Changing gears from Detroit 4.56 on a 91 Suburban - how expensive/easy to do?

[Jessie James]

I'm looking at a used [edit] 1991 Suburban and the owner has done some upgrades, listed as:

Front and rear gears are 4.56 Detroit lockers (soft locker in the front), 1 ton 14bolt full-floating axle rear axle, and ¾ ton in the front. 38" tires.

So of course it gets like 6 mpg... Ouch! However, I like the lockers.

So how expensive and how tricky is it to upgrade these to something of a more normal ratio, keeping in mind the oversized tires, and what would a good ratio be? Yeah, I know these trucks don't get great MPG to begin with, I've got a '91 Blazer, but 6 mpg is a killer. I'd love to get at least 10 mpg for example. I'll probably only use the truck occasionally on the street, and some snow duty and light off-roading.

 

[chulisohombre]

with overdrive and those big of tires.4.56 is about where you want to be.some would even say 5.13 gearing would help you a lil on power and not lugging the engine.if you go any lower in gearing you would be wasting more money on gettin the truck moving.and then you run the problem of burning up the trans overdrive.if it is a occasional use vehicle,i wouldnt change a thing and just have fun with it.are you sure its really 6 mpg.what engine is it running and how many miles.maybe the engine is tired and needs some tlc.i have a 300 horse 350 and even when im gettin on it i get 8-10 mpg.

 

[4X4HIGH]

Ok, first thing we need to know is what trans is in the rig? It could be either a 700R4 or a TH400. If it is a 700R4 I would not even begin thinking of changing the gear ratio. If it is a TH400 then you might consider stepping down to a 4.10 gear IF you do alot of freeway driving.

Next, we need to know if anyone has corrected the speedo AFTER the gear and tire size change (you cannot calculate MPG using the odometer if this hasn't been done).

At 65 mph with 4.56's and a 38" tire (going to assume a 37" diameter since tires rarely measure the stated size) and a TH400 trans you would be spinning close to 2691 rpm's (actually a little higher because of convertor slippage). At 65 mph with 4.56's and a 38" tire (once again assuming a 37" diameter) and a 700R4 trans you would be spinning 1884 rpm's.

 

[Jessie James]

The seller says it has a 350 and accelerates very quickly due to the low gearing, hence the poor mileage. I have not seen (or driven) it yet so I am just researching. I think he said it had around 125k miles? I forget now.

Ok, first thing we need to know is what trans is in the rig?

It's a 4L80E.

Next, we need to know if anyone has corrected the speedo AFTER the gear and tire size change (you cannot calculate MPG using the odometer if this hasn't been done).

I had not thought to ask that. How is that done? If I ask him, what's the correct answer going to be?

From the sounds of it, the seller has put a lot of work into the truck, and I'm waiting on pics tomorrow. If the pics look good I may take it for a drive. If that pans out, I'll see about getting it inspected and so forth.

Thanks guys, I really appreciate the help!

 

[4X4HIGH]

The seller says it has a 350 and accelerates very quickly due to the low gearing, hence the poor mileage. I have not seen (or driven) it yet so I am just researching. I think he said it had around 125k miles? I forget now.




It's a 4L80E.


I had not thought to ask that. How is that done? If I ask him, what's the correct answer going to be?

From the sounds of it, the seller has put a lot of work into the truck, and I'm waiting on pics tomorrow. If the pics look good I may take it for a drive. If that pans out, I'll see about getting it inspected and so forth.

Thanks guys, I really appreciate the help!

Unless the guy installed a 4L80E that was not an option. The ONLY year that had an option of a 4L80E was 1991.

To correct the speedo on 1990-1991 he would have had to modify the DRAC (look at the for sale post by sweetk30 and you can see what a DRAC is).

 

[Jessie James]

I'm an idiot, it is a 1991. Thanks for the tip, so it looks like it can be calibrated easily enough.

However, with all that, can changing the gears help with the MPG?

 

[4X4HIGH]

I'm an idiot, it is a 1991. Thanks for the tip, so it looks like it can be calibrated easily enough.

However, with all that, can changing the gears help with the MPG?

Changing to a higher gear ratio (numerically lower) will probably hurt mileage not increase it given the tire size.

 

[Jessie James]

Ok, I did not know that.

I think I'm just going to stick with what I know, and try to pick up another K5 that's mostly stock. This is too much for my feeble little brain!

 

[kevin400ex]

a suburban is a heavy pig. with a small block and 38's your not really gonna get much better mpg. it sits pretty high on 38's and thats killing the mpg as well as the added weight of the tires. stock that thing probalby only got 10 to begin with.