406 tpi 700r4 2.14- 2.56 gears

[kad5118]

hello
i Have a 80 pontiac lemans with a modified tpi 406sbc that's modified with a th700r4 and what ever stock gears that came with the car for that year.
it could be between 2.14 - 2.56 gear.what i want to now is.how will the car drive and what will my mpg look like on the highway,I am saving for a 3.08 gears right now was we speak.

 

[DoubleV]

It will drive anywhere from crappy to great and the milage will be anywhere from crappy to great too. Seriously, we don't know anything about your motor so how can we even guess? You have OD though, so why bother with puny 3.08 gears? Get steeper gears.

 

[Kwik_Cutty88]

I've got a pretty stout 350 with a 700R4 and also a 2.56 non-posi. Right now the best mileage I can get is around 15, I usually get around 13.5. I'm saving up for a 3.73, posi. With my cruising rpm at 1500 at 60 right now, I'm not too worried about the 3.73's.

 

[jlcustomz]

There is a code list you can look up to check the #' 's on your diff housing & see what gears it came with.

Tire size also needs to be factored in when figuring what gears needed.
The torque you should have with a tpi 406 will work better with higher gearing as it should be a torque monster that runs out of breath at higher rpm with that intake on a larger motor. Still, I'd at least go 3.23 to 3.42 on gears with average size tires.

 

[kad5118]

yea i plan on getting 3.08 gears that's what i want maybe 3.23 but that's the highest that i would go.I am not really worried about it being a dog coming out of the hole.I just don't want my rpms to be high.I so an average of 80 mph on the highway getting back and fourth from work and home.I also want the mpg as well i live 3 hour away from where my new job is.

 

[King_V]

It's likely an open rear.

If the tag is not there anymore, jack up one rear wheel, rotate the wheel TWICE, and count how many revolutions of the driveshaft/rear-u-joint happen.

Slightly less than 2-1/4 rotations, you have 2.14
Slightly more than 2-1/4 rotations, you have 2.29
Slightly less than 2-1/2 rotations, you have 2.41
Slightly more than 2-1/2 rotations, you have 2.56
Slightly less than 2-3/4 rotations, you have 2.73
etc.


EDIT: for comparison, my 1987 Crown Vic has a 2.73 and the AOD trans. The engine's designed for low-revving, but there's times when it definitely will readily pop back from 4th to 3rd gear at the slightest provocation. Still, it manages to cruise along just fine. But we are talking a motor that makes power as such:
150 hp @ 3200, 270 ft-lbs @ 2000 (single exhaust)
160 hp @ 3400, 280 ft-lbs @ 2200 (dual exhaust)

 

[DoubleV]

yea i plan on getting 3.08 gears that's what i want maybe 3.23 but that's the highest that i would go.I am not really worried about it being a dog coming out of the hole.I just don't want my rpms to be high.I so an average of 80 mph on the highway getting back and fourth from work and home.I also want the mpg as well i live 3 hour away from where my new job is.

F*ck that $hit!!!

Your engine will be the most efficient at around where it makes peak torque so just figure out what rpm range you're driving in most of the time and get gears/tire size that puts you close to that when in OD.

 

[King_V]

So, first thing's first - we need to know the gear ratio, and the tire size.

Then we need to know your engine's torque curve. Or at least where it's happiest.

Then we can do the math.

 

[kad5118]

So, first thing's first - we need to know the gear ratio, and the tire size.

Then we need to know your engine's torque curve. Or at least where it's happiest.

Then we can do the math.

well i don't know what gear i have.I do know that i have 210 runner heads 64 chamber in 2.02 in 1.64 ex
slp runners
edlerbrock lower plenum
32 lb injectors
292 duration cam
58 mm TB
ls 1 coil packs
ls 1 pcm
400 block bore over 30 over

 

[King_V]

I'll have to defer to someone who actually knows what they're talking about to figure out where that combo produces peak torque, how easily it can handle very low RPMs under load, etc.