What gear ratio is best for my truck? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

MisterE72 · 01-30-2008, 01:17 PM

Greetings folks. I'm ready to order some rear end gears for my c10. It has a 12 bolt possi, with 3:08 gears. I'd like to go with something different, that won't wind the engine at highway speeds, but has some launch. Here some numbers to think about: 383 stroker, 350 turbo tranny, and 20's with 275/35/20 tires. Any advice is greatly appreciated.

Madman1 · 01-30-2008, 01:26 PM

I am running a 350 with a Weiand 142 Supercharger, 18" rear wheels, TH350 Trans., and I just installed 3:73 gears with a Detroit Truetrac differential and I couldn't be happier. It gave me the low end that I wanted and doesn't wind up too tight on the top end. Good luck with your search!

heavy70 · 01-30-2008, 01:34 PM

3:73's would work the best, it would have alot of bottom end especially with the 383 depending on what grind your cam is.

**WorkinLonghorn** · 01-30-2008, 01:36 PM

WHat's the diameter of those tires? I don't see why,with 3.07 gears,you'd be reving too high. What TC are you running?

Shyguy · 01-30-2008, 01:44 PM

My son has a 454 in his '70 Chevy and is running 3.42 I think and likes them.

I am running a stock 350 with 3.73s and they give me what I want on the highway. Of course I tend to drive slower than some.

I am turning around 2500 rpms at 60 mph which seems to be ok.

Danny -SEMO

MisterE72 · 01-30-2008, 01:57 PM

Quote:

Originally Posted by WorkinLonghorn

WHat's the diameter of those tires? I don't see why,with 3.07 gears,you'd be reving too high. What TC are you running?

I'm guessing the overall diameter is about 25 inches. I haven't bought the converter yet. Ant recomendations on the TC?

**tll74** · 01-30-2008, 03:22 PM

I say 3.42 and here's why. First, you have tons of low end unless you over camed it. You don't have a overdrive. And at 25 inches you have a "short tire". With a 3.42 ratio, auto trans w/o lockup convertor, and a 25 inch tire your RPM @ 60 MPH will be 2757. Here's the formula to calculate RPM.
Ratio X MPH X 336 divided by tire diameter. 3.42 X 60 X 336 Divide by 25 =2757 RPM. 3.73 would be 3007 RPM. I don't think the convertor stall will affect total RPM at cruise speeds. For a lock up convertor or manual trans subsitute the 335 for the 336.
Just my thoughts.

Paying closer attension to your question I realize you are looking to lower the RPM. Your gear choice for aftermarket gears may end at 3.08. The factory options were 2.56, 2.76, 3.07, 3.40-3.42, 3.73, 4.10 and 4.56. just due the math.........

MisterE72 · 01-30-2008, 03:28 PM

Quote:

Originally Posted by tll74

I say 3.42 and here's why. First, you have tons of low end unless you over camed it. You don't have a overdrive. And at 25 inches you have a "short tire". With a 3.42 ratio, auto trans w/o lockup convertor, and a 25 inch tire your RPM @ 60 MPH will be 2757. Here's the formula to calculate RPM.
Ratio X MPH X 336 divided by tire diameter. 3.42 X 60 X 336 Divide by 25 =2757 RPM. 3.73 would be 3007 RPM. I don't think the convertor stall will affect total RPM at cruise speeds. For a lock up convertor or manual trans subsitute the 335 for the 336.
Just my thoughts.

I had never seen the formula before today. I'm a peson who works best with real data or formulas. I will definitey write this one down on the garage wall. Thanks. Mr.E

wild one · 01-30-2008, 03:42 PM

Here's a few sites for calculating tire size, RPM, one that compares your original size tire to your new tire, and one for tire size and width.

http://www.therangerstation.com/tech...nch_Tires.html

http://www.csgnetwork.com/multirpmcalc.html

http://www.miata.net/garage/tirecalc.html

http://victorylibrary.com/mopar/car-tire.htm

KFreddy · 01-30-2008, 04:13 PM

A 275/35/20 tire has a diameter of 27.6 inches.

Psycho71 · 01-31-2008, 02:42 AM

As for the TC, you should get what will compliment your cam the best. Whoever ground your cam shuld be able to point you to the correct stall speed for yur application.

As for the gears, throw some 3:73's in there, and call it good. If it twists too many rpm, just put an O/D in it. We're all made of money right? LOL.

Jay

BigSky71 · 01-31-2008, 04:20 AM

Quote:

Originally Posted by Psycho71

We're all made of money right? LOL.
Jay

Money for nothing and over drive trannys for free!

Man I wish, I need one!

MisterE72 · 01-31-2008, 10:07 AM

Quote:

Originally Posted by Psycho71

As for the TC, you should get what will compliment your cam the best. Whoever ground your cam shuld be able to point you to the correct stall speed for yur application.

