foxracing254

I know this has been posted probably a million and a half times, but my search turned up less than satisfying results. I have a 97 K1500 with the 5.7. (Ext. cab). I have 285/75/r16 tires on the stock 16's (not the 265/75/16's that come stock) so obviously they're a bit bigger. Digging around in the golve box I discovered it has the 3.42 rear end, which might explain why I'm only getting 10mpg with my foot out of it. Would the swap to a 3.73 warrant enough benefit to be worth it? I would most likely pull a whole axle out of a junkyard, I don't feel like going through the complicated rebuild process. I'm looking to get better mpg overall, I do mostly city driving with about 100 miles of highway mixed in a week. I'm thinking the reduced stress on the engine to get all that rubber moving would help. Any thoughts?

country_09

There's not much of a difference from 3:42-3:73. But the older LS motors don't do as well as the 00-12 LS motors do when in higher rpm's so I wouldn't go 4:10s. If you can find a axle with 3:73s, snag it and put it in. Should help some, not massive amounts, but some. You'll need to computer recalibrated for the new ratio and get the speedo correct. And since you have to do that might as well get a hold of blackbear tuning and see if Justin can tune your truck to get you a lil more mpg while he's at it

foxracing254

Thanks! That's pretty much what I've been reading. I've spoken with Justin before, I'd most likely go that way. Would I be able to get anything for my 3.42 rear end? (as far as resale) I don't need to break even or anything, but I don't need a rear end just laying around my garage. It's an open diff (not sure if they made posi 3.42's)

SWHouston

Fox, you'd better rethink this ! !

Remember you got TWO axles in your vehicle. You must change the ratio in BOTH to get things to drive right in 4x4. That's $$ x 2 !

I would support the change to a slower set of gears, but, replacing tires with the original size would probably the equivalent of the gear change (or close anyway).

But that aside, yes, generally using a lower ratio gear set inevitably means you have to put more throttle on it, to keep your speed up. IOW you sorta loose power to some extent. Usually, using a slightly higher ratio gear in will more likely give you better mileage, as long as you don't overdo it.

grumpychevy

Higher gear ratio equates to a lower rpm cruising speed. Also means less torque multiplication- slower get up and go. This equals better mpg. Lower gear ratio equates to a higher rpm cruising speed but gives much better acceleration off idle. This equals less mpg. Example: @ 60 mph with 2.73:1 gears you may get 1700 rpm-- or @ 60 mph with 3.73:1 gears you may get 2000 rpm. As said above if you have 4x4 you will need to change both. That being said you don't want to choose a gear that takes you out of you power curve.

foxracing254

Thanks for the input, guys. I understand that my 3.42 ratio is better for cruising, but my thought process was this: being that I drive more "city" than highway, it might be hurting my mpg's because the engine has to work harder to get the bigger tires moving. So i figured 3.73's would be a good compromise between the 3.42 "cruising" gears and the 4.10 "towing" gears. Is there any truth to that at all?

SWHouston

I agree.

foxracing254

With who?

SWHouston

Fox,

HA, ok sorry about that...
With you !

Also, you might consider trying to select a replacement Rear End which already has some type of Locking/Limited Slip Differential. I have the Eaton G80 in my Burb. I like it's ability to cut out over 20 mph.

It works like this...

PS:
There's a lot of factors involved, which collectively may make one think that a change in gear ratio will increase or decrease gas mileage. The only way to be sure is try it.

foxracing254

Thanks a lot! Yeah, I wouldn't expect to get 20 mpg or anything ridiculous like that. But I average 10, and the other 90's sliverado's I've seen are somewhere around 13 or 14, even 15 average. The only mod to my truck is the tires, so I figure that's got a little bit to do with it.