My wife and I lived in the U.S. for about 2-1/2 years, (South Carolina. New Mexico and D.C.), and really enjoyed the friendly people, the good food, the scenery and, the low fuel prices.
Where I live now, gas is over US$7.00 per gallon. The roads are twisty and hilly, which is fun, but you never really get going too fast.
In all my years around cars, I always thought that an engine built for low end torque was probably going to be pretty good for mileage too. Around here, both of those are fine things.
The fact of the matter is, my '76 is a cruiser, and I don't much care how fast it will go, I just like to drive around in it.
The diff gears are 3.08, L48, with an automatic transmission.
I don't imagine too many of you in America, with your cheap gas, have spent a lot of time trying to get better mileage. But, any and all tips are welcome.
Also, mailing me some southern BBQ pulled pork would make you a friend forever, especially if you included some of that brown sweet vinegar sauce.

 

Adjust your spending habits to have enough for fuel, like skipping the pulled pork.

 

What are you getting now for MPG? I would also be curious how you are on maintenance for the engine? When was the last tune up on the car? The 3.08 gears may help with MPG, but the automatic is hurting it. What carb are you running? Is it a quadrajet still?

 

I had to download your opening paragraph three times because my laptop refused to put gas mileage and Corvette in the same sentence.

You are at the economy level of drivetrain now: L48 / 3.08 / auto. It doesn' t get much better.

People of long ago used to say:
"If you don't like your gas mileage, put an old egg under the gas pedal".

I am sure your C3 is doing better on gas than mine: Holley Double Pumper, 3,000 torque converter, 4:11 rear.

 

A perfect tune is first and foremost as a tuned engine is an effcient one and a powerful one......the byproduct of tuning is extracting as much energy from X amount of fuel.....when you do that, you make more power and can use less fuel to do it......
The 66' in the Avatar was a 331 (327)/350 l-79 with a million hours in the heads.....stock L-79 cam....2101 Performer....factory sides...4 speed M20 and 3.36 gears.....it pulled 16 mpg all around out of an Edelbrock 650 AVS carb......perfect curve and perfect jetting.....
After tuning.....an O/D trans is the next biggest benefit......old cars usually have 3 or 4 series gears and an O/D brings them into the 21st century......plus a 5 speed swap has gear spacing that is so much better the car just feels faster.....and works less to pull itself.....
In the 80's.....5.0L Mustangs had like 2.70 or 3.07 gear sets...even with a 5 speed.....they would knock down about 18-20 all around if you kept your foot out it......it you swapped to a 3.55 set, they would pickup 2-3 MPG just because the 5 speed was happier with the shorter gear.....it worked less.....and the car went faster......ask any 5.0 guy and they will tell you!
Another big one that no one pays attention to is stance.......if your Vette's stance is nose high like a LOT of them are (78-79 were like that off the assembly line) it will not only handle bad and look like poo....but it will lift at 110 MPH.....because the car becomes a windsail, the front of a Vette is shaped such that a nose high attitude generates a TON of lift, and guess what? That lift is there at 70 MPH too......and costing power to move it..and fuel....when I lowered the front of my 69' 427/400, it was VERY noticeable how much more stable the car was, how it didn't lift at 120...and how much less strain on the engine at 80 MPH on I-35......a one inch lower front to rear or a "Cal-Rake" is the best bet.....the lower, the better......my 72' is a low as it can get in the front and still drive around here......it is planted and stable at 120 MPH too.....
How much less fuel is it using? I have no data......but it has to save some, and the other benefits make it a no-brainer......
A lot of folk will be like "If you are worried about mileage then get another car......" and that is horseshit.....a good running Vette is a fuel efficient one......and EFI isn't a magic bullet either....it may be a little better, but not much than a well tuned carb......this is fact.

Tell us as much as you know about your combo and post some pics.....

Jebby

 

That type of driving cannot be fuel efficient. See post #2.

 

I agree. Keep your car tuned.

 

Well caskiguy, I've actually learned how to make pulled pork here.
Sigforty, I've only checked on one tank of gas so far, and it was around 13-14 MPG. But, there was probably an hour of idling time while I set timing, checked for leaks, warmed up the automatic after changing the fluid and filter, set idle, etc. So, it may actually have been 14-15. I'll finish off the second tank in the next week or so, and we'll see. I bought the car knowing it needed work, and have done a major tune-up, even including a new distributor, plug wires, etc. It has an almost new Edelbrock carb, but I have no idea yet how it's jetted. The timing is to factory specs with the distributor advance coming in early.
Jebbysan, the stance is original and looks just about level. Certainly, the front end isn't up more than usual. The front springs are original, I'm pretty sure, but in good shape. I'm sticking with the engine/trans setup because by the time I bought and imported an overdrive automatic and everything it took to install it, I would have to drive about 1,000,000 miles to get my money back. I'm not worried too much about the mileage, but because the car now runs really well, it's just the next project, and as I said, fuel here ain't cheap.
Resdoggie, you're right. the winding and hilly roads here are probably half as bad as city driving, but after a while, you adapt to it and don't use the accelerator and brakes half as much as the tourists do.
I'm aiming for about 17-18 around home, and maybe low twenties on the highway.

