changing diff gears

[chrpmaster]

I have a 1976 C10 with th350 and 4.10 gears in diff.  I want to change the gears to allow better highway driving and less screaming from the engine.

How do I calc the difference in speed I can expect with the different gear ratios available?  For example if I am going 50 mph now at 2500 rpm how would I calculate the speed/rpm for different gears?

Thanks
Andy

[HAULIN IT]

Andy, Do a Google search for RPM MPH calculator, plug in your info & play around with rear gear ratios until you see what you need. You will want to use 1.00 as the trans gear (drive in the TH350) Have Fun, Lorne

[eventhorizon66]

I always just whip out the calculator and use the following formula.

(rear gear ratio X transmission top gear X vehicle speed X 336) / tire diameter = engine rpm

On non-lockup applications the torque converter stall will also affect the cruising rpm at certain speeds and throttle inputs.

so in your case it's currently: (4.10 X 1.00 X 55 X 336) / 29" = 2613 rpm

w/ 3.42 gears: (3.42 X 1.00 X 55 X 336) / 29" = 2179 rpm

w/ 3.08 gears: (3.08 X 1.00 X 55 X 336) / 29" = 1963 rpm

[Toolmaster]

http://www.ringpinion.com/Calculators.aspx

[VileZambonie]

Keep in mind how much throttle opening will it take to make the engine achieve the rpm to keep the vehicle at a steady cruise and keep up with traffic. Just using a calculator and on paper won't take into consideration how much fuel your engine is sucking down. Your best bet would be a 700R4 and leave the gearing you have.

Also just to point out terminology: differential gears are the gears that allow your axles to turn independently, your final drive gears are the ring and pinion.

[eventhorizon66]

To prove Vile's point.  My truck got about 1-1.5 mpg (on avg) worse fuel economy on the highway with a 3.08 in the rear rather than the current 3.73.  You had to push the throttle noticeably harder to maintain speed with the 3.08s at speeds of 55-65.  And passing almost always involved a downshift or two, whereas now I can pass easily in OD.  Not to mention city driving now requires much less throttle to accelerate at the same rate as before.

[VileZambonie]

A factor GM engineers obviously didn't take into consideration when they were making these trucks. You'll notice most of the ford explorers come with 4.11:1 final drive ratio and a 4 speed auto.

[chrpmaster]

Thanks for all the help guys.  Though it would be nice to get better gas mileage with a different final drive ratio I actually just want to be able to drive normal highway speeds without feeling like the engine is going to blow up! 

What year truck can I take gears from that will fit on mine?  I have the 12 bolt rear cover.

Thanks
Andy

[DnStClr]

Here's another website that gives a ton of info for you chrpmaster-

http://www.drivetrain.com/gm8.875trk.html

[chrpmaster]

Here's another website that gives a ton of info for you chrpmaster-

http://www.drivetrain.com/gm8.875trk.html

Thanks

I called several transmission shops locally and was quoted around $500-600 for the change of the final drive including bearings.  After pricing parts I was up to around $300 to $400.  I am looking at this and thinking that only around $200 for labor isn't bad.  Especially when several of the parts sites strongly recommend professional installation.  I like to turn a wrench as much as the next guy but I have never even opened up a differential before.  What do you guys think?

Andy

[SUX2BU99]

I JUST went through a very similar scenario, Andy. Only for me I was going with deeper gears instead of taller. My truck had a 12 bolt with 2.76 gears and I wanted more performance. I found a 3.40 posi 12 bolt rear end that fit the bill perfectly, for $150.  I also checked around at what it would cost me to change just the gears and locally it was around $800  That's when I went rear end shopping. Pulled diffs at the bone yards were $400 but I found this one at a small country bone yard and it was already pulled. It was covered in surface rust which creates it's own problems like rusted, stuck bolts but everything else was perfect. I spent another $100 in drum brake parts to freshen them, and about another $120 collectively for some brake tools, fluids, gasket, sealant, u-bolts, etc. to do the swap. I have it in and I'm happy with it. Might be something for you to consider. Any GM truck (not van) 12 bolt from 73 to 81 will fit, and you will want one from a 1/2 ton, 2wd to be a direct swap.  Finding a 3.08 one should be easy.

[chrpmaster]

I JUST went through a very similar scenario, Andy. Only for me I was going with deeper gears instead of taller. My truck had a 12 bolt with 2.76 gears and I wanted more performance. I found a 3.40 posi 12 bolt rear end that fit the bill perfectly, for $150.  I also checked around at what it would cost me to change just the gears and locally it was around $800  That's when I went rear end shopping. Pulled diffs at the bone yards were $400 but I found this one at a small country bone yard and it was already pulled. It was covered in surface rust which creates it's own problems like rusted, stuck bolts but everything else was perfect. I spent another $100 in drum brake parts to freshen them, and about another $120 collectively for some brake tools, fluids, gasket, sealant, u-bolts, etc. to do the swap. I have it in and I'm happy with it. Might be something for you to consider. Any GM truck (not van) 12 bolt from 73 to 81 will fit, and you will want one from a 1/2 ton, 2wd to be a direct swap.  Finding a 3.08 one should be easy.

Just out of curiosity what did you do with your 2.76's?  Interested in selling it?

Andy

[SUX2BU99]

I currently have my 2.76 diff listed on Craigslist right now. I'm sure shipping though would be huge from Chilliwack BC Canada down to Texas! I have it listed for $200.

[bob77]

Just today I swapped the 4.10's in my shortbox out for a rear end with 3.07's. The swap was pretty simple. I have a 78 that I swapped out from. Now the 4.10's are in the 78 which I am going to be selling. I have a question that may sound quite foolish, but I need to ask. What is the easiest was to determine if a rear end has posi-traction. I have heard a few "wives tales" about jacking the truck up and spinning one wheel to see which way the other spins and such. I am just not exactly certain that I am looking for the right thing. I'm sure there is a ver simple answer, please let me in on it. In selling the 78 I kind of want to highlight the rear end considering someone may be interested in the 4.10's. I'd like to make certain whether or not it is posi before I Craigslist it.
Thanks.

[VileZambonie]

Best way to tell is to pull off the meatball cover.