Hello V8 Just upgraded my 1969 Buick with 3:73 gears in the posi rear end I have in it. The engine is a 350 just rebuilt with mild upgrades nothing major. The transmission is the TH350 auto. My question is now that rear end has the bigger gears, is there anything else that needs to be done to complement that? My transmission now has to be shifted out of every gear much faster . My first and second gears now feel so short before the engine is screaming, can that be fixed or adjusted? Also when the car Speedo says it is at 70 mph it clearly is not going 70 mph . I also feel like at higher speeds say 65/70mph you can feel some significant rumbling and stress coming from possibly the rear or Tranny etc. what could be causing that? Any and all help will be appreciated .
Nothing else really needs to be done. You can try adjusting the screw in the modulator to hold the shifts longer under part throttle if it shifting too fast. Adjusting the modulator will let the trans hold the shifts longer You need to replace the plastic speedo gear in the trans so the speedo reads correctly.
The rumbling ..... well, now your driveshaft is spinning much faster at highway speeds. It likely needs to be balanced. You can try a DIY balance by putting a hose clamp on it at different positions and see if it helps..... the screw acts as a balance weight. Or take it to a shop to be balanced. I'll expand on that.... Jason means the vacuum modulator on the automatic trans. Full throttle shifts can be adjusted by modding the governor on the trans.
When I saw the 3 on the tree and th350, I assumed it was a troll post. FYI, 3 on the tree refers to 3 speed standard transmission with the shifter on the column.
You may want to invest in a gear vendors overdrive or a 200R4 if you plan to do much highway driving. Also, a GPS navigation device (Garmin, etc) has a speed readout that can help you figure out your speed.
The reason why it’s shifting sooner is because you increased the speed of everything forward of the differential, and that includes the governor for the trans output shaft, the weights are being thrown outward sooner (centrifugally) and shifting the trans sooner..... normal for the trans. It’s something you’ll have to get used too The ONLY thing I’d change is get the proper drive and driven gears for the speedo. What gears did you originally have?
The vacuum modulator can offer some very limited adjustment, maybe 2-5 MPH at most for minimum and part throttle shifts. You'll need to adjust the governor weights, they control wide open throttle shift points. B&M makes a very nice kit for doing that, but there is a lot of trial and error when doing so. I recommend you buy another governor and mod that. If you run into trouble, you can always put the stock one back in. https://www.summitracing.com/parts/BMM-20248/ CHANGE ONE WEIGHT AT A TIME and then evaluate. Same thing with the springs. To get your speedometer right, you'll need 3 numbers, 1) rear tire size and height 2) rear gear ratio (3.73) 3) number of teeth on the Speedometer DRIVE gear. The last one is the hardest. You'll need to pull the DRIVEN gear sleeve, then look inside at the DRIVE gear. It is on the output shaft, and it is helical, like a distributor gear. You'll loose fluid, so get the back end of the car high in the air to minimize fluid loss. With the transmission in neutral, turn the driveshaft and count the number of teeth on the DRIVE gear. You might be able to verify the number of teeth by DRIVE gear color. Once you have that, a simple calculation is all you need to see if there is an available DRIVEN gear that will get your speedometer accurate. https://www.tciauto.com/speedometer-gear-calculator
You didn't mention anything about your tires. The height of your tires can play a pretty big role in your realized final drive ratio, or the RPM experienced at any given speed. Taller tires will decrease your RPM. Shorter tires will increase your RPM. You need to make sure you have the tires your going to stay with and then work on getting your speedo calibrated by switching the Trans gears that control the speedo. I swapped out the original 2:92 gears in mine for a 3:73 and it made a huge difference in the responsiveness of the car. Also made traction harder to control. The downside is a higher cruise RPM and quicker shifting of the Trans but those are trade-offs I would gladly accept for the increased torque multiplication.
