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Discussion Starter #1
Trued search function. Anyone ever had to adjust or reprogram a speedometer in 91-93 buick or olds? I use my gps forr my speedo now but woukd like the cluster to read right
 

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You need the correct drive and driven gear. What ratio is your rear? What tire diameter are you running. What gears are in the VSS. The drive gear is probably an 18 tooth (light blue). With 28 inch tires, and a 3.23 rear, you need a 42 or 43 tooth gear (green, or purple). If you have a 2.73, you need a 35-36 tooth (orange or white). If you have smaller or larger tires, you must adjust the gear ratio. The driven gear does NOT have a shaft on it. The 43 tooth gears are made of unobtanium. One tooth on the driven gear will adjust the speedometer by about 3-4 MPH. A larger gear will make the speedometer read slower than the smaller gear, so if you are off by 4 MPH (car going slower than speedometer) you need a one tooth larger driven gear. You also have to be conscious about the VSS itself. There are 2 different units. One is for 40+ tooth driven gears, and one is for 39- tooth driven gears. If you have smaller tires, you need a larger driven gear or a smaller drive gear. 15-35 is the same as 18-42.


29 inch tires seem to be more accurate with the 18-36 ratio than the 28 inch tires and a 2.73 (speedometer is 2 MPH fast with the 29 inch tires (235-75-15), 6 MPH fast with the 28 inch tires (225-75-15)). 18-43 seems to be very accurate with a 3.23 and 27 inch tires (255-60-15/255-50-17). Those ratios and tires are what work on my cars.
 

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Discussion Starter #3 (Edited)
Wow haha i didnt think the 700 had a drive gear? Course i was thinkin back to my home built 87 4x4 that i swapes out th400 for a 700. i know the old th400 in it had replaceanle gears dut didnt thjinkg the 700 was replaceable. Im runnin 255/50/17 as of rite now with the 2.73 rear (hate it.) The car had from factory 235/70/15 n still never read right. I gotta do math later xuz im drinkin mow but the car runnin 74 (on gps) on highway with the 255s has speedo at about 80. Switxhin out tirez thos week to 275/40/17s cuz i have 2 sets i got on steal and the 255s are babys ass smooth
 

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275-40-15 is a 25.7 inch tire. That will make the speedometer about 4 MPH faster (84 MPH). you will probably need a 38, or 39 tooth driven gear (38.52). The formula puts you right in the middle with an 18 tooth drive gear. If you got a 17 (red) tooth drive gear, you would use a 36-37 tooth driven gear (36.38). Since you already have the 39- VSS, you could use either gear set. The only issue you have is finding the gears.
 
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