hubes

Please post before and after tranny and before and after mpg, thanks

Shark Racer

Went from 10 to 12 in the city. 13 to 18 on the freeway.

-Lead Foot Steve

carl a

went from 7 mpg with the 3 spd turbo to 17 with the 700r-4-(383 mtr)

zwede

Went from 4-speed to 5-speed with 0.68 OD. 3.08 rear gears. Highway mileage went from 15 to 21. City stayed the same.

hubes

Lead Foot Steve, whats your before and after tranny?

Simmo

Gain is probably very dependant on engine setup. If you have mp injection gain is likely to be very good. If you have big cam, big heads and big carb you may lose mpg.

markdtn

Can't speak to gain in the vette since I bought it non-running and built it the way it is. I get 17 city with TPI, 3.73 gears and a 700R4. I think it would have to be 3-4mpg better than a TH400.

mandm1200

It seems it varies quite a bit from car to car. Although I would like to have an OD, I just don't think I drive my car enough to justify the initial cost.
We like to use mpg term because it easy to understand and comprehend. Essentailly it is how much fuel is required to overcome friction/resistance. If the car is driven at the same speed except for the gear ratio of the transmssion, then everything is else can be considered the same. The friction of the air or the drag of the car is identical at 60mph no matter what transmission or gear the car is in. The resistance the tires as well as the resistance/friction or the rear end (including universal joints and so for) are all the same. The only difference I can see is how much friction/resistance is reduced by spinning an engine at lower speeds. That is where the savings are at. We all know that it takes arm power to turn the engine while adjusting the valves. We would be lucky (or extrememly strong) to be able to spin the engine at 10rpms. Imagine how much energy is required to turn it at 3500rpms. If the engine turns a 1000rpms less, gas mileage should improve. How much? I'll leave that to someone who works in that field. Most street engines should be fairly efficient at 2000/2500 rpms where an OD transmssion will operate in at 60mph. If you have a radical cam, then lowering the rpms may not improve things as much.

Shark Racer

TH350 -> TH700R4 w/ 3.55:1 rear gears. Mileage figures were for stock L-48.

-Steve

AlwaysWave

From a TH400 to a 200r4, from 17 to 21 highway. Less in city because 3rd doesn't lock up. I don't feel the swap was worth it.

LOTAHP

Do not know actual mpg. But I will give you an idea... With M-20/3.73 or TH350/3.73 I would go about 150 miles then the fuel gauge would read about 1/4 tank left. Now with a 700R4/4.11 I can 200 miles on the same amount of gas.

Also I am tacking at 2800rpm@80mph, where before I was tacking at 3500rpm@80MPH. This is not "exact" but very close.

Fevre

Just doing the swap for better gas mileage is not worth it but if you take into account the drop in rpm's for e-way driving and the increase in cruising range then you have more to factor into your decision.

pdgourno

The way I drive does not really get me the best MPG. I recently told my son that I figure I spend 80 to 90 percent of my time slowing down when I drive around town.

The best I can figure now is about 16-17 for city driving measured from topping off the gas start and finish.

I did drive 240 miles from Seattle to Kennewick and used 8.3 gallons of gas from one top off in Seattle to a top off in Kennewick. It only took a little over 3 hours to get there so once again my goal was not getting the best MPG I could. In overdrive I am around 2000 rpm at 70 - 75 MPH.

There is a lot of good stuff to be said about fuel injection and an overdrive tranny in a C3. MPG is one thing but the dependable operation of the car makes the "non original" status okay by me.

Happy holidays to the C3 crowd.

David

UKPaul

That all depends on what you have to pay for fuel!
Can't give exact figures but 2 visits to our annual Vette show with the stock TH350C used just over 1 tank of fuel each time. Same journey the next 2 years with a 200-4r took 3/4 tank. At our fuel prices the trans will pay for itself eventually. The better economy is a bonus, but being able to run at 75mph @ 2000rpm is great (especially with loud pipes that get a bit "enthusiastic" at around 3000rpm ). Seeing the looks on C4/5 drivers faces when an old C3 purrs up alongside them at high speed is priceless

w1ctc

Aside from MPG, it would seem logical that the motor would
last longer when going 1500 rpm on the hyway compard to 3000 rpm.

bobs77vet

i figured with my deal on the 700r4 i would need like 25,000 to break even cost wise, but i didn't calculate driving pleasure with a new faster 1st gear or reduced stress with an OD

redwingvette

I went from a TH400 to the 2004R and 11MPG's to 17MPG on the highway. I didn't make the swap thinking that it would pay for itself and any improvement was only a plus. I wanted the better get-up-and-go. Losing 1000rpm at 75mph diffidently was worth the money.

1971corvette

I would do it just to quote UKPaul




(Seeing the looks on C4/5 drivers faces when an old C3 purrs up alongside them at high speed is priceless )

Stoge

markdtn

and run cooler too.