 

Scrap the factory timing spec.......E-Mail Lars and get his latest paper on timing.......you will pickup power and mileage.....set the timing correctly first, then go after the jetting.

One thing I forgot to mention.....long tube headers always pickup power and 1-2 MPG......

My wife made pulled pork last Sunday.....she cooks it with Dr. Pepper.....out of this world.

Jebby

 

You could modify your carb/linkage so that the secondaries don't open.

 

I get 16 mpg with a healthy 427 small block because I have an overdrive transmission. When I first went to O/D I was running a 350 and went from 12 mpg to 20. O/D is by far the best mileage mod you can perform, but it’s going to cost you a couple grand and you can buy a lot of gas for that money.
Get your distributor advance curve set properly and you will see a big improvement as well. You should be able to do a lot better than 15.

 

As you know, the U.S. has many different companies/refineries (Mobil/Shell/Exxon/etc.)
Have you tried buying gasoline from different Companies?
I have found a difference of 3 to 4 mpg depending on which company I buy gasoline.

 

dual free flowing exhaust, 195 degree F. thermostat, lowest weight oil practical, remove the exhaust blocking flapper valve because it impedes free exhaust flow. Use type "F" automatic transmission fluid. It has more friction additives for less internal slippage. Remove the auto-tranny cooler lines from the radiator and put on a large external tranny radiator. Get rid of the heavy metal fan. I originally went to stainless steel flex fan of the same diameter with the big blades and later went to dual electric fans.

I would move the routing of the fuel line over the front of the motor and away from the hot exhaust.

 

If you put the cases of beer behind the seat, you will pick up 0.5 mpg, in aerodynamics.

 

In order of magnitude.
1. Take the weight out.
2. The biggest waste is the THM 350 tranny which has no lockup....so fuel is wasted THERE.
3. Perfect tune up.
4. Low rolling resistance tires and tire pressures.
5.Aerodynamics....while this starts to become a REAL FACTOR at 60 MPH (100KMH) the C-3 was desidedly un-aerodynamic. The D-60? Spoilers remove 15% right there.
6. Thickness of oil.
7. Engine running at the right temp. 195° F is correct.
8. FAN, you can loose 5hp and MPG for a fan that is overly over-performing.
9. Carburetor.. I lived thru the 'gas crisis.' in 1974 with a Pontiac GTO Ram-Air 3 (400Hp)....she got 9 mpg. I 'temporarily' Replaced the carb with a large two-barrel...yep! performance was gone...but it went 'up to LOL' about 15 mpg on the highway.
10. OCTANE....tuning the engine to run on low octane fuel is cheaper....generally sub 9:1 compression can use REGULAR 87-89 octane with no problem. (see #3)
11. Rear axle ratio...there is even more hyway axle ratio available...it's in the 2.74-2.84 range, which will drop the cruising rpms noticeably.

There's your top ten! (+1)

Good luck..... oh! and if you put a pulled pork sandwich on the gas pedal...you get GREAT gas mileage! lol

Unkahal

 

I've been driving 40 year old V8 non overdrive cars for close to 40 years, I wouldn't complain about 15 MPG. As Jebby said, lower that thing! 1-2 inches in the back, 2-3 in the front, if for no other reason than it will look great! Mine feels like it wants to lift over 80 as well. Lowering is on the list.

 

Your '76 C3 is not really engineered for efficiency and economy. For European use, it needs lighter weight, steeper rear gear, and a 5/6 speed manual trans with overdrive. The THM-400 auto trans is "bullet-proof"....but it is an inefficient and gas-guzzling albatross when presented with tight, winding and hilly roadways.
Converting your C3 to get those features would be costly and would likely not be cost-justified for the miles you intend to put on the car. One thing you can do with the present set-up is to use 2nd range [manually shifted] rather than just putting the car in Drive and leaving it there. Since the engine has so much low-end torque, it probably doesn't downshift when needed for economical driving. Going up hills at low speeds in 3rd range is just drinking fuel for no reason.
If you find someone with a decent C3 with 3.73 or 4.11 rear gear and a 5/6 speed manual O/D trans who wants to trade cars, you might think seriously about it. Wish you the best with your car. Sorry about the price of fuel over there.....

 

FWIW.......I stayed in the Smoky Mountains for one week. The "Gasometer" (MPG dash display) showed that my Cad. SRX was averaging 11 mpg while driving on mountain roads/sightseeing.
On the other hand....the 600 mile trip TO the Smoky Mountains averaged 29 mpg on "level" Interstate highways.

 

You are in France.
You have limited part supply.
You have limited gas stations.
Its a sportscar.
Its a Vette.
Leave it alone and enjoy it.

 

This says it all
When I bought my 72 it was setup the same- base 350 TH400 308 I drove it everyday. Mostly flat roads, no hard use, the car averaged 11-12 mpg. I rebuilt it with a torquer cam, headers, better timing, carb, shift kit, etc, it gets about 16 combined now. My base motor 300hp m20 336 69 got 18 cruising at 65mph for 8 hours.