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Speedo calibration

4K views 5 replies 4 participants last post by  JCamasto 
#1 ·
I went from 215-65-16 tires to 225-60-16 that are a little shorter. Did a mileage check and got .8 reading in a mile. Can speedo be adjusted to tires, loosing money as this is a taxi.
 
#2 ·
Your speedo seems to be way out.

Changing from 215/65-16 to 225/60-16 is only a difference of about 2.7% but your mileage check is 20% low.

Best way to check is by resetting speedo trip recoder to zero, resetting trip computer to zero, then checking them against distance recorded on a sat nav over 10 miles or more.

My own speedo is pretty accurate, compared to sat nav it reads about 2mph fast at 70mph - less than 3% error.

Thinking about it, 215/65 = 139.75mm radius while 225/60 = 135.00mm radius so worn 215/60's should be virtually the same radius as new 225/60's. Sounds like the new tyre size is not the problem.
 
#3 ·
Your speedometer/odometer should have gone the otherway. Given that there is roughly a 0.4" inch difference (with your new tires being shorter), your new tires will need to rotate more times per mile, and as such, your speedometer should be registering high.

Could it be that your van was originally equipped with 14" or 15" tires? If that happens to be the case, then the two 16" tire sizes that you've had on the van are both larger in diameter than the tires your van was calibrated for.

Regarding whether you can recalibrate your odometer, I don't honestly know.

Keep us posted.
 
#6 ·
As far as taxis billing per mile traveled (and all else being equal/same as before): you should be making incrementally more money with smaller wheel/tires... assuming your billing is based on vehicle miles indicated, and not using a gps to calculate...

-Jim
 
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