rearend gears

[cjb70chevy]

i got 70 chevy with 12 bolt, tires are 235/55/15. stock gears in it are 2.73's. i'm going to be putting in either 3.73 or 4.10's but i don't know. question is how can i high will the rpms be at cruising down the highway. i want the higher gear but i don't want the rpm's to high if i want to cruise down the highway

[cdowns]

273 you have high gears now// a larger number like 373 411 etc are lower gears

[GASoline71]

Yup... you want lower gears (numerically higher). 4.10's re gonna get you around 3,500 RPM's at highway speeds with a non-overdrive tranny.

I have a TH350 and 3.73's in my rig and run just under 3,000 RPM's at highway speed.

Gary

[~Whitey~]

This is 4:10 gears, 350 motor, turbo350 transmission. 2wd shortbed. PS... that's with 275/60/15 tires.

[69GWC]

Truck would scoot around pretty good on 3:73 and not turn a lot of rpms, but if you had a overdrive the 4:10s would be nicer to have.
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[70CHEVYBB]

3:73 should be around 2700 rpm and 4:10 will be around 3000 rpm at 55

[cjb70chevy]

thanks for the advice, much appreciated...

[1972SUPERSB]

Here is mine with 3.73 gears with 255/70/15 tires and TH350 trans.

[SS Tim]

Since 2.73 gears were not stock in '70 are you sure that is what you have? '70 Stock 12 bolt gears were generally 3.73 or 3.07 with a 4.11 option. Without any driveline info it is kind of difficult to say what is best for you. Here is a basic calculator that should help.
http://www.chevellestuff.com/tools/rpm_calculator.htm

[robnolimit]

get an OD and use 4.10's. best of both worlds.

[cjb70chevy]

it has 2.73's in it. i have serviced rearend 2 since i've had it and checked each time. i am the 4th owner, found 2 previous owners and no one has said they changed it. the trucks an all origional 70 chevy 402 bbc th400 trans a/c cab. its possible the 1st owner changed it but idk. the first owner had it for about 17 years.

[SS Tim]

Interesting, So it tooth counts and divides to a 2.73:1 ratio? Did you check the axle codes? Is it on the SPID as a special ratio? Is there a build sheet. Everything that comes to hand is showing a 3.07 to be the stock/only gear. Have to go and dig out my 70 engineering specs book tonight and see what that has to say.

Seems like a lot of work for about a 250 RPM drop (from 3.07s) on the freeway, but if you are running fast, a lot, it could be desirable.

http://67-72chevytrucks.com/vboard/s...d.php?t=561137

[cjb70chevy]

the code on axle is scratched down far enough i cant make it out. everything i've lookd up no truck had 2.73's. i just assumed that because it has a big block with a 3 speed automatic they would of wanted the lower gear for highway driving. the truck index in the glovebox doesn't have anything on the rearend gears. i really appreciate the help but no need to go to to much trouble as i would like to run a different set of gears.

[SS Tim]

Well then a set of 3.54 or 3.73 gears would be about the best compromise with your stock trans.. But a Gear Vendors O/D and 4.11s will put the grin factor into your truck. You end up with six speeds and the ability to past a gas station or two along the way.

[oldspowered67C10]

Use this page to expiriment with:
http://vexer.com/automotive-tools/speed-rpm-calculator
I have 3.08 with 295/35-20 and a th400, 3000 rpm@80mph.

[70blackfish]

if you use your truck to haul a lot or tow a lot. use 4.11 if not go with 3.73

your tires are not very tall..

[Already Gone]

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[cjb70chevy]

only thing i pull with it is the jetski or a buddies boat, and the haulin my buddies boat the fatty really wasn't grunting to hard. what does an overdrive usually run?

[cjb70chevy]

and the th400 is heading to the trans shop to get built/ an with a shift kit

[Fitz]

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

Enter your trans type OD/no OD, tire size, rear gears and MPH. Use 1 for high range, low range and 3rd range (your high range is 1:1, you don't have the others) It will calculate RPM for you or, put in RPM and it will tell you resulting speed in every gear. You will likely see you do not want 4.10's unless you add an OD. 3600 revs for 65 mph is going to be REALLY loud and with the current occupant at 1600 Pennsylvania Ave. gas isn't gonna get cheaper......

[cjb70chevy]

i agree on the gas not getting any cheaper thats why i drive my saturn that gets 37mpg. but for me atleast. my truck is the one thing i do to have fun/helps me forget harsh reality's of life for just a little while. i honestly don't care about if its good on gas, not the way i'm building it. that and its got a factory big block lol. thanks for the link tho

[hyatt]

Not a truck but I have a bigblock chevelle with a 400 turbo and 273s I think it is the perfect combo The bigblock has plenty of low end torque an 67-70 mph cruzing is effortless Engine runs cooler and uses less gas and still smokes tires at redlight If it were my truck I wouldnt change a thing

[Ends]

most rear end specialists i have talked to tell me that if your tire is 30" from the ground to the top of the tire or higher that a 4:10 gear ratio will act much more like a 3:73.

[cjb70chevy]

i got a set of 235/55/15's now but am planning on putting 20's all the way around, and i'm always looking to the future, thats y i'm wanting to put a bigger gear. just want to do it once.