I have a stock base model 1978 corvette c3 that's almost completely untouched. I thought I had read while researching this vehicle that it could get around 20 mpg which while not great for modern standards I thought not that horrible especially with an old V8. Turns out it gets about half that and is about accurate based on further research. So what I was hoping to use as a daily fun commuter car is now turning out to be a bit on the pricey side for a daily driver.

I haven't been able to find much on this maybe because it's heresy but I would actually like my corvette to have a decent fuel efficiency, not amazing but better. I would like to keep performance but driving 60 miles a day at 10 mpg is fun to me but not to the wallet or the wife. I'm hoping that maybe I can find something reasonable to do. I'm not an extremely mechanical person I got one of these vets in untouched driving condition because I wanted to learn.

The top tip I found was to upgrade to I believe an Edlebrock 650 Carb.

I was also thinking that by upgrading my exhaust system to side pipes I could get better airflow and reduce power loss to gain a couple of mpgs. Also I was considering removing the other anti pollution bits as they no longer are required for my vehicle.

Some other things I heard was having my car alignment correct helps a little. And then for extreme cost getting an overdrive transmission.

What are your suggestions?

 

First things first make sure its running coorectly , tuned up and contact Lars about adjusting the carb and timing. If its still terrible it might be time for a rebuild or performance parts to help correct the low compression from the factory.

 

Fuel economy and performance in an old school carburetored configuration does not go together. I put an 04 LS 5.3 in my 93 Silverado 2wd, 4:10 gear and I only get 15 mpg, thats with the 4L60E trans. So I dont see much of a gain possible even with the freshest tune possible.
I guess anything is possible though.

 

What is your rear end ratio? Performance or fuel economy??? Pick one!

I have an 82 CE....2.87 rear gears, fuel injection, .70 4th gear, lock-up in all gears except 1st, lightened 250-300 lbs, more aerodymanic = 26mpg hi-way....that's as good as it gets with a C-3.

 

Yes I do expect I'm going to have to change my configuration from factory stock. Expecting to get greater gas mileage using stock is like asking pigs to fly without a catapult. I only mention what I currently have as that is my starting point. Considering 41 years of improved tech I would imagine that with the right upgrades I can get both better mpg and performance. I'm just trying to look at what options I have to make this a car that I can enjoy for many years to come. I am expecting to eventually do a frame off resto on my car. I'm just looking for tips to get the best bang for the buck.

On a separate topic I spent the last week driving my new vet, and then today got back into my old corolla. That corolla felt like an SUV.

 

IF you can find and old complete DPFI setup from say an '88-91 vette, and then an overdrive automatic....or a similar vintage complete engine, roller cam/etc, serp drive....


ditch the cat converter, probably blocked up by now anyway....side pipes ...eh, it's a style thing, let your wife burn her legs on it, and get back to us.....

 

Evolution of my '75:

Stock '75 L-48 4 speed 3.36 rear = 10 mpg
True dual exhaust, recurve distributor = 15 mpg
Mild cam, carb tuned by Lars = still about 15 mpg
Tremec 3550 5 speed with .62 OD and mild rebuilt 350 (~350 hp) = 16 to 18 mpg on highway

Now, 496 big block, EFI, 6 speed T-56, 3.73 gears = ~23+ on highway, 10 to 12 mpg around town.

 

Fuel efficient Corvette is an oxymoron

 

That has the aerodynamics of a brick wall and weighs 1k lbs more than a vette. Did you have it tuned to removed torque management and clean up the tune? What mpg did the truck get before the ls swap?

My 406 with 3k stall in a 700r4, 355s gets about 12 in the city, it would be higher with a lower stall (but thats no fun).

 

I'd never heard of a recursive distributor or even considered that a distributer would be a source of some mpg gains.

It's interesting to hear about true dual exhaust as well. I've heard contradicting opinions on any gains there.

 

youll never double the mpg in a c3 just tune it up well and throw an extra 10 bucks in your gas budget.
A fat lady can dance but dont mean she looks good doin it lol

 

If its got high gears and is running right, I see no reason it shouldn't get 20 mpg on the highway.

