Swapping the LS into my new 83 with 2.73 gears

[ivorton74]

So I bought an 83 c10 and parting out my 74 with the LS swap. The entire powertrain is being swapped into the 83 but it has the factory 10 bolt rear with 2.73. My buddy told me it would good for gas mileage but exactly how bad would be acceleration wise? Eventually i want to swap it out for 3.55s or something better.

Heres a pic on the 83...the 305/350 already pulled

[ivorton74]

My 74 waiting to be parted😔

[mongocanfly]

My old 86 had 2.73s in it as well along with a sm465 trans...it wasn't that bad on power and not great on gas with the 305..but would sure have been nicer if it had lower gears...the 3.55s would be a good choice but remember you need to consider final trans gear and tire size to get best gear ratio for what you intend to do with your truck..my 99 5.3 truck had 3.73s in it with 265/70 series tire and was fine..

[ivorton74]

Im trying to make it into a nice fast street truck. The 74 has 3.08 gears with 275/60/15 tires n it was ok off the line but not great. It did have good top end tho. I also want to go forced induction eventually n i dont think that 2.73 is gonna cut it.

[mongocanfly]

I've got a 6.0 in mine with a .73 overdrive trans and 4.10s...very quick...rpms around 2200 at 65mph..very fun to drive

[jessemthompson]

Your buddy is wrong about the mileage. With 2.73s and the overdrive the engine will not be in its operating range. That paired with the aerodynamics of a brick, Your power and mileage will both suffer.

[ivorton74]

Yeah that rear end has definitely gotta go...hope i can find a nice posi somewhere

[72MARIO]

If you stick with 275 60 15 3:73 -4:10 would be a great combo I have 3:42 27" tire 6.0/4L80 and its to low of rpm for towing.

[69gmcc10]

There are some trans/diff ratio/tire diameter programs that help you calculate the rpm you cruise down the highway. After you know tire size and final trans ratio you want to use one of those calculators to find what diff ratio to use, shoot for somewhere around 1900 to 2300 with an overdrive trans, that is going to be the sweet spot for efficiency for an LS engine. 4.10, 3.73, 3.55 are pretty normal ratios for LS engines.

[GMC-YA68]

Originally Posted by jessemthompson

Your buddy is wrong about the mileage. With 2.73s and the overdrive the engine will not be in its operating range. That paired with the aerodynamics of a brick, Your power and mileage will both suffer.

bingo...do 3:73

[ivorton74]

I have a 4L60E out of an 02 pickup and a 5.3 from a 99...whats the final drive for an 02 trans?

[ivorton74]

Whats the final drive for a 4L60 from an 02 pickup? You i actually didnt mind the old 3.08s...it still had good power off the line but if i got thru the trouble to swap out my rear end i want to get it right

[mongocanfly]

4l60e 4th gear ratio is 0.696

[ivorton74]

How do u do the calculations then? Sorry for double post earlier btw...my ipad is messing up

[Hart_Rod]

Originally Posted by ivorton74

How do u do the calculations then? Sorry for double post earlier btw...my ipad is messing up

At 70 mph in OD, I shoot for around 2000-2200 rpm.

Check this out: http://spicerparts.com/calculators/t...rpm-calculator

http://www.wallaceracing.com/gear-speed.php

[69gmcc10]

A 275/60r15 tire is about 28" tall, as stated the final drive of a 4L60E is .696, here is a link to a calculator to help you figure your drive speed at various rpms and ring/pinion ratios.

http://spicerparts.com/calculators/t...rpm-calculator

It looks to me that with the 28" tall tire and a 3.55 ring/pinion you would cruise down the interstate at 74 Miles an hour turning 2200 rpm, 64 mph is about 1900 rpm. With that diameter of tire I would do the 3.55 ring and pinion to keep the rpms in the most fuel efficient range. depending on how you look at it 3.73s will decrease those speeds by about 5 mph or increase those rpms 150.

[69gmcc10]

Funny, Hart Rod, you beat me to it.

[MalibuSSwagon]

This is a great calculator to use.

http://www.grimmjeeper.com/gears.html

I am still running 2.73 gears with my 5.3L, I do have an SM465 inbetween and it's ok....I would tow and everything with it. But I am changing to a TKO600 soon so 2.73's are out and 3.73 are going in.

[ivorton74]

Man that calculator is awesome! So did some calculations based on the .696 final and 2000rpm with the 28 inch tires...
2.73=87mph
3.08(my 74 has this)=77mph
3.42=70mph
3.55=67mph
3.73=64mph
4.10=58mph

Im liking the 3.42 and 3.55 the best....I can see the 2.73 is way too high for that final drive...probably be kicking in n out of overdrive constantly

[homemade87]

Originally Posted by ivorton74

Man that calculator is awesome! So did some calculations based on the .696 final and 2000rpm with the 28 inch tires...
2.73=87mph
3.08(my 74 has this)=77mph
3.42=70mph
3.55=67mph
3.73=64mph
4.10=58mph

Im liking the 3.42 and 3.55 the best....I can see the 2.73 is way too high for that final drive...probably be kicking in n out of overdrive constantly

Keep in mind that the 4l60 4th gear is the .696 final . But remember you also have the tcc lock up that will drop the rpm another 200 to 250 rpm as well at just cruise speeds light throttle .

[mongocanfly]

I'm thinking you'll have a bigger grin with the 3.73s...

[foamypirate]

Originally Posted by homemade87

Keep in mind that the 4l60 4th gear is the .696 final . But remember you also have the tcc lock up that will drop the rpm another 200 to 250 rpm as well at just cruise speeds light throttle .

The .696 final ratio assumes the TC is locked up, otherwise you'd be dropping the RPM physically lower than the gearing allows, which of course, is impossible, barring catastrophic failure.