I have 1979 cutlass with a 1976 olds 455
the motor was built and bored .30 over
makes roughly 460 hp
all done by performance shop 10 years ago by previous owners

I got 150 miles to a tank which holds 18 gallons

is this within the normal range for mpg ?

thats 8.3 mpg pretty bad

What are you getting?? Per tank

 

My 468 averages about 11. I'd say you need to do some tuning, but it depends on how you drive, too. 80mph taching 3000rpm will suck down more than cruising at 2000.

 

Give some more specs for the engine. On the face of it, that sounds like the lower range of acceptability for a 460 HP engine but it's hard to say until you know what's in it.

 

My bone stock 455 in my 76 H/O got 11-12 MPG on the trip to the OCA Nationals this year. That is with the stock rear end. Pretty thirsty.

 

You have to fill up and drive, then fill up and divide gallons into miles. I am guessing you didn't drive it to empty so you had gas still in the tank.

 

My top down thrice weekly 50 mile highway /back road drive typically sucks down at least 4 gallons. 1971 stock 455, TH400, 3.23 rear.
12.5mpg, but well worth it.

 

I filled it to 65 litres so I’m assuming it had 4/5 litres left in tank so 70 litre tank

I’m in Canada so everything is in litres but I have a auto meter guage set up so those are in
miles

Now that’s all city drinving

 

here’s the build sheet. Sorry I’m not sure what it all means

 

Keep in mind, bad traffic/excessive idling is a killer in a carbureted car.

We burn twice as much fuel per hour idling vs fuel injected.

 

Confirm the odometer accuracy.

 

1976 Cutlass Supreme 190 hp 455 TH400 = 6.9-8.3 mpg city only
1968 Hurst Oldsmobile 390 hp 455 TH400 = 4.8-5.7 mpg city only

Automobile Catalog.

Your mileage is clearly on the high side of things.

 

wow 1968 5.7 mpg max.
probably on a first name basis with the gas station attendant. Dammm

 

What gears are in your rear end?
I have a 1979 Hurst/Olds that was high performance before I went back to stock with it. I had a 1971 Olds 455 with Edelbrock heads, 10:1 compression, Comp cams extreme energy cam, roller rockers, and other components in it. It was making about 400+ hp and it was a blast on the street. Never took it to the track, though. I had a built TH350 behind it after the first one didn't last more than a month behind the 455. I also had 3.73 gears in the 8.5 inch rear end. Mileage was roughly 8-10 MPG if I could keep my foot off of it. Highway mileage wasn't any better... maybe 12. Turning 3400 rpms at highway speed got old really fast. There's a reason my kids called it the noisy car and didn't want to ride in it. I pulled the motor and rear end, sold them, and put a 350 and the stock 2.73 posi rear back in. The car is much more enjoyable and probably almost doubles the mileage I had been getting.

 

I think I have the 3.73 gears as well
it’s not to bad on the hwy
just over 2000 rpm at 60 mph

it’s got a 1984 Camaro ss 4 speed

it’s also a bit loud but not crazy.
I am looking at options for a quieter muffler or adding resonators before the mufflers

 

That’s not necessarily true. Sorry.

 

When you have cars like these, you are not measuring how good your mileage is but rather how bad it is. Years ago I had a 74 Corvette with a bone stock 454. It was in the single digits.

 

After my car was totaled she sat one year. But was started consistently. My fuel gauge is inaccurate/bad, so I go off of mileage and precisely how full the tank is on a level surface. I literally look at the gas location in the fill tube with a flashlight. She eats 1 gallon per 1/2 hour of idle. From full to full. I keep a diary of startups including duration of run time and fill-ups. Comparing that with notes from others is how I came to the carb vs injected conclusion. The general number I was seeing for a fuel injected V8 was 1 gallon per hour at idle. Now you may have different parameters but that's how I concluded that observation.



As bad as that was there was worse in original muscle era. Automatic Hemis were down to 4.4 miles per gallon, city.


All in all if you read all posts and consider your exact driving conditions you are in the correct range.

You seem to be matching what a 1976 Olds 455 would do mileage wise but at double its original horsepower. That's a win ! The massive diet that the 79 underwent vs a 76 is part of it. Negative 600lbs including your weight gain from the 455.