As for the gears, throw some 3:73's in there, and call it good. If it twists too many rpm, just put an O/D in it. We're all made of money right? LOL.

Jay

I'm on a teachers salary.

But I love my job.

**tll74** · 01-31-2008, 10:19 AM

Sorry Guys!!!!!!!! I had a couple figures mixed up and wrong. The 336 and 335 are wrong. The 336 figure applies to the manual and lock up auto's.
The other figure for auto's w/o lock up is 355. Did this off memory at home. Sorry for the mistake. good day

jim zag · 01-31-2008, 10:49 AM

I have a 383 with a turbo 400 and stock torque converter. My truck has 3.07 gears and 255 70 15 tires. The tires are 29 inches tall. That stroker engine makes so much TORQUE, I can spin the tires any time I want. Here is my suggestion. Try it with the 3.07 gears first. You might like it the way it is.

KFreddy · 02-01-2008, 11:21 PM

Though Jim is new to the game; post wise anyway, welcome Jim... I have to back him up on this one. If your engines worth it's salt in TQ #s a 3 speed tranny with about a 3.08 gear will make you want to propose marriage....

Just Ask the guy in the, temp tag, 2008 Dodge Charger SRT8 that tried me in a little 30 to 90mph romp... My 3.08's and ZZ-BIG staved off the "Hemi" Dodge whilst smoking the 29' inchers all the way 'till the next red light. He couldn't even look at me as we sat side by side at a dead stop at the red light, never mind try me again. I was gonna give him an E for effort and a T for nice try, but he just fiddled about with his radio...

The moral of the story, 8 mpg priceless...

And my A/C works...

Longhorn Man · 02-02-2008, 01:44 AM

Quote:

--------------------------------------------------------------------------------

Though Jim is new to the game; post wise anyway, welcome Jim... I have to back him up on this one. If your engines worth it's salt in TQ #s a 3 speed tranny with about a 3.08 gear will make you want to propose marriage....

VERY well said, and I was about to say something about that. The 3.07 gear gets a good cruise RPM, and the 383 should have enough torque to not even be phased by the taller gear.

mrein3 · 02-02-2008, 02:39 AM

Why not 3.73s and a well built 700r4? I'm thinking that is the best of all worlds.

KFreddy · 02-02-2008, 04:52 AM

Quote:

Originally Posted by mrein3

Why not 3.73s and a well built 700r4? I'm thinking that is the best of all worlds.

I think that 3.73s and a 4 speed is a nice set up, however the original post mentioned a 12 bolt posi, 3:08 gears, 383 stroker, 350 turbo tranny, and 20's with 275/35/20 tires.

01-30-2008, 01:17 PM	#1
MisterE72 Keep Your Nose Clean Join Date: Dec 2006 Location: California Posts: 496	What gear ratio is best for my truck? Greetings folks. I'm ready to order some rear end gears for my c10. It has a 12 bolt possi, with 3:08 gears. I'd like to go with something different, that won't wind the engine at highway speeds, but has some launch. Here some numbers to think about: 383 stroker, 350 turbo tranny, and 20's with 275/35/20 tires. Any advice is greatly appreciated.

01-30-2008, 01:26 PM	#2
Madman1 Registered User Join Date: Mar 2007 Location: America Posts: 9	Re: What gear ratio is best for my truck? I am running a 350 with a Weiand 142 Supercharger, 18" rear wheels, TH350 Trans., and I just installed 3:73 gears with a Detroit Truetrac differential and I couldn't be happier. It gave me the low end that I wanted and doesn't wind up too tight on the top end. Good luck with your search!

01-30-2008, 01:34 PM	#3
heavy70 Registered User Join Date: Dec 2007 Location: choctaw, oklahoma Posts: 100	Re: What gear ratio is best for my truck? 3:73's would work the best, it would have alot of bottom end especially with the 383 depending on what grind your cam is.

01-30-2008, 01:36 PM	#4
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,887	Re: What gear ratio is best for my truck? WHat's the diameter of those tires? I don't see why,with 3.07 gears,you'd be reving too high. What TC are you running? __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

01-30-2008, 01:44 PM	#5
Shyguy Junior Member Join Date: Dec 2006 Location: Southeast Missouri Posts: 2,436	Re: What gear ratio is best for my truck? My son has a 454 in his '70 Chevy and is running 3.42 I think and likes them. I am running a stock 350 with 3.73s and they give me what I want on the highway. Of course I tend to drive slower than some. I am turning around 2500 rpms at 60 mph which seems to be ok. Danny -SEMO

