Need help on gears for my 87 R10 and new 383

[UATahoe]

I am about to install a 383 motor into my 87. It has a 700r4 tranny and the current rear is an open 2.73. I will never be racing the truck obviously and gas mileage is my min concern but I read a few posts here where people actually gained mpg going to a 3.42 or 3.73 gear ratio. Given the new motor and additional power should I change the gearing or am I going to get the best mileage out of the 2.73's? Thanks for the input!
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[Keith Seymore]

3.73 gears will be perfect for that truck, with the overdrive trans.

K

[UATahoe]

Will it actually drop my mpg or help it? I always thought going to a 3.43 or 3.72 would hurt gas mileage. I appreciate all of the help.
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[UATahoe]

Should also add I am running 31x10.5 tires and its a 2wd.
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[kelsfine69]

I would go 3:42 since you have the overdrive trans M2C

[INSIDIOUS '86]

Mileage in city may be better but you may lose some highway mileage.
Where does your engine feel strongest? Not fastest or anything but the happy spot where it cruises?

3:42-3:73 is your range. 3:42 aren't bad for that big first gear and racing. 3:73 gears may actually over gear you on the track and make you lose time.

So we need to know some engine specs too
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[Jonboy]

With the 31's, I would look at the 3.73's. If you will be sicking with that tall of a tire, 3.90's may not be out of the question, either.

[UATahoe]

Engine specs:

The crankshaft is by "EAGLE" products, with 5.7" long I-beam rods and SpeedPro pistons with moly rings, giving pump gas friendly (9.2to1) compression ratio. A custom ground hydraulic roller camshaft and lifters, spec's (210/220`@.050" with .500" lift on 112`LSA) with matching valve springs with a power range of 1500-5000 rpm's.

Expecting 360hp and 410 ft.lbs. torque.

Haven't installed the motor yet. Should receive it in about 2 weeks. The truck will never see the track but will be a daily driver with a mix if city and hwy driving hence the goal to maximize mpg. How will the stock gearing do as opposed to a different ratio?

Thanks again for all the info and help!
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[Keith Seymore]

Originally Posted by UATahoe

Engine specs:

The crankshaft is by "EAGLE" products, with 5.7" long I-beam rods and SpeedPro pistons with moly rings, giving pump gas friendly (9.2to1) compression ratio. A custom ground hydraulic roller camshaft and lifters, spec's (210/220`@.050" with .500" lift on 112`LSA) with matching valve springs with a power range of 1500-5000 rpm's.

Expecting 360hp and 410 ft.lbs. torque.

Haven't installed the motor yet. Should receive it in about 2 weeks. The truck will never see the track but will be a daily driver with a mix if city and hwy driving hence the goal to maximize mpg. How will the stock gearing do as opposed to a different ratio?

Thanks again for all the info and help!
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The stock gearing with those tires will kill it. You will need at least the 3.73's.

K

[UATahoe]

That's what I was thinking on the 2.73. And I was hesitant to change thinking it would kill gas mileage. How do I figure out which gears I need? I think 3.73 are probably best but picking the right set in summits website is confusing. Don't really know the specs on my rear end.
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[UATahoe]

Keith, do i need to look into a posi as well?

[ont_guy_38]

Motor sounds similar to my setup, i have a turbo 400 with 4:10 gears. A 700 tranny with 373 gears would be about perfect. If you are getting new gears installed you might as well put a posi in at the same time instead of paying twice, as you will probably find you will need the posi.

[UATahoe]

373 definitely seem to be a popular vote. Now i just have to figure out the details of rear so i can get the right set.

[Keith Seymore]

Originally Posted by ont_guy_38

Motor sounds similar to my setup, i have a turbo 400 with 4:10 gears. A 700 tranny with 373 gears would be about perfect. If you are getting new gears installed you might as well put a posi in at the same time instead of paying twice, as you will probably find you will need the posi.

x2.

I always like to have a "non-open" diff.

K

[UATahoe]

Suggestions on which diff to use and brand of gear? This is like rolling a snowball down hill. LOL. Never as easy as just swapping out the motor. LOL

[ont_guy_38]

I got a eaton from a 8.5 nova/camaro in mine, works awesome

[Ziegelsteinfaust]

Use this.

http://www.richmondgear.com/101032.html

I like to set my drive rpm at 75 at 2200 rpm. That way I can use a 2000 rpm converter, and that leaves me enough of a hole shot not to need the extra gearing.

So I would use a 3.90 gear which summit sells for the 8.5.

[gchemist]

I use engine torque curves to set the ratio. Do you know the projected curve? Put cruise speed RPM at max torque possible.

[Scotty-D]

My 383 is very similar to yours and I also have a 2.73 rear gear in a LWB truck, although I swapped in a th350.

The 2.73 rear is horrible. Even as torquey as the motor is, it's a real yawner at low speeds...and I just have a 29" tire. Ill be going to 3.42's and a 28" tire shortly and I expect it to be a night and day difference.

With your taller tire, I'd say the guys above who suggested the 3.73's are spot on.
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[UATahoe]

What is the over drive ratio?? We are definitely moving into the way over my head category. My main concern is trying to balance mpg and performance and i would be willing to sacrifice a little performance to help on gas mileage.

[Scotty-D]

.70 for a 700
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[UATahoe]

gchemist, i do not know the projected curve. i can try to find out what has been typical though.

[UATahoe]

Got it. So with a 3.73 ratio,31" tire and a speed of 75mph, i would be turning at 2122rpms with the 700 tranny? I guess that goes back to finding the sweet spot of the motor at cruising speed. 2100 seems pretty goood though. Looks like right now with the 2.73's i am running at 1553 rpm at 75mph. That seems way low. I wonder how that will effect hwy mileage??

[UATahoe]

3.42's totally out of the question? Found a brand new set here locally with a brand new posi unti as well for 350. They are Yukon brand never installed. Seemed like a really good deal.

[INSIDIOUS '86]

3:42 will give you some better highway mileage but you may be a little low on rpm on the highway with them tall tires. City mileage will be about the Same. Add a posi if you want to get anywere anytime soon. Richmond and Yukon gears are god long lasting gears. Eaton posi is recomended as well
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