Randalla Posted January 17, 2023 Report Share Posted January 17, 2023 My 620 has a stock 5-speed manual transmission and differential. I'm running 225/50/17 wheels and tires, which has my speedometer under reporting by as much as 20 mph??? I have not yanked the current speedo driver out to see which one I have, but I'm wondering, for those of you running oversized Titan wheels/tires, which one you have swapped in and how accurate it is. Quote Link to comment
datzenmike Posted January 17, 2023 Report Share Posted January 17, 2023 20MPH doesn't help much, sorry. Drive with a second car at a constant 50 MPH and accurately read what the speedometer is reading. (not GPS) This difference X2 is a percentage based on 100 MPH. Raise the rear of the truck so the transmission oil flows to the front and hopefully does not pour out when removing the speedometer sleeve. Have something handy to catch it just in case. Unscrew the speedometer cable but only loosen it for now. Remove the 10mm bolt that secures the pinion sleeve and using the loosened cable tug straight out. Wiggle but do not twist the pinion or the plastic gear may be crushed. What color is the gear, we need to know this to calculate how many fewer teeth are needed to turn the cable faster. 225/50R17 is 25.86" diameter wheel which is so close to the stock 26" diameter that I can't see how the speedometer is so badly off. Possibly the 5 speed is from something else with a different pinion gear? Maybe the differential isn't 4.375? A number of things could be different from stock. The '77 should have a Red 20 tooth gear but better to check to be sure. Quote Link to comment
Randalla Posted January 21, 2023 Author Report Share Posted January 21, 2023 Pulled the speedo driver out today, and it is a red 20 tooth version. I'm assuming I'll need one with less teeth to get a higher reading on my speedo. Quote Link to comment
datzenmike Posted January 21, 2023 Report Share Posted January 21, 2023 Have someone drive ahead of you at exactly 50 MPH and see what your speedometer is reading. From this number an amount out is known and it's easy to figure out how many fewer teeth are needed to correct it. Quote Link to comment
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