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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
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| | | | Joined: Apr 2016 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2016 Posts: 52 | I have installed a 72 GM 4 speed and a 79 pickup 3.08 rear end in my 54 3100. My speedometer now reads 40 miles per hour when my actual speed is 60. Is it possible to make the original read correctly? | | | | | Joined: Apr 2016 Posts: 52 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2016 Posts: 52 | Did a little research and can possibly answer my own question.. I think the difference between my speedometer reading and actual speed is too great for a simple gear change. Looks like I'll need a "mechanical ratio adapter" Still open to your ideas. | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | The Speedometer Shop can make the ratio adapter and Jim Carter Truck Parts supposedly can make the ratio adapter. Here are other sources. | | | | | Joined: Feb 2001 Posts: 1,094 'Bolter | | 'Bolter Joined: Feb 2001 Posts: 1,094 | That is a factory gear ratio, I would think you could get the correct gears for the transmission. lot of vehicles produced with a 3.08 rear end and a standard transmission.
Tommy
59 apache 1/2t
261 short stepside
| | | | | Joined: Apr 2005 Posts: 2,832 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 2,832 | Both TCI and the Transmission Center have drive and driven speedometer gears and boxes where you can plug in your gear ratio and tire size to get the correct gear combination. I use my gps to get the true speed and then see what percent the speedo reads fast or slow. I can then count the teeth on the existing drive/driven gear and pick a combo from the listed gears that will speed up or slow down the reading by that percentage.
Evan
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