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It may be helpful for you to note that when I pass roadside automatic speed detectors (not Police ones) that 30 mph shows as 27mph, Same happens at 50mph when it shows around 47mph. This is with the bog standard factory fitted wheel. I have made no change to the tyre wall height with my new tyres.

Lots of people will tell you that there is an approximate 10% deviation from normal speed built into any car and the above has made it more clear for me.

Imagine your car is 100% absolute perfectly calibrated with new tyres. Later when they are bald and worn you would not be showing a true and accurate speed. ie the formula used to measure the circumference of the tyre would now produce a slightly lower reading.

Now, if you were to fit a larger wheel ie 16" then naturally the circumference should increase for the same depth of tyre that was previously fitted ie 75mm

This is only a theory but it may well be that when your speedo is factory calibrated, it will be set to the highest speed, to accomodate the widest standard wheel that can be fitted. Perhaps that is why there is a 10% deviation in the rumour.

What is the widest standard wheel and tyre that can be fitted to the Jeep.
 

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Most cars are built to +2/-4 tolerances. So yo may be going up to 2mph faster than indicated or 4 mph slower. (I've run into some cars that were +2/-6, and for police vehicles they usually have tighter tolerances).

For the renegade, if you dig around here, it's been discussed and it seemed the goodyear wrangler rs-a tires on the TH were spot on (at least when new). Which would mean most of the other tire options were slower than displayed. When I moved to the pirelli scorpion ATR, I got the expected ~1mph discrepency at 60 and at around town speeds it was below the resolution of the speedometer. At about 14k on the tires, it's reading very close to spot on.

Based on past experience, a lot of vehicles seem to jsut calibrate to the largest OEM tire for all the trim levels. Except for trucks, and trucks usually have a huge variance between multiple tire options.
 

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The difference shouldn't really be an issue. I once saw a chart that showed the deviation based on tire size and really there was no change until you got up to +60mph and even then it was minor. And that was just going up a size to fit a/t tires on the TH.
 

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Dealers can do it. It is called "Pinion Angle Adjustment". nothing to do with the actual angle of the pinion, but the size of the tires that are mounted on the vehicle.
Depending on the year, make and model, it uses the tire size (using my tires for example BFG KO2), i.e. 245/65R17, or the revolutions-per-mile, i.e. 704 to correct the speedometer.

Some independent repair facility's have the capabilities of making changes to the Pinion Angle (dependent on their scan tool).
 

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Depending on the year, make and model, it uses the tire size (using my tires for example BFG KO2), i.e. 245/65R17, or the revolutions-per-mile, i.e. 704 to correct the speedometer.
I knew there had to be some other way to add an accurate figure into the calculation that I was trying to explain above. Your explanation better enables me to understand what I was meaning :D

Circumference of any tyre (measured in inches or feet), at any stage of wear multiplied by the calibrated revolutions-per-mile, would give an accurate speed when measured by the Dealers vehicle PC.

Now it all makes real sense to me.

Big (y) to IDoMyOwnRacing. You are extremely knowledgeable. TY.
 

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I was told there is no provision to recalibrate it yet. When it happens, if ever, it will probably be a dealer update.
I went from p215/60/r17 up to p235/65/r17.and mine is exactly 4 mph off from 25 to 75 so if i set it at 69 I'm running 73
 
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