Gearing the rear end 86 Short Bed

[DarkKnight]

Hey all,
I am sure this has been asked over and over.
Family project...aka The Dark Knight. An 86 Short bed 2wd.
Boys will drive it to and from school...(Yeah right)!!!!
Its geared 3:08 and I would prefer 3:42 or 3:73 for more off the line acceleration, however, it will see mostly highway. Looking to find that sweet spot betwen fuel economy and power, so which way one to lean towards?

[65standard]

Depends on what trans you have. If you have a turbo 350/400, I would just stick with the 3.08. If you have the 200r/700r, I would go with 3.73 gears

[SkinnyG]

I have 3.07 and TH350, and do mixed city/highway on a short (15 minute) commute. I ordered 3.73, and will eventually install a 700R4. Main reason is the way I built my engine and the torque converter need more gear.

The 3.07 gears are fine on the highway, but lackluster everywhere else.

Some say that 3.42 is a good compromise if all you have is a TH350.

[piecesparts]

Here is my learning curve for you to evaluate:

My son bought an 86 SWB with a 3.08 rearend gear. We changed the tranny over to a 700 R4 and the resulting RPMs cost him a lot of gas mileage. We then had RPMs so low that the engine was working harder to keep the truck moving. Gas mileage went from 14 MPG to 8 MPG. We then put a 3.73 geared rear end under the truck and now the gas mileage is back to 14 MPG. If you are putting a 3.42 or 3.73 geared rear end under the truck, I would suggest a change in tranny to an OD style to get the best optimization of your fuel. If yopu are keeping the tranny as a non-OD tranny, then do not go too deep to keep your RPMs under control. My son's truck travels down the highway at 2300 to 2400 RPMs at 70 MPH, with a 28" tall tire on it.

[greg64]

I'd like to know the engine and trans before making a recommendation, also the tire size.

[Keith Seymore]

3.42 w/o overdrive ;

3.73 w/ overdrive trans.

K

[DarkKnight]

Thank you.
Plans are to stick the 400 we have in it, engine 350 ci.
As for tires, 15 s...but not sure yet on actual tire sizes...yet.
Truck is stripped to the frame and everything is being looked at.
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[greg64]

I've got 3.42s and a non o/d transmission (sm465). With stock tires, the combo works well. 60 mph is about 2600 rpm, and that's about as fast as I go. I'd like to have Keith's o/d transmission.

[franken]

The 1:1 trans and ~3:1 rear gives you a compromise at both around town and freeway driving.
If you go to a lower rear, you'll be sad on the freeway.
Get an OD trans and a low 3.73/4.10 rear and you're happy but can't buy food.
A TH400 is a great trans but ~30% power loss in a 350 truck is asking to be disappointed w/ power.

[DarkKnight]

Thanks again all!
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[D13]

wait... you want your teenager to have more acceleration and tire spinning capability?
GM built millions of 3.08 geared trucks (like almost all V-6 and 6.2 diesels).
Think of your motor's cam range, aim for the bottom end of the torque curve at normal driving (so on my motors it's 1650 at 55, ~2000 at 70), then run the gear / tire calculators to find the optimum gear for MPG. Then decide how much you want to trade off for performance.
As a point of reference my 77 has manual trans and 2.73's, my 87 has 700R4 and 3.08's, my 3/4 ton has really tall tires and 3.43's and even with the 250/3OTT will pull anything but is not happy over 60mph.

[DarkKnight]

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[DarkKnight]

Well...that's what the other half is saying....until she sees the bill to put this truck back together..haha...my bet is they drive my current 2012...
Appreciate everyone's input.
If I remember...didn't GM make a 3:23 gear as well?
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[wilkin250r]

Originally Posted by DarkKnight

Thank you.
Plans are to stick the 400 we have in it, engine 350 ci.
As for tires, 15 s...but not sure yet on actual tire sizes...yet.

You don't have an exact tire size, but you have an IDEA, correct? Is your truck essentially stock suspension and tire size, around 31 inches tall? Or is it lifted, maybe 33~35" or bigger? Is it lowered? (which could either be bigger, or smaller tires, depending on the look you're going for)

I believe most of the replies in this thread are based on the assumption of stock tire size around 31 inches tall. You might not know your exact tire size, but if you're lifted or lowered, let us know so we can answer you accordingly.

[DarkKnight]

For now it will be mostly stock height, so yes, around 31 in tires...Thank you all again
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