Any idea what gear ratio I should use?

[thejunkman2005]

So, I'm building a truck...

Here's the specs.
First,
6.2 diesel
T400 tranny
3+3 frame
standard cab
12ft dump
Corp 14 rear axle
Maybe disk brakes?
19.5 wheels with 225/70 tires
narrow width duallies

Truck use,
Will quite often haul fully loaded bed, over 6k in the bed.
Will nearly always have a 12k equipment trailer hooked to it.

Most mileage will be highway, speeds from 55 to 65 and up if possible.

So, what do you think I should run for gear ratio? MPG is very important, along with pulling power.

4.56s? NO way, 60 top speed
4.10s? Possibly, but I've never owned a diesel and dont' want to over work the diesel on the highway.
3.73s? I have these in a dually axle, do you think this would work?
Maybe even taller gears? 3.08s or something? I don't think have these, but if a there's a chance they will be the best option, I'll consider buying them, and having them installed.

I'd like to know what people are running for gear ratios and how they fair out.

Thanks,
Len

[arveetek]

In order to even begin to pull that kind of load at highway speed, you'll need at least 4.10 gears, and 4.56 would be better, especially with 19.5" wheels, but don't expect to go over 60 mph. Do you know what the diameter of the tires will be?

3.08's and 3.42's are suitable for grocery getters and mild towing at the best. They'd be worthless for towing that large load.

3.73's would only work if you had low-pro tires and a high-torque engine, such as with a 5.9L Cummins.

To be honest, I think you're expecting WAY too much out of a 6.2L equipped truck. A 6.2L just can't push 25K or more down the highway with any satisfactory results.

Casey

[Craig M]

I think you are asking a lot from a poor old 6.2. That much weight, freeway speed and high mileage! Good luck! With 6,000# in your dump bed, your truck will be slow, but be able to get around. Not fast, but moveable. Add a trailer with 15,000# and you will really be slow. Transmission and rear end better be heavy duty. If you really want to pull that much, better go with the 4.56. That will get the vehicle moving, but you are correct, top speed will be horrible. Remember the 6.2 was built as a good mileage low power diesel, not a torque moster heavy hauler like the current diesels. If you want to stay old school a built 6.5 turbo with a 6 speed manual might be better for you.

[thejunkman2005]

right now I'm running a
K2500 1985
350 4 bolt
torque cam
edelbrock intake
Holley carb
dual exhaust

And it'll run 55-65 loaded on the highway, average MPG is the 9-11 range. I'm not hoping to have massive amounts of power, just enough torque to pull on the hills, and better MPGs..

I even considered a T700R4, but I have my doubts any year 700 would hold the weight i need it to. I also considered a 454, but I don't want to buy stock in Exxon to get me around.

So, I should stick with 4.10s? probably a nice medium. My current truck runs about 2500 rpms at 55 with 3.73s. I'm hoping the diesel will do a bit better than that.

thanks for the input,
Len

[arveetek]

right now I'm running a
K2500 1985
350 4 bolt
torque cam
edelbrock intake
Holley carb
dual exhaust

And it'll run 55-65 loaded on the highway, average MPG is the 9-11 range. I
thanks for the input,
Len

Are you pulling the same trailer with the 3/4 ton? I'm assuming you don't have a dump bed on it or have it loaded with 6K of weight either.

A 700 would not do you any good, as you would never be able to use overdrive. Even though I'm happy with the power output of my turbo'ed 6.2L, I still can't use overdrive when towing large loads, even with 4.10 gears....it lugs the engine too much.

I would estimate you'd only get around 10 to 13 mpg with the rig you've described hauling the load you've estimated. The worst my n/a 6.2L ever got was 13 mpg pulling hard. Now that I've upped the power, added a turbo, and tweaked the fuel output, I've seen it drop to 10 at full throttle towing. Mild towing (8500 lb RV) it gets about 12-13 mpg. Empty, I get about 18 mpg.

Casey

[thejunkman2005]

thanks for the tips. No, my 3/4t doesn't have a dump, but has a steel flat bed. I was getting 13-14 when I had a stock aluminum intake and stock carb. But since I upped to the edelbrock and holley it dropped quite a bit. I dont' race on the highway, I usually chug along at 2500 rpm(about 55mph), maybe upto 2900rpm coming home with an empty trailer. But I still only get 9-11mpg.

I'm hoping the diesel will give me slightly better milage, and slightly more payload power.

I'm not expecting miracle, just a little better.

I might start the truck with 4.10s, then depending on how it runs, I'll swap it to 3.73s if I think I have the rpms and power to allow it.

I didn't know if I could go down to the low 3.xx.

The engine is a J series, coming out of a 1985 Chevy 1t, 14gvwr truck that was owned by our local power company. I was hoping to swap over to a stick shift, but finding the parts to make it work seem to be eluding me, so I'm just going to stick with the t400. I was thinking of running a 4wd T400 and transfer just so I could shift into 4lo to help pull up some of the tougher driveways that I quite often travel.

Len