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Speedo accuracy

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Check www.f-fody.org

There's a speedo calculator program link there.  You type in the tire size, transmission gear ratio and rearend ratio and it will give you MPH every 100 or RPMs

By the way, you only need to put in one transmission gear ratio if there is only one you're interested in, such as top gear.

Check it out...


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My 2000rpm speed is 76 mph.  I checked the odometer since my post against about 20 'mile' markers on the highway today and I get 1.1 miles on the odo per mile which coincides with the apparant 10% increase in my indicated speed as opposed to yours.

Could I have an incorrect gear for the speedo in the trans or is it all done via a signal to the ECM?

Anybody know the 'fix'?

Thank you.

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Tell the dealer about your findings and he can reprogram your computer module for you...

It sounds to me like you may be programmed for a 3.42 rear gear ratio.

You are saying that your Avalanche registered approx 22 miles over a 20 mile section of highway right?

I would like to think that the final gear ratio in the tranny is the same as the 1/2 ton pickups but I don't know...

If your mph's are higher and your odometer is registering more miles than you travel, then I'm guessing they programmed the wrong settings at the factory.  The 2 things to check with the dealer are to make sure 3.73 and not 3.42 gears are programmed along with 265 tires and not 245 tires.  Either one of these would cause the problems you have stated.

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