changing gears

Discussion in 'GM Powertrain' started by theburban, Mar 6, 2009.

  1. theburban

    theburban New Member

    The previous owner of my burban was going to use the truck as a commuter (he is a big guy and needed the room) anyways, while he was dumping money into this thing he had the differential gears changed to 3.08. I use the truck to tow a 26 ft travel trailer and have always had issues on hills. The engine in the truck is the engine that was in the car i used to use to tow the trailer before the tranny went out in that car. I know that the engine was more than capable of pulling that trailer over a mountain pass at the speed limit. In the burban im stuck limping up a hill at 30 mph, sometimes slower, in first gear with the engine bogging. It was suggested to me that the gears in the axles were to blame for my problems so i called Randys Ring and Pinion. They told me that even though the ratio made it seem like it was a lower gear it was actually a higher gear that was prolly installed to save gas (but not to tow). They recomended that i change the gears to 3.73 which is what most trucks come with stock. They tell me that once i get the gears to 3.73 that i should be able to tow with no issues. I have the gears for the rear axle and im picking up a 3.73 front axle this weekend. Does this answer sound right to other people? Im worried that after going through all this work i will still have my problems. The transmission is a turbo 350 with around 50,000 miles on it. The engine is a chev 350 with a #1407 performer series carb and was rebuilt around 10,000 miles ago with a RV cam (at least thats what i asked to be installed). The tires are about 32 inches. The axle thing makes sence to me (kinda) i guess i just need some reassurances.
    Last edited: Mar 6, 2009
  2. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    I'm no expert in gearing, but this sounds right. If the gearing was changed before, that could very well be the source of the problem. Why did the previous owner change them?
  3. theburban

    theburban New Member

    Im guessing that he was wanting to get lower RPMs at higher speeds for highway fuel efficiency.
  4. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    The 3.73 gears will make a big improvement. Is your speedometer off or was it reprogrammed when the gears were changed?
  5. bry2500

    bry2500 Epic Member 5+ Years 1000 Posts

    I would definitely blame gearing. Especially since the previous owner had changed the gears for commuting. The engine and tranny are fine for towing.

    The only concern I would have is if the kick down for the tranny from the carb is installed correct. This would definitely give you a bogging issue when you want to accelerate under load. But you mentioned that you were in first gear so then gearing is probably the main issue.
  6. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    Definitely change the gears. The only thing I might have done differently, especially if this burbans primary reason for being is to tow that trailer over the mountain, would have been to put 4.11 in instead of 3.73's.
  7. theburban

    theburban New Member

    The speedometer is a whole other issue. It worked well when i got it and then all of the sudden was 10 MPH off. I had it reset at a shop and it worked well until the needle started bouncing drastically. I replaced the cable and head unit. I have had to replace the speedo head three times total and now its off and slightly bouncing again. Now i just use a GPS as a speedometer.
  8. theburban

    theburban New Member

    Yes, when i need it i will take the shifter and throw it all the way down to help me get a leg up on the hill.
  9. theburban

    theburban New Member

    I dont do mauntain passes often i was just using that as a reference. Primarily the hills that i go up are at most a mile long just steep. As im coming up to the hill i will floor it to try and get some steam and might hit the bottom of the hill at 70, but by the time im at the top im down to 30.
  10. phoebeisis

    phoebeisis Epic Member 5+ Years 1000 Posts

    If you are planning to tow, then 3.08 is like everyone else said-too tall.

    How many RPMS do you turn in top gear at 60 mph? I'm guessing 1500 rpms?
    1500 rpms would be good for a travel vehicle/highway vehicle road trip vehicle, since the lower RPMs would probably improve the MPG a tiny bit and the motor noise would be less.

    1500 RPMs at 60 mph isn't so good when you vehicle weighs 9000-11,000 lbs like yours must with the trailer in tow.

    Switching to the 3.78 should bump you up almost 350RPMs at 60 mph-maybe 1850-1900 vs 1500.

    If it was a pure heavy tow mountain roads vehicle, you might want even more RPMs-like Mr Shortys 4.11 which would bump you up 500 RPMs at 60 mph.

    Your 3.78 should work fine for normal towing with occasional mountain towing.


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