Which gears should I run?

[BLOWN72]

I am building a new 383 for my truck and should be somewhere 400-425hp. I have a th350 trans and truck already has posi. I want something that is a good aggresive gear. I will be driving mostly intown so not worried about highway speed that much.

[rsavage]

4.11 - gives me about 50 mph with 28" tires at 25-2600 rpms in my wagon with a TH350.

[1972RedNeck]

Depends on what you want to do.

[68 TT]

Originally Posted by BLOWN72

I am building a new 383 for my truck and should be somewhere 400-425hp. I have a th350 trans and truck already has posi. I want something that is a good aggresive gear. I will be driving mostly intown so not worried about highway speed that much.

Your tire diameter will have a fairly big impact upon the gearing you will need. What are you running for tire size out back?

For a city driving only performance setup with a T-350 you can easily run 4.10 gears with no issue in a car with 28" tall tires so that is a safe bet for a similar truck. You can run even more gear but it really isn't necessary unless you are drag racing and need every little bit that the 4.56-up gearing can give you. The 4.10 gear with 28's is still usable on the freeway for short trips if needed without excessive wear & tear on parts.

Taller tires will negate the effects of the low gearing so you can step the gearing up to compensate. MPH x gear x 336 / tire diameter = RPM

Throw your tire diameter and gearing in the equation to see where you are at and what the changes to any part of it will do to your RPM at speed.

[hugger6933]

Get a little more motor so you don't have to run as much gear. A gear somewhere in the 373 to 309 range will do ok if you have a little more motor than the 450 or so that the plan is calling for. When we have a plan say for 450 to 500 hp it is always gonna fall on the 450 side or so it seems and so it feels in the pants seat anyway. Be on the safer side plan on 500 to 550 so maybe the build can take in the other 50 you needed in the first place. if any of that makes any sense. Jim

[jocko]

3.73:1 is a great compromise. If you have a 400 horse 383, you'll have no issues being "aggressive" with 3.73 - and you can still take it on the highway. If you were doing a lot of highway driving, I'd recommend a 3.42, but sounds like that's not the case.

Concur w/68TT, tire diameter plays a big role here. Would help if you posted that or at least the tire size.

Also - is the end of the road for the driveline? i.e. is there a chance you'll put a 4 spd o/d in place of the TH350? If yes, then plan the rear around that future upgrade and definitely go with a 3.73 (the factory usually did, for a good reason), or even lower numerical (higher) gear because the o/d transmissions have deeper 1st gear ratios, and it multiplies the rear end more than a TH350. Best of both worlds.

4.11 is a great drag gear for a hot rod, or a great hauling gear for a heavy 3/4 or 1 ton, but I tend to lean toward a 3.73 for all around streetability - and it will do what you want for being aggressive, while maintaining some level of highway drivability.

[bbcmudtruck]

My truck came with an "unknown" built 350 with posi 373 gears. It will straight get up and boogie for a small block. I would imagine your 383 to have more torque. I wouldn't see the need to go to a 4:10 ratio. If your currently running 3:73s, I would try them out before you swap gears. Also, I'm running a 28" tall tire. It will light them up on command.

[62k10&84k20burb]

5.17s!!

[msgdsrf]

I have a 383 at about 450+hp and run 3.73 gears with a 29" tire. It will light up the tires while at cruising speed when I put it to the floor, smoke them from a stop. Can drive freeway very resonably at 65 mph and 2700rpm. Good compromise (not really comprimising that much actually).

[Longhorn 70]

Where does your cam want to run? Shouldn't you be within the power band of the cam whilst driving at the speed you normally drive most?

I like the 4.11's with a 700R4 and cam that likes 1900 or so.....Smooth, effortless acceleration and great cruise down the freeway at 2200 or so.

[engineer_gregh]

So lets put the math to work. 65MPHx3.07x336=67049/28(275x60x15)=~2400 RPM. My factory tack is off, it reads ~3100RPM at that speed. I knew it was not accurate but I didn't think it was that far off. Did I do that math right?

[engineer_gregh]

Does anyone know the formula for a Turbo 400 transmission, is it the same as the turbo 350?

[CC69Rat]

Stall converter ?

[engineer_gregh]

Stock converter. Big block 402 .30 over, Comp Cam 268 HE, 3 angle valve job. Trans was rebuilt back to stock specs last year.

[engineer_gregh]

After doing a lot of reading on the internet I came across a site that will calculate RPM based on three inputs; tire diameter, speed, and rear end ratio. This might help you Blown72 as you try and decide what to do. It also backs up the formula 68 TT added. Very helpful thread for me, thanks everyone.

http://www.advanced-ev.com/Calculators/TireSize/

[CC69Rat]

I think I'd do 3.73, just my opinion. Do you know what gear is in it now?

[cubber]

I have a 450+ hp 383 with a TH350 and 27.2" tires. I am running a 3.o8 gear, which will spin the tires any time, from anywhere below 50 mph, and will cruise 70-75mph at about 2800 rpm.

[engineer_gregh]

And Cubber's truck sound awesome! When he comes to the cruise ins i can tell his truck by the sound without ever even seeing it.

[jocko]

Originally Posted by engineer_gregh

Does anyone know the formula for a Turbo 400 transmission, is it the same as the turbo 350?

TH350 and TH400 are both 1:1 in 3rd gear. Concur, your tach is off. Assuming you have 3.08 gears with a 28" tire, below are your cruising RPMs at 65 and 75 mph, respectively, for a TH350 and TH400 (only the 1st and 2nd gear ratios are different between the two, 3rd is still 1:1 for both). If you're showing 3100, I'm guessing your truck originally had 4.10s (or, at a minimum, your trans has the speedo gears for a 4.10). 4.10 RPM @65mph=3198 (and 3.73 RPM @65mph=2909).

[engineer_gregh]

Thanks Jocko. I used my timing light with digital tach to see just how far my in dash tach was off. In dash tack at 3100 RPM's only shows 2400 on my timing light. I did this in my driveway with the tranny in park of course. I knew it was off but had no idea it was this far off. Guess I need a new tach.

[Longhorn 70]

I am running 29.5" tires (or there abouts) with a 700R4 and a 4.11 rear end. I run about 2250-2300 at 65. I think at 75 it is about 2500 or so. I don't drive that fast often.

I used to be 3100 at 65 with the TH400.