Tremac-C2 Conversion Gas Mileage Survey

[Greg S.]

I have been reading all the old threads discussing the installation issues and merits of installing a Tremac 5 speed in a C2 Corvette. The one topic no one has discussed is the gas mileage benefits of the Tremac. I'd like those with this conversion to weigh in and respond to this thread with their typical gas mileage numbers.
Include typical mileage before and after installation.The engine size and horsepower and the rear end ratio should also be included in the survey so we can compare to our own situations.
Thanks!

Greg Samson

[Gary R.]

Re: Tremac-C2 Conversion Gas Mileage Survey

Also include who had issues with these trans, like shifting into 3rd over 3k RPM, leaks from the slip yoke, shift rod binding,etc.

[Greg S.]

Re: Tremac-C2 Conversion Gas Mileage Survey

Yes, please feel free to post anything relevent to installation of these tranny's. The ultimate question I would like to answer is whether or not the tranny will ever pay for itself simply by the gas savings.

[Paul J.]

Re: Tremac-C2 Conversion Gas Mileage Survey

I have been reading all the old threads discussing the installation issues and merits of installing a Tremac 5 speed in a C2 Corvette. The one topic no one has discussed is the gas mileage benefits of the Tremac.

Greg Samson

Gas mileage? Who cares! 30 minutes ago I sold a Toyota Corolla. That person was concerned about gas mileage.

[Patrick H.]

Re: Tremac-C2 Conversion Gas Mileage Survey

The ultimate question I would like to answer is whether or not the tranny will ever pay for itself simply by the gas savings.

If you drive it as your daily driver, year round, for several years then the answer is yes. Driving these old cars occasionally or only during the summer months, the answer is no.

I computed the difference between a gas and diesel engine in the same car (VW, my friend was looking) accounting for higher cost of the diesel motor and price of the fuel. It took over 100,000 miles to have equity, and this is when gas was about $4/gallon. Similar numbers apply to the 5 speed conversion of an older Corvette.

Patrick

[Duke W.]

Re: Tremac-C2 Conversion Gas Mileage Survey

Unless you do a lot of highway driving, installing an O/D transmission will likely never pay for itself, especially if you already have a 3.08 or 3.36 axle. The primary benefit of an O/D transmission is probably a reduction in "busyness" and noise during highway cruising if your car has short gears.

Fuel economy can be better maximized by working on the spark advance map. You want the most aggressive centrifugal curve you can get away with without signficant detonation with a total initial plus full centrifugal of about 38 degrees (small block) along with a 16 degree full-time vacuum advance that is properly matched to the engine's manifold vacuum characterisitics.

Further improvements may be had by experimenting with carburetor jetting to get the engine on the ragged edge of lean surge at cruise.

Radial tires kept at relatively high pressure will reduce fuel consumption 5-10 percent compared to bias ply at OE recommended pressure.

Shift at 2000 RPM. Base engines should pull from well under 1000 revs in any gear and about 1000 for SHP. If not, you've got a lousy setup/tune.

Using the above techniques, most vintage Corvettes should see a ten percent gain in around town fuel economy, and about five percent better on the highway.

Early tail pipe emission controlled engines will see even better gains, because their spark advance maps were set up for emissions, which actually increased fuel consumption.

Engines without exhaust emission controls had better spark advance maps, but many can be further optimized, even with today's slightly lower octane gasoline.

As far as highway mileage is concerned, if you have a short geared engine like a SHP with a 4.11 axle, installing a 3.36 will drop cruise revs about 20 percent and reduce highway cruise fuel consumption in the range of 5-10 percent. Further reductions in revs yield diminishing returns.

The power requirement increases with the CUBE of speed, so maintaining as moderate a pace as possible on the highway reduces fuel consumption and the chance of getting ticket.

Using the above techniques, my L-76/3.08 axle/radial tire SWC would knock out about 22 MPG at 70 MPH cruise and mid to high teens around town, and with the more aggressive centrifugal advance curve it had a lot more low end torque than OE and no problem pulling in top gear from 1000 revs.

Duke

[Stuart F.]

Re: Tremac-C2 Conversion Gas Mileage Survey

I gave this same conversion about 5 minutes consideration a few years back at a winter meet as I drooled over the beauty of one of those Tremec beasts. I run a 3.36 with an L-76 engine in my 63 and I looked at it as a possible way to get lower/wider spacing on the first four gears for a little more low end flexibility - nah!!

I have since made changes to improve my low end through carb mods and ignition advance changes such as Duke suggests. Also, I think you would be adding a few extra pounds on the front end. Then too, I'm not afraid to tear into my Muncie both internally and it's shift linkage, but that Tremec looks a little intimidating, both to install and work on in the future. You'd have to use a good hefty transmission jack - no laying on the floor with it on your belly with that one!

Stu Fox

[Timothy B.]

Re: Tremac-C2 Conversion Gas Mileage Survey

I was at Carlisle last week and I think there was a guy selling overdrives for the regular Muncie four speeds. I stopped for a moment but did not get the dirty from him on what other changes to first, second , third are.

I think it's a waste of $$ except if you have low gears and want to drive distances.. Probably just as cheep to buy another rear for spare. Not that big a deal to change after you have never seized all the fasteners..

[Bill M.]

Re: Tremac-C2 Conversion Gas Mileage Survey

My '92 gets about 30 mpg at 70 mph in 6th gear (1500 rpm at 70).

I wondered how it would do if I ran it in 4th gear (3000 rpm at 70), so I ran long enough in 4th gear to get an average mpg on the onboard computer: 17 mpg.

This driveline is optimized for the relatively mild LT1 engine.

[Mark W.]

Re: Tremac-C2 Conversion Gas Mileage Survey

To answer your question, I installed a Tremec 5 speed about 3 years ago in my '66, 327/350 HP with a 3.70 rear end. I did not buy it for the gas mileage improvement and don't ever hope (or care) to recover the cost. I only checked the mileage with the Muncie a few times before the transplant and only a few times after, but I was getting about 14.5 with the Muncie and now get about 18 with the Tremec. These were all freeway miles at about 75 mph. I also sold the Muncie for $800, which helped re-coup some of the cost of the Tremec. The Muncie came out of a '69 or '70 Firebird so there was no point in keeping it.