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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: r1087 on April 08, 2014, 03:06:31 pm

Title: speedometer gears
Post by: r1087 on April 08, 2014, 03:06:31 pm

anyone know how to find out what speedometer gears i would need for an '87 r10 2wd 305; automatic w/overdrive. tire size 235/75-15. looking for tooth count and or colors for both drive and driven gear. truck is in storage so its not readily available for me to take apart and look. also search function wont work for me on the site for some reason. any help would be appreciated.

Title: Re: speedometer gears
Post by: BADAZ chevy guy on April 08, 2014, 08:47:54 pm

Here's what you need to know. The ratio of the rear differential, and the height of the rear tires. (Measure from ground to center and x that by 2) don't just measure from ground to top, you'll get the wrong number.
Then go here>>> http://www.transmissioncenter.org/GM_Speedometer_Gears.htm

Title: Re: speedometer gears
Post by: Captkaos on April 09, 2014, 09:19:15 pm

http://73-87.com/7387garage/drivetrain/spdgears.htm

Title: Re: speedometer gears
Post by: hatzie on April 11, 2014, 02:48:34 pm

I made a spreadsheet that calculates RPM & speedo gears based on entered tire size, axle ratios, and transmission ratios. 
At the top right side there's a table with ratios from some popular transmissions & transfer cases.
At the bottom left are the calculated speedo gear tables.
It's in excel 2003 format so the free Open Office 4.0 spreadsheet application will read it.
https://www.dropbox.com/s/o6x3430fjrrwob2/RPM_Gear_Axle_Transmission_TransferCase.xls (https://www.dropbox.com/s/o6x3430fjrrwob2/RPM_Gear_Axle_Transmission_TransferCase.xls)

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