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Hello, my speed gauge reads 70, but is really going 68, I have a superchips calibrator and need to know if I should go up a tire size to slow it closer to 68?
 

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If the gauge is reading faster than you're going, then it thinks you have bigger tires, not smaller. You want to tell it they're a little smaller.
 

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Almost all speedometers are a little optimistic when delivered by the factory and have been for eons. A 2mph difference is very minor and almost impossible to get out entirely.

For one thing the "nominal" tire size for the majority of stock JK/JKUs is 32", but the tire when new is more like 31.5" and then as the tire wears it gets smaller and the corresponding speedometer indicates the same speed but the actual speed is a little slower.

If you have the factory nav, you can't use that speed, for instead of calculating the speed from the GPS signals (as stand alone navs do) the speed in the system is an input from the Jeeps computer (and the same as the speedometer).
 

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Hello, my speed gauge reads 70, but is really going 68, I have a superchips calibrator and need to know if I should go up a tire size to slow it closer to 68?
I just deliberated changing my calibration using the AppcarDiag FCA software.

The PCM had tyre circumference of 2429mm whereas the stock tyres are actually 2558mm (around 5% under). My aftermarket tyres are 2489mm so not too far out but I changed it anyway.

I'm sure they do it so that the speedo under-reads. In Sydney, we are unofficially allowed 10kmh variance on our speed. In nanny state Melbourne it is zero so having a speedo that is 5% under limits your chances of getting a ticket and gets the manufacturer out of hot water if anyone was to claim that their speedo was at fault.
 
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