• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Stock tire size

Share this page

Morris

Morris

Yoda
Offline
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?
 
Chalk, driveway, tape measure
diameter=circumference/pi
 
The stock tyre is a 145SR13 which is a 145/80R13 in modern terms. This equates to 562mm OD or 22-1/8".
 
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 said:
https://www.1010tires.com/tiresizecalculator.asp

odometer ratio will be the same as the speedometer ratio
Click to expand...

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.
 
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"
 
regularman said:
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"
Click to expand...

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!
 
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:
 
Back
Top