Planning on doing following change to my '71 350-2 Skylark Custom : from 225/60R14 to 245/60R15 front, 275/60R15 rear.. Do I have to change the drive gear as well or only the driven gear? What was the drive gear for a standard TH350 and 2.56 rear?
Use the calculator here to figure out what gear changes to drive and driven gears you will have to make based on tire height , gear ratio etc. https://www.tciauto.com/speedometer-gear-calculator If your changing tire height only, not gear ratio, changing the driven gear will get you where you need to be... JIM
I don't think there is a standard DRIVE gear. You'll need to open it up and count the teeth through the DRIVEN Gear sleeve hole. The rear tire is all that matters. The 275/60R-15 is 28" tall The stock tire size is about 26.6", so your current 225/60R-14 are 24.6" tall, so your speedometer is way off now. If you jack the back of the car up high, and pull the sleeve out, you can turn the driveshaft with the trans in neutral, and count the DRIVE gear teeth. The gear is helical so it isn't the easiest, but it is doable. Mark one tooth with some white grease and turn the driveshaft. Do it a few times so you get it right. You just need the three numbers. Tire height (28"), gear (2.56), and the number of teeth on the DRIVE gear. Then just pop it into the formula to get the DRIVEN gear needed. https://www.tciauto.com/speedometer-gear-calculator
Thanks Larry, wanted to avoid this procedure initially. Just couldn't find a specific number as for the TH400 in the manual.
Really no way to avoid it. If you get the back of the car up, you can avoid as much fluid loss as possible. You'll have to pull the sleeve anyway to change the DRIVEN gear. Might as well be sure.
More reading: http://www.v8buick.com/index.php?th...eter-trans-gear-question.354151/#post-3026279 http://www.v8buick.com/index.php?threads/speedometer-gear-help.342103/#post-2894847 Jim
Ok, just finished the Job. Ok, just finished the Job. I have a 18 tooth ( green ) drive gear and a beige 35 tooth driven gear.. According to the computation I will need a 33 tooth driven gear for my 275/60R15 Setup right?
Yes, 33, but no such thing. 34 teeth is the closest. You can change the drive gear easy enough if you want.
Ok, I hate threads from wich you never hear about their outcome.Today was early christmas eve and the new wheels arrived. Here is before and after. I ordered a 34 tooth driven gear and hope it will get me an almost accurate speed reading.
hope the white lettering remains white unlike mine which are turning brown on the rears. Had the new fronts replaced due to the brown letters. Replacements are staying white, so far.
I purchased the front BFG T/A tires in early 2019. In a few months the white letters had all gone brown and would not cleanup even with toluene. I tries many other cleaners but toluene is the very strongest u can buy. With the Covid, it took until about June 2020 to have the dealer replace them. They are staying white. I purchased a pair for the rears Oct. 2, 2020. They, too, are going brown but I cleaned them in November & they went white, now browning. Other than cleaning the letters when they get dirty, there is no other maintenance required. good luck.
Good to know . Will keep an Eye on it. The car won't drive till march so I will keep the blue coating on.