01-30-2008, 03:22 PM	#7
tll74 "Junkyard Dog" Join Date: Jan 2008 Location: Springfield Oregon Posts: 2,529	Re: What gear ratio is best for my truck? I say 3.42 and here's why. First, you have tons of low end unless you over camed it. You don't have a overdrive. And at 25 inches you have a "short tire". With a 3.42 ratio, auto trans w/o lockup convertor, and a 25 inch tire your RPM @ 60 MPH will be 2757. Here's the formula to calculate RPM. Ratio X MPH X 336 divided by tire diameter. 3.42 X 60 X 336 Divide by 25 =2757 RPM. 3.73 would be 3007 RPM. I don't think the convertor stall will affect total RPM at cruise speeds. For a lock up convertor or manual trans subsitute the 335 for the 336. Just my thoughts. Paying closer attension to your question I realize you are looking to lower the RPM. Your gear choice for aftermarket gears may end at 3.08. The factory options were 2.56, 2.76, 3.07, 3.40-3.42, 3.73, 4.10 and 4.56. just due the math......... Last edited by tll74; 01-30-2008 at 03:33 PM.

01-30-2008, 03:42 PM	#9
wild one Registered......... Join Date: May 2006 Location: Livermore,CA Posts: 288	Re: What gear ratio is best for my truck? Here's a few sites for calculating tire size, RPM, one that compares your original size tire to your new tire, and one for tire size and width. http://www.therangerstation.com/tech...nch_Tires.html http://www.csgnetwork.com/multirpmcalc.html http://www.miata.net/garage/tirecalc.html http://victorylibrary.com/mopar/car-tire.htm __________________ 1966 C10 panel-400 small block/TH 350 combo Nothing is fool proof to a talented fool! Popcorn: Pops at exactly 444 degreese Expands to 44 times it's original size Has a 4 year shelf life

01-30-2008, 04:13 PM	#10
KFreddy Active Member Join Date: Apr 2005 Location: Miami, Florida Posts: 286	Re: What gear ratio is best for my truck? A 275/35/20 tire has a diameter of 27.6 inches. __________________ "The Great State of Florida"

01-31-2008, 02:42 AM	#11
Psycho71 Registered User Join Date: Jan 2006 Location: Kaufman, Tx Posts: 827	Re: What gear ratio is best for my truck? As for the TC, you should get what will compliment your cam the best. Whoever ground your cam shuld be able to point you to the correct stall speed for yur application. As for the gears, throw some 3:73's in there, and call it good. If it twists too many rpm, just put an O/D in it. We're all made of money right? LOL. Jay __________________ You only need two tools in life - WD-40 and Duct Tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape. If a hammer doesn't fix it, you have an electrical problem! Slot Cars, 1:24 Scale, 100MPH@100,000RPM, fastest things on wheels!

01-31-2008, 10:19 AM	#14
tll74 "Junkyard Dog" Join Date: Jan 2008 Location: Springfield Oregon Posts: 2,529	Re: What gear ratio is best for my truck? Sorry Guys!!!!!!!! I had a couple figures mixed up and wrong. The 336 and 335 are wrong. The 336 figure applies to the manual and lock up auto's. The other figure for auto's w/o lock up is 355. Did this off memory at home. Sorry for the mistake. good day

01-31-2008, 10:49 AM	#15
jim zag ENGINE BUILDER Join Date: Apr 2006 Location: Shortsville n.y. Posts: 344	Re: What gear ratio is best for my truck? I have a 383 with a turbo 400 and stock torque converter. My truck has 3.07 gears and 255 70 15 tires. The tires are 29 inches tall. That stroker engine makes so much TORQUE, I can spin the tires any time I want. Here is my suggestion. Try it with the 3.07 gears first. You might like it the way it is. __________________ JIM ZAG

02-01-2008, 11:21 PM	#16
KFreddy Active Member Join Date: Apr 2005 Location: Miami, Florida Posts: 286	Re: What gear ratio is best for my truck? Though Jim is new to the game; post wise anyway, welcome Jim... I have to back him up on this one. If your engines worth it's salt in TQ #s a 3 speed tranny with about a 3.08 gear will make you want to propose marriage.... Just Ask the guy in the, temp tag, 2008 Dodge Charger SRT8 that tried me in a little 30 to 90mph romp... My 3.08's and ZZ-BIG staved off the "Hemi" Dodge whilst smoking the 29' inchers all the way 'till the next red light. He couldn't even look at me as we sat side by side at a dead stop at the red light, never mind try me again. I was gonna give him an E for effort and a T for nice try, but he just fiddled about with his radio... The moral of the story, 8 mpg priceless... And my A/C works... __________________ "The Great State of Florida"

02-02-2008, 02:39 AM	#18
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,254	Re: What gear ratio is best for my truck? Why not 3.73s and a well built 700r4? I'm thinking that is the best of all worlds. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

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