Well,if I leave the motor running while I'm at the pump it will never fill up.

 

My stock 69 427/390hp w/ 3.36 gears and an M21 trans. gets a little over 15 mpg in mixed use. Now, with premium down below $2 a gallon, that's almost acceptable. I was at the S.F. International Auto Show the day after Thanksgiving and most of the manufacturers aren't claiming much more than that for their around town gas mileage.

BTW, ever since I found a mechanic to sort out my Q-Jet and get the secondaries working, the miles per gallon went to hell but the smiles per gallon are much, much higher.

 

76 l48 small block stock regular four speed and 336 rear end...best highway was 16, around town less. Put your foot into the secondaries and you need to be watching the road as the fence poles start moving by pretty quickly, not the gas gauge which moves very fast. It probably averages 14-15 on the highway at 65 but get it to 75 or 80 with 255r60 15" tires and 336 rear end and you're turning more than 2500 rpms pretty quickly. My trailblazer does 65 at 1800 rpms with 17" tires and gets 22-23 on the highway with its 2008 four speed auto. Vette is never going to get there without new gears, new rear, carb perfect. But...for an occasional use car....it's not worth all that and it stays happy as is.

 

 

I was at 20mpg... 70mph, 1975rpm... Back to 15... Tune up time ... 100 smiles per gallon though...(been using spg for years on my T-Bucket)

 

Mine same except 3.55 rear end. Struggle to bet 12mpg. Even when I stay off it. Tuned well. Don't know why such a poor mpg.

 

My 68 has a 3.55:1. I hadn't driven it for about 20 years before I started to renew it to get it on the road again. I thought it had a 3.08:1. The diff was rebuilt by Kenny at Tom's differential (RIP). If I'd known it was a 3.55:1, I'd changed it to a 3.08. It revs 3000 rpm at about 70 mph. Terrible gas mileage. The cost of gasoline is not an issue to me since I spend a lot more dollars on the car than I do gasoline. I just do not like the engine wear and tear of 3000 rpm at 70 mph. To keep the 3.55, I'm going to need to buy a Tremac 5 speed. I'm sort of planning on that. Have to convert the crossmember to being removeable. I don't want to ever put another 5 speed in with a non removeable crossmember.

I do feed my 68 with a little dose of $10 a gallon race gas. It's non leaded. The ZZ4 in my 68 has a high compression ratio which I can't remember now. The ZZ4 will live on 91 octane, but a little extra I think is welcome for the engine. I bought 10 gallons of unleaded since I can also pour it in my 08. I think leaded race gas was a tiny bit cheaper.

A mileage factoid: I had a L88 427 BB clone installed in my 69 coupe. The car had a 3.08:1 differential and with a 1:1 top gear transmission, (Doug Nash 5 speed, but the 5th gear output was the same 1:1 as the stock Muncie). The piston domes had been milled down to 11.5:1 compression as compared to stock 12.5:1 compression. Stock L88 cam and big port heads. Hooker sidepipe headers - low restriction. With a 20 gallon tank, I could drive to Las Vegas, with about 1/4 (??) tank of fuel remaining. Vegas is about 290 miles from here. What I'm trying to say here is that a high performance engine can actually be a good mileage engine. The things you do to maximize HP can often be the things that also improve engine efficiency. When I drove to Vegas in the car, I had to load it up with Torrance airport AvGas. With a 1/4 (??) of tank of AvGas remaining in Vegas, I could fill it up with the best commercial octane gas and drive back to LA without pinging on acceleration.

 

I used to get over 20 with the T56 and the stroker. Now that I'm running the Procharger I get about 17 or 18.

 

Don't care. If I got 10 gallons to the mile, I'd still drive it.

 

My all-stock base-350/TH400 '71 delivered 18.3mpg @ 65mph on a 120-mile round-trip to a show last Summer. I was pleasantly surprised.

 

16 Corvettes since 1977, and I dont think I've ever calcuated the miles per gallon in one of my cars.

What's next? Luggage space?

 

I don't check mileage: 355 SBC, 750cfm comp carb, big solid lifter bump stick, M22 and 4:56's and a 7000 rev limiter - I just know that to have fun I'll pay by the gallon!

 

Last fall I got about 18mpg driving to Bowling Green, Ky. Got closer to 20mpg when I didn't use the AC.

 

Stock 1981, 350 engine, 350 tranny no overdrive, computer controlled QJ, all emission items and ECM still on and operating, about 13-14 mpg running locally. Have not took her on a long road trip yet so unsure of trip mileage. Using hi-test at $3 plus around here hurts some but the smiles are worth it.

 

On a good day - 8. On a bad day - a little less. Original 327, 4.11s. A lot more smiles per gallon here too!

 

What would you be turning for rpms going 70 mph on the Interstate cruising? My l48 350/180 with M21 turns about 2850 rpms at that speed plus or minus a little with 3:36 rear end. I presume yours would have to be higher?

Lance

 

After i tuned the 74 stock motor,i took it for test run... the most difficult part was keeping it at 60... i had to ride the right lane and felt i was holding back traffic... i usually get around 16-18 on the hiway doing minimun 70...

 

Just recently did a carburation day with Jeff Smith from Car Craft, used a LM-1 on my car as well as two of my friends cars, got about 21 mixed city/hwy on the return trip from LA to San Diego.

The 496 Nova and the dualquad 454 belair both got 14 or so on the return trip.

 

Before I slit my throat at my apparent carb tuning deficit for my 14.5 mpg on my '76 L48 four speed with 3:36 rear end I thought to look at your profile and see what your car was. It says it's a 78 L82 engine...nice looking, btw....and a tremec 5 speed tranny, right? Don't know what rear end you have in it but I would kill to have the five speed to get the last gear's worth of lower rpms which should translate to much better mileage.

What is your rear end in the car and what is the fifth gear's ratio on the tremec 5 speed if you don't mind sharing? I hope it's right that it is a 5 speed.

You might be saving a life over a lousy tuned Quadrajet with facts to tell me how well my carb is actually doing compared to different gear ratios and their effects!

Thanks.
Lance

 

The rear gear is a 3.70:1 with the 5th gear it calcs out at about a 2:50 in 5th gear (0.68:1 OD). Yes freeway alone I get better mileage, the 21 quoted was city/fwy combined. My RPM's at 70 went from like 3500 to 2500.

However the 1st gear is lower and I suspect the 5spd actually hurts my city mileage...TBH I have only tracked mileage maybe 2-3 times in the time ive owned this car. I should point out the 1st time I calculated mileage was before I had the 5spd, then with the 4 spd and the 3.70 I was able to knock down 20 on one freeway leg of a cross country trip, that was driving for mileage.

I see you are in Chester VA. I bought this car when I was living in Roanoke.