zrc3john

Presently have a 3:70 rear in my 73 modified 350 4sd. Corvette.....

I'm running 255-60-15 all around.....Can't stand the revs on the highway on trips over 20-25 miles...Parkway driving is fine, expressway where everyone is doing 80, not so fine....

If I do go to a taller tire (275) it should drop the revs around 130 RPM or so......Don't know if that's going to make a difference...

Just wondering if anyone has ever went from 255-60 to a 275-60 and was somewhat happy with the change......

Like taking the convertible on long drives....I mean I have a ZR-1 for any real long distance trips, but there's something about a stingray convertible...

fishslayer143

130 rpm is not much.. have you considered Over Drive Transmission

flyeri

275 is wider and may cause a problem. I'd suggest some careful measuring. I think you will be disappointed with the result though. In all seriousness, if you want to enjoy driving the car, the 5 speed conversion is the best modification you will make to your car. I was very skeptical because I'm pretty thrifty.... no frugal...no cheap. Driving at 55-60 just wore me out in about 25 miles. Now we drive 200-400 miles on Saturdays. It is a night and day transformation. Many others will will tell you same thing.

zrc3john

Yes,my original ratio was 3:36 until old age just got the best of it......Found a deal on a complete 3:70....
Down the line I may just go with a 3:36 if I hold on to the car......Have a 383 I may drop in also....

AboveTheLogic

3.36:1 and a 4-speed still annoyed the heck out of me.

OD transmission seriously, its a completely different car. Making use of the torque on the highway is really great.

7T1vette

O/D transmission is the best solution. 3.08 rear gear with stock transmission is what is "comfortable" on the highway. I have 3.36 in my C3 and went to the O/D trans. Now it turns 2050 rpm at 70 mph and gets about 20 mpg.

Street Rat

One word - Overdrive.

derekderek

Also, apparently you do a fair bit of highway driving in this car. Most people just driving around town and driving the car shows. You need a fifth gear.

cv67

taller tire is money down the drain. Burn up that visa you wont regret it. the 255/60 looks great on these, leave it

PainfullySlow

Heck I am personally looking for a 373 or even 411 since I have a 6spd going in. If going to an OD trans is not an option for you, perhaps trading with someone who has/is doing the swap since they can easily handle the ratio but as pretty much everyone else has suggested: if you can go with an OD trans, do it.

cv67

If you know someone wtih a 700r in a hotrod go drive one youll be sold instantly. It really transforms the car youll drive it more often

Hated the 3 spd autos, and a 4 spd with gears was only acceptable when the speed limits were 55. 65 will get you killed these days

OldCarBum

I'm currently running a big block TH400 with a 3.36 rear end. At 70 I'm turning 3070 rpm. I'm stroking my 454 to 496, swapping to a 3.08 rear end gear and dropping in a close ratio TKO 600 with a .82 final gear, which should put me at 2100 rpm at 70 mph. My hydraulic roller cam's operating range is 2000 - 6000 rpm which will put me in the sweet spot cruising down the highways.
Start doing your research now before you go any further. You mentioned dropping in a 383 in the future.
Everything has to work together to make a great performing car.
Some over drives with a .63 or .7 final gear may put your rpm's at 1700 or less at highway speed, which can cause issues with your engine especially if you are planning to build a high reving 383.
Choose your engine mods, trans and rear end gearing carefully.

c3_dk

7T1vette

You really don't want to install an O/D transmission in a car with less than a 3.36 rear gear. With a 3.08 gear and 5-speed manual trans, you would need to stay in 3rd gear around town and 4th in suburban areas. Otherwise, the car will really be 'lugging' at less than 1500 rpm's.

A 3.55 or 3.70 rear gear go great with a .67 O/D high-range gear. You will get great starts in the lower ratio 1st gear and comfort/mileage at highway speeds in O/D.

OldCarBum

I do want a 3.08 rear gear with a close ratio 5 speed. I don't want a .67 overdrive 5th gear.
Shifting from 4th to 5th the rpm difference is over 900 rpm, talk about lugging. At 70 my cruising rpm will be in the 1700 range with a 3.36 rear gear. Way to low for my engine set up.
Shifting from 5th to 4th would make by butt pucker.
I rarely drive thru town and don't care if I'm only in 3rd gear for the 2 miles from my house to the interstate then on to the foothills or mountain roads.
The gearing with the TKO600 is perfect for a 3.08 rear gear around town.
How could it not, it's manual transmission and I can choose whatever gear I want.
Plus with my 600+ ft lbs of torque I don't think I'll have to worry about any around town acceleration.

AboveTheLogic

Tech specs of this page:
http://www.tremec.com/menu/tko/

Different ratio trans have different max torque specs, interesting.

zrc3john

Right now I'm at the crossroads with this car...Owned it now for 16 years.....Picked up a C-4 ZR-1 a few years back and put 10 grand in mods bringing it up to over 500 HP......

The 73 is just a different ride,more fun in the street with the off line grunt and torque the motor produces...If I keep it I know I will have to swap in a 3:36 rear or a 5 speed if I keep my 3:70 set up....Really like the 3:70 set up but on the highway it just beats me up after a while...

As far as a 5 speed,I am not familiar with which one is a good all around tranny....If anything I would look for a used one and try to get a deal.......Its either that, or go with a 3:36 rear and sell my 3:70...

Will see....

BLUE1972

I would go with the Silver Sport kit for the 5 speed.

They give you all the parts and the conversion does not require cutting up the car. Great Tech support.

It made my 71 a pleasure to drive - best of both worlds with the 3:55 rear. I chose the .6 overdrive, and 600 foot pounds of torque - it will never break..... 2000rpm = 80 mph.

OldCarBum

a

OldCarBum

Yes, American Powertrain produced the TKO 5 speed which will take a little less torque.
The TKO 600 5 speed is the newer model and is built stouter for bigger torque numbers at a little higher price.
Their 6 speed with take 700 ft lbs torque.