trainor

Right now with my 3.08 gears I'm at 2600rpm at 65 mph. 3.55 will increase rpm to 3062 a difference of 512 rpms. A set of 3.73 gears will increase rpms to 3217, a difference of 560 rpms. Thats quite a bump in rpms on the highway. Will I notice that much more of a difference between 3.55 and 3.73? And will this car be driveable on the highway? I'm makin a decision today on gears so let me hear it. Iv'e got a 350/350 with a turbo 400, Thanks guys.

mds3013

I have 3:55s with th400 also. I never thought about it until I started driving vehicles with overdrive tranmissions. I would not want to go any higher than the 3:55s myself. Car does ok and gets about 15mpg running at 65-70 mph. I am considering a trans. change but will first need to decide manual or auto.. mike...

AZDoug

When i wasn't running 4.56 in my '61 (327/350HP), I ran 3.55s.

No problems at all, even driving cross country.

Doug

johnt365

i am curious what are your goals for the car. My car is in the same situation. I just ordered a new torque converter 2800rpm and will report the changes if you still have the original stall.

trainor

Thanks Mike, I agree that sounds like a nice gear. Did you have 3.08 in it pryor or has it always been 3.55? If you did change it over did you notice a big difference?

Paul L

I drove a 1979 L-82 with TH-350 and 3.55:1 for 17 years (up to 2003). It is still on the road today. No harm done at 3,200rpm at 60mph. The TH-400 final gear may be a little different but the message is the same. No problem with 3.55:1.

trainor

Having just had the motor done, I'm looking for more of a launch out of the hole. I've got right at 350hp and this thing is so slow from a stop. After about 40 feet its all out but bottom end isn't there. I've got the stock stall with a turbo400 with a shift kit. Just don't want to screw up the highway drivin, gettin about 16 mpg right now.

johnt365

That is my exact situation. I opted to try a different converter first. I know you asked about gears but the goal is to get the car in the power band quicker and a converter can help there also. I took measurements of my rpms at different speeds and will compare to the new unit.

mds3013

Always 3:55. mike...

trainor

Please keep me in mind when you get that converter in , was thinking today about doing the same thing. I spoke with Continental converters and they said that just the converter alone isn't a good idea, but I should do gears too. Then I talked to Coan converters and they said to just do a 10" converter and it will work sweet. Let me know as soon as you get that puppy in, I'll be waiting. Thanks, Tim

johnt365

You got it. I ordered an 11 in from PTC. I also emailed Kiesler for a quote on one of their "budget" model 5 speed conversion kits. At $4k its safe to say we have two different ideas of the word budget. It will be 2 weeks.

jb78L-82

78 L-82 4 speed with 3.70 stock gears. With stock diameter tires the car turns 2,860 RPM at 60 mph, 3,500 RPM at 70 MPH, close to 3,900 RPM at 80 MPH on the stock speedometer. Personally, after 28 years of ownership, I find those high rev's annoying especially after driving modern cars with overdrive. Even my 1994 Mustang GT with I believe, 3.23's, turns only 2,500 RPM @ 80 MPH, 2001 Pontiac Grand Prix turns 2,500 RPM @80 MPH, or my 2008 Chrysler 300 turns 2,500 RPM @ 80 MPH. I would go with the 3.55's at most. Someday, I am going to with a 5 speed OD like Keisler which will lower my rev's to 2,200 RPM at 80 MPH. Can you imagine the increase in MPG's and the reduction in noise? Part of the problem is shorty headers, 2.5 true duals and Monza Turbo mufflers at those revs!

fauxrs

Same deal here, 3.70:1 in my 78 L82 4 speed equipped car which I have driven on several long trips and the high revs do eventually get to you. I did make the conversion to the TKO 5 speed OD tranny and it by far was the best drivability change made to the car.

ajrothm

Install a GOOD stall speed converter first..... It won't affect your freeway RPMS that much, if any....yet it will wake up the launch big time..... Hit up ATI, Ultimate Converter Concepts, PTC, Coan..... Any of them will build a nice one for your combo...

I had a local guy build me one and my car ran great with 3.08s....It would still 60' high 1.50s and cruised the freeway at 70mph@2900......

I now have 3.36s and cruise 70 mph at 3000 rpms. 27" tires. I only picked up a few hundreths at the track with the gear swap so......

But a SB needs some gear.

I would go 3.36 and a good converter if you want an efficient combo and decent freeway cruise...... All that being said, I am looking into billet 200R4s od trannies...... I would like to be at 70 mph @ 2000 so I can better my 9-10 mpg on the hwy.

drwet

I have 3.55's in my '79 and I would spin about 3200 rpm on the highway with the original TH350. I didn't mind the noise so much but the mileage sucked and it would always run a little warm. I put in a 700R4. The mileage went from 13 to 20 mpg. If you want deeper gears, plan on putting in an overdrive.

milo30

Is it driveable on the highway....yes. Is it annoying, especially with loud pipes....YES! They cam with 3:70s from the factory so it will do just fine but your gas mileage is going to suffer some.

I have 3:70s in mine and don't plan on changing but I am planning on going to an OD trans just for mileage and comfort.

captl27

Take your car out put in 2nd gear (auto trans) and bring it up to the rpms you think you will be running and that will feel somewhat like what your car will do when you change gears.

7T1vette

If you go with more than a 3.55 rear end (with an auto tranny), you will be disappointed in the performance (your torque converter already provide decent torque multiplcation at lower rpm ranges), highway economy, and highway comfort (noise/heat). If you want more gear, you will have to also add an overdrive tranny to bring down highway rpms, noise, etc.

gkull

If you think you need more gear you are not making enough power

What cam are you using to only make 350 HP? 350's are very streetable up to about the 430 hp range.

9.5 inch TC's are kind of like the same idea as lighter weight flywheels

Oldguard 7

I agree with the 3.55 gear crowd. It is a happy medium As for the torque convertor, I have never done so but it seems to me a 3200 stall speed convertor regardless of the diameter will be a pita to drive on the street and I'm sure the heat generated due to slippage is cooking the fluid and the clutches even with a cooler.