 

I can go with that statement.

The coefficient of wind drag is really not that bad on a stock C3 Corvette. If you travel a lot at highway speeds a transmission with overdrive will help enough to be noticed.
A stock Quadrajet rebuilt and correctly tuned will be as efficient as can be and stand up against any brand carb for fuel efficiency. Correctly sized, correctly inflated - low Rolling resistant tires coupled with minor adjustments like tuning which includes a a clean air filter, properly gaped plugs ( read for defects like improper timing, fuel/air adjustments changing to an efficient motor oil) and a well working PC valve will add up. Additionally, there was an Article in Corvette News years back about how wind drag is greatly increased for the efficient C3 running above 40 mph with the windows down and even greater drag when the T-tops are off. Under normal driving at highway speeds, this is a time when using your AC is beneficial to gas mileage.

In my opinion, any gain in fuel efficiency due to any installation of performance parts or modifications is lost upon the costs involved in such changes. You will wait many years to see even a thousand dollars of your improvements paid for by gas savings.
(I know this next sentence makes me sound like a tree-hugger, but I have been called worse things) -

That doesn’t say you shouldn’t care if your car is running efficiently. Any efficiency gained, however small may reduce your carbon footprint and save measurable resources when collectively added with other like minded drivers.

Rich

 

Alma. you didn't say. auto or 4 speed? as Z06 said, overdrive trans is the single biggest improvement. if 4 speed, there are Borg T5 from 88-92 Camaro and Firebirds that are a fairly straightforward swap. 200 or 700r automatics. can you do your own work? and if you really want fuel mileage, a 4.3 from a late 90's S10 is cheap and easy to find. but this is only if you can wire and plumb the efi yourself. or they are rare, but for a few years Chebby made a 4.3 V8. went in Monzas and the like. 3.5 bore 3.5 stroke 267. and nobody will want to shoot you when you open the hood...

 

The biggest improvement may come from how you drive.
If you drive in a spirited manner, driving gently may be the best way to get mpg gains.

 

If you do mostly city driving and you don't "baby" it, 10-12 mpg is what to expect with a carbureted engine. On the highway, (unless it has a lower rear gear ratio) you should expect 15-18 mpg, if the carb is working properly and your engine tune is good.

Fuel mileage goes up when the engine is running steadily at moderate rpm's. If your engine is running at 2000 rpm at 70 mph, you could get 20+ mpg with a carbed engine.

 

You have to keep in mind that you're talking about a 41 year old car, that appears to have a "heritage" that you're unfamiliar with. My wife had a '79, which she bought brand new, and kept until 1997. While it wasn't any where near an "economy car", it certainly achieved more than 10-12 MPG, in average driving.

Having said that, and including the questions that you posed, A) a "true dual" exhaust is far more efficient than a single exhaust, but may not be legal where you live, as it involves removing the OE exhaust, which includes a catalytic converter. Question #1, is your converter original? If so, it's probably shot, by now, and quite possible now a restriction in the exhaust tract, which will cost you both performance and MPG. Installing dual exhausts, if legal, will also require that your carburetor be adjusted (richen the jetting) to accommodate the additional air flow.

"Recurving" the distributor means altering the timing advance curve, most frequently brining in more timing advance, at a lower RPM. HOWEVER, again considering that your car is 41 years old, is the timing advance function of your distributor currently functioning correctly? If it's not, you're losing both performance and MPG. While the OE advance curve may not be optimal, it's still better than none at all. Therefore, I recommend that you make sure that your car is operating as it should, before considering modifications.....

 

While the drag coefficient was good for its day, the C3 had 5% higher drag coefficient than the C2, according to Karl Ludvigsen's Corvette: America's star-spangled sports car. The 1968 C3's drag coefficient was 0.450 with headlights down. The Base C7 Stringray is 0.30. While the frontal area and lift also effect total drag, the C3 is pushing a fair amount of air.

 

I was wondering about that, thanks for filling us in. Do you know the CD for the later bubble back C3s?

 

I do not believe the non sugar-scoop 1978-79's were that much better. There was an improvement in 1980 though.