Change rear gears or not?

[worthirt_99]

I'm thinking about changing the diff gears in my truck. It has 3.55s in it now. I'm thinking about 3.73s. Is there a big difference, or should I just leave them alone?

[grinch4om]

you won't see that big a difference in 3:55-3:73's and your mileage may suffer. Now 4:10's, you'd see a difference

[454HO]

What transmission do you have? How do you use the truck?

[worthirt_99]

I have a 700R4, I'm pretty easy on the truck as far as use.

[454HO]

The 700r4 has a 3.06 first gear and 0.70 overdrive.

So with 3.55 rear your starting ratio is 10.86 and final drive ratio is 2.14

Going to 3.73 gears would change starting ratio to 11.41 and final to 2.61

With a 28 inch tall tire, rpm at 70mph would go from 2100 with 3.55, to 2200 with 3.73.

If you tow a trailer, haul heavy loads or drag race, then the starting gear would be noticably better with the 3.73's. And I doubt your highway milage would suffer at all. In fact, it may even get better eventhough you would be turning more rpm. This is because the engine would not be lugging.

[Pyrotechnic]

It all depends on what the power band of the motor is and at what RPM it is most efficient at. Also depends whether the power at the efficient RPM can move the truck at the desired speed.

A 2002 Silverado with the 4L60E and the towing package will come with 3.73 gears and 30" tall tires. This is roughly equal to 3.42's and 28" tires. Corvettes with 27-28" tires have come with 3.42's or 3.55's for a long time.

Generally, if your motor is stock or at least still has some guts down low if you have modified it, anything from 3.42 to 3.73 will work well with a 28" tall tire.

[Captkaos]

You may see improvements on both ends like Greg stated. It isn't enough to warrant a change in my opinion, but I don't know the reason why you are thinking about it.

[worthirt_99]

I was thinking about it since I have to bust the diff open anyway.

[454HO]

A new set of gears is cheap compared to the labor to install them. If you are opening the diff because your current 3.55 gears are bad, then the 3.73 may be a good option for you. If you are going in because of the carrier, maybe switching from open diff to a posi, then the 3.73 gears would be an incremental cost. You have to be the one to decide if it's worth it or not.

[shrtbx]

Im thinking about making the change soon as well. I have 3:08s right now coupled with a 700 r4 and 350 motor. I drive every weekend to san diego so im trying to figure out if 3:42s or 3:73 are the right choice. I keep reading that mileage might be better when the od transmission is hooked up with one of those two ratios. Been reading up on it and so far its 50/50 as far as opinions go. What do you guys think.

[piecesparts]

I have a couple trucks with 700 R4 trannys in them. The first truck started with a 3.42 gear and then I changed it to the 3.73 gears. The second truck had a 3.08 gear and they end result was the 3.73 gears. Both trucks benefitted from the 3.73 gears and the second was the most dramatic, of course. the first truck had the 3.42, but with the 700 R4 installed it had some cruise control and acceleration problems on the highway. At about 75 MPH, the truck would downshift more than I wanted it to, on the highway (in the hills). This truck got a solid 16 MPG on the hgihway with a 383 stroker motor and the 3.42 gears. the truck now gets 14.5 MPG and it has the kick that I wanted it to on the city streets and does just fine on the highway.

The second truck was getting a sloppy 11 MPG with the 3.08 gears, 700R4 tranny, and a 350 motor. The change to the 3.73 gears raised the MPG to 14 and gave the life back to the truck. It now acceleerates easily and does not shift down to much on the highway (in the hills). There is a good RPM that you want to get to with a SBC and I believe that it is around 2100 to 2500 RPMs for the best combination of driving and gas mileage. There can be some mileage gains, but you give up a lot on performance to gain it. A set of 4.11 gears will give you some serious pull, but the engine will scream at you on the road.

I can put either one of my trucks at 100+ MPH and still run around or under 3000 RPM. Plenty of room to accelerate for more.

[shrtbx]

im getting close to 15 mpg on the freeway with the 3:08 gears i have but im wondering if the guy who had it before me swapped out the original gears. Strange thing is there is no real pop on the street. i really cant see me getting more than 17 mpg with the upgrade. I guess the worse that can happen is i drop a mile and gain more zip.

[Pyrotechnic]

Originally Posted by piecesparts

the first truck had the 3.42, but with the 700 R4 installed it had some cruise control and acceleration problems on the highway. At about 75 MPH, the truck would downshift more than I wanted it to, on the highway (in the hills). This truck got a solid 16 MPG on the hgihway with a 383 stroker motor and the 3.42 gears. the truck now gets 14.5 MPG and it has the kick that I wanted it to on the city streets and does just fine on the highway.

Is this a flat tappet or roller cam motor ? Seems like flat tapper motors like to run a few hundred RPM higher at cruise than a roller cam will.

[piecesparts]

Originally Posted by Pyrotechnic

Is this a flat tappet or roller cam motor ? Seems like flat tapper motors like to run a few hundred RPM higher at cruise than a roller cam will.

I have a Comp Cams roller setup with roller rockers. I do run a shorter profile tire than others, but am here to tell you, the tires are relatively close to the same diameter as the 275/60/15s that came on it. I do not have a lock up on my torque converter and that costs me just a little bit on RPMs, but not very much. It does not matter much where I run the truck---75 MPH gives me a nice RPM and 100 MPH gives me a little more RPM, but still has the kick to pull hard.

[slugish]

3 speed 350 trans. 3.42 > best of both worlds

4 speed 700-r4 trans. 3.73 > best of boyth worlds peroid