gear ratio ?

[krazy_texan]

ok guys here is the deal i have my 1990 suburban i have been driving for sometime now. it had 3.73 gears in the rear but the ring gear broke a few teeth so i had a set of 2.73 axles laying around i through the rear one in my truck to get me back on the road and now i am hating my fuel mileage it is far worse than it was with the 3.73. i have since acquired a set of 12bolt 3.08 axles and now i am at a toss up with what to do. should i throw the 3.08s in the truck and drive it or should i spend the money rebuilding and re gearing the 3.08s to either a 3.73 or 4.10. just so you guys know that my plans within the next year are to remove the 3 in lift in the truck and go with a 6 in lift and 35in tires and i want to be able to pull a 20ft camper or my car hauler with ease. so what should i do gear wise with my truck. what are you guys running in your trucks.

[K5owner]

I have 4.10' in mine on a 4 inch lift on 35 inch bfgs and I should have gone 4.56's. On a towing rig I would def gear up higher numerically.

[Cabnchassis]

I have 4.10s and they are perfect for a stock truck up to like 31s, they might even still be comfy on 33s but my tires are 265/75/16s which are basically 30s. My cruising rpms are right where I'd want them economy wise. I would say go with the 4.56s too if you really want to run 35s. And then start upgrading the durability of your front end.

In case no one has told before these trucks don't like big tires much. The independent front diffs and everything that's attached to them hates big tires. Be prepared to break stuff wheeling on big tires in gm independent especially in a half ton gm truck with stock 4wd parts. They like break things like cvs, axles, and even diff gears. When you put those 35s and the lift on there stop trusting your tie rods too. They're just ok for stock trucks especially half tons, so they are getting a ways out of their class with tires that size. I'd carry a full set of them if I was wheeling lifted on 35s and my truck is a 3/4 ton and has a tougher front suspension set up.

[krazy_texan]

My truck is a v1500 suburban so it still has a solid front axle but I am planning on rebuilding the front axle end to end when re geared

[Cabnchassis]

Changes everything. I don't know how I missed that V there but I assumed it was a c/k series and that wasn't fair to your truck, its way more durable up front than that

Having a straight axle talk to a gear shop before you make any decision. Most independent trucks won't ever have the money put into them to maximize the size of tire you should be able to run with 4.88s.You have the durability to give you a little more tire freedom. A good shop should be able to calculate your cruising rpms just going off your tire size, trans, and diff gearing. You may find that 4.88s are better suited to what you're doing. With 4.56s depending on your transmission you may still be a little too high (or not thats why talking to a pro is must to get it right), or 4.88s may just give you a little more cushion to go up a tire size or two in the future. Those are also two pretty common gear ratios in bowties so finding the gears sets at a wrecking yard isn't out of the question either although you'd still need to rebuild both axles.

There won't be a lot of cost difference between the two but if you end up geared too high you won't forgive yourself for a good long while. Personally I won't touch a truck that has been lifted if it wasn't properly re-geared. It's total deal breaker because of the wear it causes. On the other hand if it's done right I'm immediately more interested.

Another reason to have the gears calculated is mileage.These things aren't good on fuel anyway but the wrong gears and it'll be a demon you can't keep fed either. Properly lifting your truck will also ensure that you at least don't lose any value and will usually gain value in proportion to the quality of lift and how far through the suspension, gearing, and such that you've gone. Lastly if it has a stock 700r4 you need to do everything in your power to not give it one more reason to commit suicide. It's ok trans on a stock truck, but as soon as you start upgrading the rest of the truck and don't upgrade it, it becomes when not if it lets you down.

[K5owner]

Krazy; what trans does that suburban have? Is it the TH400 or the 700R4? I know the 4L60E wasn't out yet and the earliest you could get one was 91/92 I think. If it is the TH400 then stick with the 4.56's with 35's if it is the 700r4 then I wouldn't hesitate to run 4.88's or even higher depending on your towing needs. The 700r4 when built right will be fine for that application. What you don't want is it hunting for gears when towing especially going uphill. That will kill the trans in short order. Also make sure you correct your speedo for the gear and tire change.

[rdauphinais]

I know im new here but i think you should go with 4.56 there is hardly any price difference between that and 4.88's, but with the 4.88's the gas mileage won't be great. If it was me than i would just do the 4.56 gears now and be able to just throw some 35's on there later. but if you are planning on running say 38's, than i wouldn't go any less than 4.88's

here is a chart if it helps: http://www.4lo.com/calc/geartable.php

[krazy_texan]

I'm running a stock 700r4 but I am manager of a transmission shop so I already had plans to pull the trans in the spring and have my builder go through it and upgrade I have a differential guy that is going to teach me how to rebuild the diffs I'm going to rebuild the transfer case myself and I'm also going to put 4 wheel disc brakes on the truck. I have also been doing some reading and research other than on here and it looks like 4.56 gears is probably the best route to go for me.

