratios etc.And I think it also mentions speedo error %

Stuart.

 

[Trevor Jessie]

Chalk, driveway, tape measure
diameter=circumference/pi

 

[ChrisS]

The stock tyre is a 145SR13 which is a 145/80R13 in modern terms. This equates to 562mm OD or 22-1/8".

 

[58Custom]

Rolling diameter is what you want. Unloaded diameter will be larger than rolling diameter. Measure the rear tires from the gound to the axle centerline and multiply by 2 to get rolling diameter. This takes into account the weight compresssion of the sidewall.

 

[JPSmit]

https://www.1010tires.com/tiresizecalculator.asp

odometer ratio will be the same as the speedometer ratio

 

[Morris]

Thanks guys!


Error ratio is 3%

 

[Pythias]

https://www.1010tires.com/tiresizecalculator.asp

odometer ratio will be the same as the speedometer ratio

not quite true.....i changed the rear end from a 4.22 to a 3.9, introducing more error than was already present.. now i have a 7% error in speed plus or minus, but a 15% error ratio in the odometer, as measured thru 20 plus miles of interstate, using both a stopwatch. .. these speedo's aren't all that accurate.. at least MINE isn't.

 

[regularman]

I never even thought to check my odometer. I will check it next time against the GPS and see how far off it is. I know my speedo reads 7mph slow and 45 is really 52. I need to fix that because it is off the "bad way"

 

[Pythias]

I never even thought to check my odometer. I will check it next time against the GPS and see how far off it is. I know my speedo reads 7mph slow and 45 is really 52. I need to fix that because it is off the "bad way"

a CONSTANT 7mph slow? no matter your speed?.. take off the needle and put it back on in the right place! simple fix for that.. if its a percentage error, that's quite a different problem.. if its a steady 7%, someone changed the rear end from a 4.22 to a 3.9!

 

[bugimike]

...which is exactly what Kim did!!! :wink:

 

[regularman]

Ha, no I changed from a 3.90 to a 3.55 and then added a 5 speed. I need to do some more testing to see how linear it is, but that is what I am going to do frist is adjust the needle. I will make it right at about 60 and then hopefully it will be close enough through the rest of the range. I got gearing for days now. One for every occasion :lol: