Axle Ratio Help

[Car'n'Guitars]

Originally Posted by lap15

Here's my .02

My 86 C10 (HD half ton 6100 GVW) has been a work truck all it's life with a 305, SM465 & 3.08 rear axle. It's handled everything you could throw at it- city, highway, heavy loads, light loads and towing- while providing 15mpg. (Top speed is about 65mph)
I would leave it alone, unless it breaks.

That's what I was thinking too. But how far is the SM465 compared to the 700R4.

[Edahall]

Keep in mind those 2 previous posts are referring to trucks without overdrive.

[Car'n'Guitars]

Originally Posted by Edahall

Keep in mind those 2 previous posts are referring to trucks without overdrive.

That's what I thought, I think I'll wait until something breaks and then redo the whole drivetrain. The truck has never had anything replaced or rebuilt to my knowledge.

[lap15]

Originally Posted by Car'n'Guitars

That's what I was thinking too. But how far is the SM465 compared to the 700R4.

Totally different I suppose. But I would still leave it alone until you MUST fix it.

[piecesparts]

As you can see, everyone has a particular thing in mind that fits their needs. I have a 400+ HP 383 stroker engine with a 700 R4 in my truck, I have a set of 3.73 gears and I run the truck on the highway at speeds around 75 MPH and I get 15 MPG, with the ability to jump out and hit speeds greater than the speedo can read without any issue. I do run a 28" tall tire, but my truck is lowered, so that is what I like. This truck started life with a 305, 3.43 gears, and a TH350C tranny (no OD, but it did have a lockup converter) The mileage that I get now is about the same as the original MPG , but it was an enemic truck without any true acceleration capabilities.

With the experience that we had with my son's truck, I would never ever go with an OD tranny and 3.08 gears behind it, especially with 30" or taller tires. You should make a choice of 3.08 gears and a standard (non-OD) tranny or use an OD tranny and a set of deeper gears. The OD tranny and a set of 3.42 gears is a pretty common mix in the later model trucks and it works well with the fuel injected engines. My 1991 Silverado got 20 MPG with a 350, 3.42, 700 R4 setup. My DD that I now have is a 2011 5.3, 3.42, 6 speed auto truck that gets around 20 MPG, also. The fuel injection is a big part of this combination.

[Car'n'Guitars]

Originally Posted by piecesparts

As you can see, everyone has a particular thing in mind that fits their needs. I have a 400+ HP 383 stroker engine with a 700 R4 in my truck, I have a set of 3.73 gears and I run the truck on the highway at speeds around 75 MPH and I get 15 MPG, with the ability to jump out and hit speeds greater than the speedo can read without any issue. I do run a 28" tall tire, but my truck is lowered, so that is what I like. This truck started life with a 305, 3.43 gears, and a TH350C tranny (no OD, but it did have a lockup converter) The mileage that I get now is about the same as the original MPG , but it was an enemic truck without any true acceleration capabilities.

With the experience that we had with my son's truck, I would never ever go with an OD tranny and 3.08 gears behind it, especially with 30" or taller tires. You should make a choice of 3.08 gears and a standard (non-OD) tranny or use an OD tranny and a set of deeper gears. The OD tranny and a set of 3.42 gears is a pretty common mix in the later model trucks and it works well with the fuel injected engines. My 1991 Silverado got 20 MPG with a 350, 3.42, 700 R4 setup. My DD that I now have is a 2011 5.3, 3.42, 6 speed auto truck that gets around 20 MPG, also. The fuel injection is a big part of this combination.

Yeah I think I would like to go to 3.73 without changing the motor or trans. But I'm not sure yet whether I should just wait until something breaks and build the drivetrain I want, or to just change the gears outright.

[piecesparts]

Once you put the 3.73 gears in, you will wish you had done it a long time earlier. The change is amazing.

[JPalmer81]

I would never go higher (numerically lower) than 3.73, but that is me personally, to me 3.73 is hands down the best all around ratio and will pretty much perform well with whatever setup you have and would make a HUGE difference in your truck and I can't emphasize the HUGENESS enough. Your truck just has to be a dog with the current set up. I would almost put money that your truck came with a TH350. 3.08 will only work if you have a diesel engine of some sort in there. th350 th400 and sm465 all have the same final drive ratio of 1.00:1.

[piecesparts]

I am onboard with JPalmer 81 in this matter.

[beamn7]

Gotta say, I agree too. My 85 was a factory 305/700r4/3:08 truck and it idled down the freeway! Great way to build up carbon not to mention suck the mpg, was about 10mpg as previously mentioned. I now run a 355 / 700r4 / 3:73 (w/auburn limited slip carrier) and 31x10.50 tires and I am VERY happy with it. 2150 rpm at 70 mph and 14.5 mpg.
3:73's in my opinion are a great all around gear. They also keep the trans from being too "busy" shifting in and out of OD all the time. I have 3:73's in my 05 as well and am very happy.