[dfrank]

If it is a 3 speed trqnny than go with 4.10's if it is a 4 speed overdrive than you can run 4.56's without ****ty mileage. I am running 4.10's with 35's and have no problems getting up to speed. Then again I also have 450hp/470tq so that may play a role in it as well.

[Cabnchassis]

There is no comparison between a stock th400 and a stock 700r4. The 700r4 has a well documented history of failing if pushed. Add power, or add lift and big tires and you basically just put a fuse on it. The TH400 was GMs 3/4 and 1 ton automatic trans for pretty much the entire life of the square body. You can build a 700r4 to do a whole lot, but it's expensive and you're stuck using the specialty parts you built it with. Th400s are cheap and plentiful. The th400 has a long resume of pulling heavy loads in stock form. Not sure why you think it wouldn't handle 4.88s either, since it came factory with 4.56 or 4.88 gears on cab and chassis trucks.

Any tranny that is forced to shift constantly will die early, which is one more great reason to talk to a pro before you make the change.

Also I wouldn't trust that chart too much it's pretty generic and doesn't work at if you have overdrive. A th400 is 3 spd, 700 is 4spd and 4th in a 700 is a higher ratio than third in a 400. How do they have the exact same RPM figures?

[K5owner]

Originally Posted by Cabnchassis

The th400 has a long resume of pulling heavy loads in stock form. Not sure why you think it wouldn't handle 4.88s either, since it came factory with 4.56 or 4.88 gears on cab and chassis trucks.

True but who wants to cruise highway speeds while towing with 4.56's on 35's buzzing around 3300+RPM with a TH400? Mileage will be horrible, (not that it is great anyway) as will the driving experience on long trips. Especially when you can get a 700r4 built that will live a long life under the conditions in which the OP needs it for. Will it be as strong as a well built TH400? Nope but it can be done and since krazy_texan is the manager of a transmission shop I am thinking he can get it done for a really good price to boot.

[Cabnchassis]

My point was if he had a th400 in it there is no reason to spend a ton of $$ building and installing a 700. Replacing a trans is a lot more expensive than losing a little mileage. Since he has the 700 building it is his best option and of course 4th gear is nice, but a whole lot of people live without it just fine. Yes mileage would drop with 4.56s or 4.88s and yes it would be wound up, but that was the tech from those years, you roll with it or you spend a bunch of dough to change it. My burb is 6.2/th400/4.10 so I know the 3 speed struggle at highway speeds, but even a 5 mpg improvement would take a lot of years to pay you back for a trans build and swap.

That chart is pretty much a big piece of four letter word. It's for manual trans gearing which means a 400 would probably run even higher RPMs than it lists, and a 700 dramatically lower. It's ok as a generalization but you still have to completely guess what actual cruising rpms would be in any auto trans.

[mattybilt79]

Your best bet in my opinion is to find some 3/4 ton axles to start with instead of regearing and rebuilding your stock ones. The axles are literally bolt in and have the axle ratios you need in stock form. Find a 14b full float rear must be out of a 3/4 ton 1 ton axles have different spring pad widths unless you want to cut them off and weld new ones on. Front axles in 3/4 tons are literally the same as 1/2 tons but with 8 lug outers and lower factory ratios. Why not change out gears spindles rotors on stock front axle? Because your more likely to find a set of matching axles in a vehicle already. If you want an even stronger front axle find a Dana 60, couple minor changes to steering to deal with or go to a crossover set up but you will never break it with 35s and most came stock with 4.10 or 4.56 gears. Stock axles should never cost more than 200 bucks a piece a 60 front can go as high as 1500 or more depending on where you live but I've seen them as low as 500. For your situation if it was me I'd go up to 4.56 ratios 3/4 8 lug axles and build up the 700r4. The trans will be happier with the lower ratio won't have to work as hard and make sure your transfer case is in good shape too should be a 208 chain drive, always a good idea to make sure chain isn't stretched and reseal if tranny is out.

[K5owner]

Owning these trucks and upgrading them never makes you any money, you drive and own them because you love them. I swapped a 700r4 into my CUCV and took out the TH400 and it cost me little. Now if I had dropped in a 4L80E then yes i could see you spending a decent amount in doing so as the controller for those trans are pricey. The entire swap including finding the correct converter and trans was just over $700 bucks after I sold off the 400 and converter. Well worth it as the 6.2 went from 16-18 MPG at 55-60MPH screaming down the fwy and out of my powerband to 18-22 MPG while at 65-70MPH in the correct powerband and at a normal highway speed that wasn't going to get me run over on the freeway. It is all personal choice. On as heavy and big as a burb is riding on 35's and with a SBC I would stick with the 700r4, 4.56's (for reg driving) to 4.88's(for towing) and enjoy what that package can give you. Better yet would be a 6.0 with a 4L80E swap out of a newer GMC or chevy and then use some 4.56's.