OldCarBum

I'm doing research on differential upgrades for my 73, 496, TKO-600RR (.082) build.
I noticed that some manufactures offer a 3.23 ring and pinion gear set for a 63-79 Corvette.
I know this was not offered OEM but I am interested in the 3.23 ratio because I am considering either a 3.08 or a 3.36 and the 3.23 will put me right in between and where I would like to be cruising down the highway at 70 mph.

What do you think?

gkull

With your tire diameter what is the rpm's at 70 in od

jim2527

How much highway driving do you do?

GTR1999

That is not a common vette ratio as you know. I don't believe Tom's offered it so it is probably imported. If it was my car I would be using Tom's- US Gear gears only but your call. There is not much difference between the 308 & 336 so for a automatic I would use 308, 4 spd 336. OD trans 373

derekderek

Difference between 3.23 and 3.35 is about 4%. 10% would change 2000 to 2200 RPMs. 4% raises 2000 to 2080. I dunno if that is enough diff between 2 diffs to be worth going with a custom ratio.

OldCarBum

I'm planning to run 255-50-17's and at 70 mph my gear calculator shows I'll be turning 2287 rpm's.
The operating range for my camshaft is 2000-6000 rpm's.

OldCarBum

Mostly highway driving, mountain, foothill and coastal twisties as well.

OldCarBum

I'm just starting my research and I've heard Tom's has high quality parts, however I also heard Tom retired and they are no longer in business.
Is that true?

GTR1999

Tom died last January, his wife Jo is still running the business and I have used their new gears about 10 times since. They are still the best and still 5 cut, I did notice the set that came in last week was no longer in a Tom's box but a US Gear box.

gkull

Tom is gone and his wife is keeping the doors open. You can buy US gears all over the place. I have US 3.55 gears in my spare rear end. I have tried two rear end ratios with my .64 OD TKO 600. IMO. IF you are road racing the .82 OD is the best choice along with a 4.11 or even a 4.30 rear end. The best rear end for a road racing .64 OD is a 3.55 which I chose or possibly a 3.73 and you never use OD while out on the track. That allows you to do 150 - 160 in 4th gear . The best rear end for the street with a .64 is a 4.11 or a 3.90 for a sub 6000 rpm type motor. I don't race my vette anymore and put the 4.11 back in .

The .82 for the street is really a dilemma! I was looking at your .82 OD and this calculator. https://shiftsst.com/speed-analyzer Low rpm at cruising isn't always the best MPG. The motor has an efficiency range. If the rpm is too low you have to have your foot a little farther down on the go pedal to maintain a good cruising speed. I really like 4.11 and our 2.87 first gear for taking off. Very easy on the clutch. I really don't mind driving at 2370 rpm to go 70. I never have to shift down to pass. I also hardly ever drive that slow.

OldCarBum

The gear calculator I've been using shows with a 3.36 rear gear, .82 TKO-600 RR trans, 255-50-17 tires I will be turning 2370 rpm's at 70 mph, which is why I'm looking at either the 3.36 gearing or the 3.08 which shows with the same set up I would be turning 2181 rpm at 70 mph.

OldCarBum

I have talked with West Coast Differential who have built a couple differentials for me in the past and they told me that they have purchased from Tom's in the past and that Tom's is top quality.
I will call Tom's and talk with them.
Thanks

76strokervette

In my opinion you will need to upgrade to 30 spline inner axles and 31 spline outer axles at your power level.
There are many threads on here to fill in the blanks on the details and machining required.

Sky65

I'd use the 3.36.
Great 1st gear overall ratio.
2.87 (1st gear) x 3.36 (rear gear) = 9.64
9.64/2.20 (M21 1st gear ratio) = 4.38 rear equivalent ratio in 1st.
According to the trans ratio calculator 2379 rpm at 70 mph in 5th.

Tom

Metalhead140

I have 3.36 with my tko600, works pretty well with my built 388 small block - it could be slightly easier to get off the line with a shorter (higher number) ratio, as it does require a very small amount of revs and/or a slight slipping of the clutch, but not much, it's not a big deal, and no problem if launching the car hard. Keep in mind that my engine is biased more towards higher rpm, so for a milder cammed street engine it will be even less of an issue. It's not anywhere near the point that I've ever stalled it. It will turn the tyres at any time/speed if you mash the throttle in 1st gear (or in 2nd, and sometimes 3rd without semi slick tyres) so I'm not sure that having a shorter (higher number) gear would offer much performance benefit for the events that I use the car for - I use the car for hillclimbs, which with my current gearing I launch in 1st, shift to second, and on many courses I leave it in 2nd gear, which is good for over 80mph. On higher speed courses I use 3rd gear which is good for 120mph. I use 4th gear only on circuits, 4th ​is good for 150mph. Having the taller gearing (lower numbers) means less gear changes, and seems to work pretty well for the events I run in. This is with ~26" diameter tyres and an engine that will pull 7000rpm. On the highway it will cruise at a bit over 2500rpm, where it's very happy, but a taller set of street tyres can drop another couple of hundred rpm.

OldCarBum

Laughing Coyote

Thanks for the info guys. Still got time before I tackle that part of the car, but it gives me an idea on which way to go.

vettebuyer6369

Post appears unrelated to 5 year old thread.