Gas mileage & gearing, again....

[Jays86]

Ok, I'm sure this thread had been beat to death, but I have a few questions on which route to go with my truck. I have an '86 K10, 5.7, 700r4, 10k on the engine and it has an Elgin E923P cam, 3" flowmaster, MSD replacement distributor still connected to the factory ECM, but all stock otherwise. I just had the carb completely rebuilt AND rebushed because the shafts had some play, so it's running pretty good otherwise. It does fall on it's face a bit with the A/C on, but I'll get that part fixed. It also seems to have a radiator leak near the bottom hose, but it only seems to leak after turning the truck off and when I had the A/C on. It's a daily driver, I may take it on some trails once in a great while, I may tow with it once in a great while, but it's a daily driven city/highway driver. My main question is, what gears to run? According to the build sheet, it has 3.08's, but I want to confirm that by lifting it and counting the revolutions. It obviously came with 235/75/15's according to the build sheet, but currently has 32x11.50x15's on it. Unfortunately, the tires have a ton of tread, but the truck sat for a while and the tires are now so dry that I can see the steel belts. I'll most likely stick with the same size that's on there now, but I was wanting to lift the truck a little, and go to a 33" or a 35" tire. I'm still up in the air on how far I want to lift it, I was curious on everyone's thoughts on that as well, and I want to get the best mileage/power I can. I don't have a tach, and had read that it might be hard to add one to my specific truck because of the ECM? I haven't found the factory rpm @65 with a 700r4 and a 29" tall tire, just charts based on I'm assuming a non OD trans. Any help is greatly appreciated!

 

[bucket]

In my experience, I've found that gearing in one of these trucks when lifted, doesn't make a big difference in mpg, but it does make a big difference in power. With the OD trans, you can go steep with the gearing to get good power and mpg will be about the same as with tall gears. Need to pick a tire size and go from there.

 

[skysurfer]

You're currently turning 1500 rpm at 66.2 mph.

33" tires with your current setup would be 68.3 mph at the same 1500 rpm.

The stock 29" tires = 1625 rpm at 65 mph.

33" tires and 3.73 gears = 1750 rpm at 65.8 mph.

33" tires and 4.11 gears = 1900 rpm at 64.8 mph.

35" tires and 4.11 gears = 1800 at 65.1 mph.

All numbers assume the trans is in od.

 

[Jays86]

Very good points, I may just do a small lift and stick with 32's. Do you guys think 3.73's would be low of gearing for the 32's? 1600 rpm's seems like a pretty low cruising rpm, what would be the perfect all around cruising rpm for mileage and power? I know the engine in stock form without the cam I have in it made peak hp around 3800, and peak torque around 2800.

 

[bucket]

3.73 would be ok, but I'd choose 4.10's. Even 4.56 would work well and your trans would be much happier. If you go larger on tires, I'd certainly go with 4.56.

 

[Jays86]

3.73 would be ok, but I'd choose 4.10's. Even 4.56 would work well and your trans would be much happier. If you go larger on tires, I'd certainly go with 4.56.

I'm thinking I may go with 4.10's, 1900-2000 rpm's at highway speeds shouldn't be bad. I'm wondering if Chevrolet really got the best mileage they could at 1600 rpm's at 65 mph, that seems like a pretty low rpm to me.

 

[4WDKC]

I'm thinking I may go with 4.10's, 1900-2000 rpm's at highway speeds shouldn't be bad. I'm wondering if Chevrolet really got the best mileage they could at 1600 rpm's at 65 mph, that seems like a pretty low rpm to me.

I have 342s and 33s on flat ground the truck probably gets great mileage but and kind of an incline or to pass and the truck kicks out of overdrive and down to 3rd.. I have 410 rear axle waiting to find a matching front before I install.

 

[Jays86]

I have 342s and 33s on flat ground the truck probably gets great mileage but and kind of an incline or to pass and the truck kicks out of overdrive and down to 3rd.. I have 410 rear axle waiting to find a matching front before I install.

That's the main reason I want to go with 4.10's, I'm not far from the hills and there's a ton of cool places by me to do some off-roading when I want and I'd rather have the gearing to pull me up hills and in some trails. The next thing I need to look into is some better axle shafts. I really don't want to swap out the front and rear complete axles. I know everyone says the 10 bolts are junk, but the truck isn't going to be a rock crawler or serious off-roading vehicle, I just need it to hold up to minor off-roading when I choose to do so.

 

[4WDKC]

When you compare prices of new gears and install if you can't do it yourself you quickly cover cost of 3/4 ton axels with the gears already set up.

Sent from my SAMSUNG-SM-N900A using Tapatalk

 

[Jays86]

When you compare prices of new gears and install if you can't do it yourself you quickly cover cost of 3/4 ton axels with the gears already set up.

Sent from my SAMSUNG-SM-N900A using Tapatalk

The only bad part about that is I'm having to trust someones word that the axles are even good.

 

[4WDKC]

The only bad part about that is I'm having to trust someones word that the axles are even good.

The same can be said about someone that says they know how to set new gears. At least with the new used axles you can get them cheaper than 1k and if they are bad you have a fall back plan of your old axles.

 

[Jays86]

The same can be said about someone that says they know how to set new gears. At least with the new used axles you can get them cheaper than 1k and if they are bad you have a fall back plan of your old axles.

True, that's just too much work though, and I don't have the extra space for an extra set of axles laying around. I'll just stick with my original housings, find a place to swap the gears, and switch to some 30 spline axles instead of the 28's. I'm not doing any hardcore wheeling, just some occasional light wheeling, and maybe some towing.