Ive preached for years that this is a problem that requires a SYSTEMIC approach. If all the sales person is going to ask is "how much power do you have" and default to "deep 1st gear, deeeep overdrive" you're leaving a lot on the table.

Rear Gear x 1st gear should land somewhere in the 9 - 10:1 range
As Tom (Sky65) pointed out the classic 10:1 example is a close ratio (M21/M22) and 4.56 rear gears. A more "docile" combo would be 4.11s and an M21/M22 (9.04:1 total gear reduction)

If you can handle the .80 OD I would leave the 3.55s alone and switch out the TKO5oo for the "600" with the 2.87 1st and .80OD; from a performnce standpoint, its the best gearbox for most applications, and it doesn't have any features that prevent it from working well with a mild smallblock - the big difference is the 26 spline input gear I believe, realistically I wouldn't hammer a 10 spline input with 500 real HP either LOL

Most of the guys I know with the 500 TKO tend to just skip 1st gear and start off in 2nd if theyre kinda lazy...and more than one have complained that the .64 OD is useless unless they're on the high side of 70 anyway. Not a fault of the gearbox, its an OEM derived gearbox with a 350hp powerplant and fuel injection, so the 3.2x 1st gear and deep OD works for fuel econ testing.

As GTOguy pointed out; just cuz you CAN lug the engine down to 1500rpm, does NOT mean its efficient there.

For those of you that don't know; the 5th gear drive is CUT on the mainshaft itself. So to change the OD and keep the 1st gear, the OP would need a .80OD TKO600 mainshaft, and the matching driven gear, an overhaul kit and probably 5hrs of labor...which still may be a better option than buying a whole new 600

One of the best Tremec-backed TKO service shops is Hanlon Motorsports in Elverson PA. Bob is a magician with this stuff and Im sure he deals in cores; in addition to being Tremec-approved. If you need a Canadian recco; I'd talk to Brent at Gears-To-Go in Abbottsford, BC. He's one of the most knowledgeable guys I know and he may have a line on a unit or parts already in-country.

Nate

 

I've got an Excel file I use for this kind of thing. I calculate (best is to get manufacturer specs) 25.86" tall tires that should have 780 revolutions per mile. At 1:1 (4th gear) you would have to be running a 2.25:1 rear to be at 2,150 RPM.

This tells me something is not right. Either the speedo is way off or your transmission has an overdrive 4th gear. If you have the TKO 600 with the 0.82 OD, and were going about 51 mph, then your numbers match. I get 2,141 rpm in 4th and 1,755 in 5th (0.82:1 gear) at 51 mph with the description you gave me AND 3.08:1 rear gears. I can't get your numbers to work with the TKO 500 ratios.

You can guess your rear end ratio by jacking up the car and rolling the rear wheels one revolution while counting the driveshaft rotations. You can do ten revolutions to make this easier if you want. You might also want to put some tape on the driveshaft to make it easier to count the revolutions. The car obviously needs to be in neutral, but you also need a partner on the other side to make sure the wheels are both turning forward.

You need to be careful that there is no binding at full droop with the rear suspension. If you feel anything, support the rear of the trailing arms to keep the wheels from hanging so low. If you have side pipes, you also need to make sure you don't catch your finger between the pipe and the wheel when you do this. You will get a nice scar and a tetanus shot if your pipes are far enough back.

Let us know what you find when you check gears and verify speed with a GPS.

Are you 100% sure you have the 600 and not the 500? I can't see how you could got from 2150 to 1750 from 4th to 5th with the 500. The 0.64 5th ratio says you would drop from 2550 to about 1750 when shifting from 4th to 5th.

 

The car is in the shop will find out the exact rear end ratio when it's out, not sure when, my mechanic will do the work at home in his spare time.

Replacing the Posi clutches, rear spring and shocks.

My mechanic did the test you described a few years ago and came up with 3.55. The rear end housing indicated 3.73.

checked speed to-day with my Garmin.70 mph , GPS had me at 68 mph in 4th gear.


Once the car is apart, I should know more.

 

I had your tires as 225/50/17 so my numbers were off. Tires are 26.74" tall with 754 revs per mile.

70 MPH in 4th would be 3,166 RPM, and 5th with a 0.68:1 overdrive would then be 2,153 RPM.

Here's my chart based on the tires and TKO specs

 

Back when my '64 was stock (327/250hp, Powerglide, 3:36 Posi rear), it was amazingly balanced. I still keep a log with mileage at each fill-up, oil change, mechanical work, tire life, etc.

All through college and into my career, the car got 15 mpg in city AND freeway driving. Thirty mph or 70, it made no difference. Now was that BALANCE or what?

 

My first Corvette was a 1967 327/300 with 3.08 gears. Best mileage it got was 20 mpg on the highway. I got gas at the freeway onramp, drove to San Diego and got gas at the offramp. All freeway speeds and no traffic. This car did get worse mileage on the city streets and with more spirited driving, but I recall it was more in the 13-18 range most of the time.