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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: g money on July 13, 2014, 08:35:56 am

Title: speedometer recalabration
Post by: g money on July 13, 2014, 08:35:56 am

I have an 83 c10 and swapped the rear end out of a blazer with pozi-traction but now my speed o buries out at 85 when im doing fifty, are there any adapters out there that ACTUALLY WORK? Any other ideas?

Title: Re: speedometer recalabration
Post by: DanMcG on July 13, 2014, 10:56:03 am

You should be able to change the speedo drive gear if you know your tranny and gear ratio

Title: Re: speedometer recalabration
Post by: rich weyand on July 13, 2014, 11:39:20 am

Here's where to get the speedo driven gear for the tranny.  You can either calculate what you need from tires and gear ratio, or look at the one you have and figure what percent change you need from your current reading.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm

Title: Re: speedometer recalabration
Post by: BADAZ chevy guy on July 13, 2014, 04:42:43 pm

This is a quote from the web site rich weyand just posted. There's some really good stuff there, but this is something a lot of guys miss. Tire height ISN'T just from the ground up!
"To determine your tires true diameter in relation to determining what speedometer gears you need, measure from the ground up to the center of the axle then multiply by two. The actual run out of the tire can be as much as 2 to 3 inches less than what the circumference of the tire measures. In effect your tire has two diameters."

Title: Re: speedometer recalabration
Post by: rich weyand on July 13, 2014, 05:29:46 pm

Tire diameter for purposes of speedometer calibration is rolling radius times 2.

Title: Re: speedometer recalabration
Post by: Stewart G Griffin on July 14, 2014, 06:37:13 pm

Another idea is to go with GPS.