Rear Gears and Cruise RPM

[66 C10 383]

I'm building a 66 C10 and would like some opinions on a decent rear gear ratio to look for in my truck. Here's what my engine and tranny combo will be.

383 with TPI based SuperRam intake
9.8 compression
AFR 190 heads
280 XFI Comp Cams hydraulic roller cam 230/236 dur @ .050" lift on 113* lsa (2000-5000 rpm range) using 1.5 rockers instead of the Comp recommended 1.6 rockers
http://www.compcams.com/Company/CC/c...?csid=176&sb=2

Estimated hp/ torque is 450 - 500 on both hp and torque

Transmission is an Tremec Aftermarket T56 with .62 6th gear

Tires will probably be 255/60/15 - 27" diameter

Can't decide if I need the 4.11 gears or the 4.56. I'm thinking 3.73 would put my combo in too low of an rpm for the cam range but I may be wrong there ...

This will hopefully be a daily driven combo with a somewhat concern for fuel mileage. About 90% of my driving (about 800 mi a month) will be at 55 - 60 mph and the other 10% at 65 - 70 mph.

Using Summit Racing's calculator, my rpm's for different speeds would be as follows:

http://www.summitracing.com/expertad...tio-calculator



255/60/15 = 27" tire height

Aftermarket T56 with .62 6th gear

3.73 x .62 = 2.31 final gear ratio 55 = 1581 65 = 1868 70 = 2012

4.11 x .62 = 2.55 final gear ratio 55 = 1745 65 = 2062 70 = 2221

4.56 x .62 = 2.83 final gear ratio 55 = 1937 65 = 2289 70 = 2465

What do you guys think, 4.11 or 4.56?

I need some guidance here ...

Thanks

Doug

[clinebarger]

4.11...To keep the driveshaft/pinion speed down.

[66 C10 383]

Thanks for the reply, I'd never even thought about driveshaft speed or that it would be a concern at these lower speeds. I was more concerned that it would be too low for the 2000 rpm operating speed of the cam.

[storm9c1]

I think 4.11 is just right for your combo and cruising. On a light truck, you could even get away with 3.73s if you don't mind downshifting on small hills if the engine lugs or pings. But with 3.73s, I think the truck may feel a little sluggish off the line. The 383 will be quite forgiving though, even with that cam.

Heck, I have 3.54 gears in the Dana 70 on my C30 and I have a 4-speed OD trans. With an empty bed, it can cruise at 55 all day long and it's a heavy truck. It needs to downshift as soon as it sees a hill though. My timing is set conservative and I run premium gasoline. But someday I'll need to upgrade to 3.73 or 4.10s as well. My setup isn't great on steep hills fully loaded (we are talking about 12K lbs here). Sometimes I need to go all the way down to 2nd to pull a big hill. But my point is that I'd never be able to get away with that without the extra low-end torque available from the 383.

[crazy longhorn]

I would go 4.11 also Longhorn

[kieth]

Originally Posted by 66 C10 383

I'm building a 66 C10 and would like some opinions on a decent rear gear ratio to look for in my truck. Here's what my engine and tranny combo will be.

383 with TPI based SuperRam intake
9.8 compression
AFR 190 heads
280 XFI Comp Cams hydraulic roller cam 230/236 dur @ .050" lift on 113* lsa (2000-5000 rpm range) using 1.5 rockers instead of the Comp recommended 1.6 rockers
http://www.compcams.com/Company/CC/c...?csid=176&sb=2

Estimated hp/ torque is 450 - 500 on both hp and torque

Transmission is an Tremec Aftermarket T56 with .62 6th gear


SEND ME YOUR E MAIL ADDRESS IN A PM ON THIS BOARD AND I WILL MAKE A CUSTOM CHART FOR YOUR TRUCK ENGINE COMBO..........WE NEED TO LOOK AT TORQUE WITHIN A USABLE RANGE OF SPEEDS SO LET ME KNOW HOW YOU DRIVE. I THINK THE 3:73 WOULD BE GREAT BECAUSE IT WILL LOWER YOUR CRUISE RPMS WAY DOWN BUT SINCE YOU ONLY HAVE 27" RUBBER IT WILL STILL COME OUT OF THE HOLE LIKE A CHAMP. ONLY DO THE 4:11 4:56 GEARS IF YOU ARE GOING TO BE RACING.
THE REASON FOR THE E MAIL ADDRESS IS I CANNOT POST A EXCELL SPREADSHEET ON THIS FORUM......KIETH

Tires will probably be 255/60/15 - 27" diameter

Can't decide if I need the 4.11 gears or the 4.56. I'm thinking 3.73 would put my combo in too low of an rpm for the cam range but I may be wrong there ...

This will hopefully be a daily driven combo with a somewhat concern for fuel mileage. About 90% of my driving (about 800 mi a month) will be at 55 - 60 mph and the other 10% at 65 - 70 mph.

Using Summit Racing's calculator, my rpm's for different speeds would be as follows:

http://www.summitracing.com/expertad...tio-calculator



255/60/15 = 27" tire height

Aftermarket T56 with .62 6th gear

3.73 x .62 = 2.31 final gear ratio 55 = 1581 65 = 1868 70 = 2012

4.11 x .62 = 2.55 final gear ratio 55 = 1745 65 = 2062 70 = 2221

4.56 x .62 = 2.83 final gear ratio 55 = 1937 65 = 2289 70 = 2465

What do you guys think, 4.11 or 4.56?

I need some guidance here ...

Thanks

Doug

OOPS LOOK ABOVE IN THE MIDDLE OF YOUR POST FOR MY REPLY KIETH

[kieth]

Remember you have 6 gears, if you want more torque just downshift, but I think this 450 should take off in 6th gear......with no PROBLEM

I always try to look at speed cruising down the highway and make sure the truck will not fall on its face. here is what my spreadsheet shows:

3:73 2000 rpm = 69.47 mph

4:10 2000 rpm = 63.2 mph

4:56 2000 rpm = 56.80


more torque than you can use.........is available will your engine cruise at 2000rpm ? if not just drive faster......ha ha kieth

THIS WILL BE QUIETER AND MORE ECONOMICAL ON THE STREET AND STILL PLENTY FAST......OFF THE LINE.

[66 C10 383]

I do appreciate all the responses guys, thanks a bunch.

I have the engine built but it's not in the truck yet. When it is time to rebuild the rear diff, I just want to make the best choice I can with new gears.

With the torque this engine should be putting out, that's not much concern. I live in So. IL so there's not many hills to worry about but a downshift every now and then wouldn't be a problem. I don't plan on towing or hauling much on a regular basis either.

With a 2.97 first gear I don't think my street tires are gonna stand much of a chance ... with the 3.73 that would be a 1st gear ratio of 11.08 and the 4.11 would be 12.20

I know it's possible to get too far down in the power band and the engine is less efficient than if it was geared with higher numerical gears. I was just afraid I'd be in that scenario of constantly lugging if I went with anything much lower than 3.73/4.11's. Just trying to make it as efficient to drive as possible.

kieth - I am sending you a PM of my email. Thanks !!!

Doug

[crazy longhorn]

Tire size does also affect the overall ratio....27" is a bit short on the tire. Just to give an idea, I ran 4.10 richmonds, with a 6 speed (Richmond) box. On a 28" tire, she was real "jumpy" with that 3.28 1st gear. I opted for a 29" tire, to tame her in town? The Richmond has a .76 OD gear, with the 3.42 gears that I am currently running, I am into the powerband(about right @ 70mph). The old truck will pull 105 @ 3150 on the tach (6th gear)...cruise all day Longhorn