1976 455: Extra urban / City / highway / avg combined.
mpg (U.S.): 13-15.6 / 6.9-8.3 / 11.5-13.8 / 10.7

 

1974 was possibly the absolute pit of the smog era. Lots of problems making those cars run efficiently. It probably wasn't very fast, either.

 

Remember....it's not about miles per gallon, but rather Smiles per gallon

 

I had a 74 L82 4 speed
I was actually in the market for a pre 77 corvette but couldn’t really find any that were in decent shape and most importantly with a 4 speed most are auto

I almost bought a 1986 corvette they are a steal of a deal right now in Ontario Canada we’re talking 8-11 k to get a nice runner
however I was fighting with myself about the body style of the c4 corvette

I really like the Gbody style as well
that’s why I got this 1979 cutlass
drivetrain and body are in good shape
working on getting the door trim window track some interior bits back together

some electrical etc but overall I really like the car

with a nice paint and new shoes it will be a nice looking car

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Same here. 1972 Olds Cutlass S 1970 high compression 455 TH400 3.08 rear. Averages 10 to 11 mph. Cost of admission I guess. Runs like a top btw.

 

Well, at least you're taking it well. If my 455 could only manage 10-11 mph, I'd be pretty upset.

 

Well, there's trade offs. My daily driver in the 1980's was a 1971 Cutlass Supreme, 455 mild build, TH400 and 2.56 rear end gears. I regularly got 17-18 mpg on the highway and one long trip averaged 21 mpg cruising at 65, that was with four adults and a trunk full of suitcases. But those rear end gears were anything but snappy around town. If you do a lot of highway driving maybe an overdrive transmission would make it fun around town but help out on the open road. John

 

Big engine with lots of horsepower = Big fuel consumption. Always. If people want to drive their classics regularly (which I'm a proponent of mind you), fuel economy and the type of fuel matters at least somewhat, unless you don't mind paying for 8mpg of premium. 460 horsepower gets you to the grocery store just as fast as 140hp, though it might be a bit more fun.

 

The 455 in my 72 seems to get about 12.2 doing 60 mph down the interstate. I figure I have 200 miles range.

 

More engine details might be helpful. What intake/carb? Distributor & timing? Exhaust?

Very debatable, but I see heavy, older style pistons/rings installed with .003 clearance (as mfg always specified) that many folks like to see at .005-.006 to limit friction & scuffing. Cam efficiency? That’s one for the more knowledgeable among us.

FWIW, my takeaway is that this car needs some tuning optimization. The engine appears to be a solid foundation, the car is relatively light weight. Favorable timing & advance curve, carb optimized for efficiency without giving up performance, to me that means a Qjet, but making the most of what you have is the idea. I think that combo could do a bit better, how much? Who knows!. Overdrive with the 3.73 would be a big help outside of town but one can burn a whole lotta gas for what a trans swap can cost.

My personal example, for comparison here, is my car that is a pretty efficient runner, a bit smaller than 455 (434), moderate cam, light rotating assy (mid 1700 bob weight), generals are in my sig. Gets appx 15mpg avg with mixed, “normal” driving, on a few occasions it has logged ~20mpg on highway leaning trips, usually ~80mph on interstate but gawd it’s tough to exercise discipline. The best indicator is on the occasional track day, drive a bit over 100 miles on meandering country highways and a stint on the interstate, take 6-8-10 passes, drive home and still have as much as a quarter tank left, but the OD trans plays a big role there as well. Red car is a mild 350 build (9:1, big valve heads, very small HR cam, Performer/Qjet, small headers, 2.5” X pipe) with 3.08 & OD so it’s in another whole class of economy., on 89 octane gas.

I’ve built & tuned some carbs for people, the ones that get to the track shaved no less than a second off their quarter mile and gained several mpg along with much better response and drivability. Not patting my own back, it’s what some conscientious tuning can achieve, despite dialing in a carb being a time consuming, tedious, exercise in trial & error, though well worth it for a typically miscalibrated, maltuned engine. (ya, I’m not sure whether those are legit words either)

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No, it ran pretty good. It had 270 net HP and still a lot of torque. It had 3.08 gears. It was considerable faster than my 75 455 H/O. 1974 was really a tough year for manufactures though. They had all cars so detuned to get me emissions.

 

I join others' opinions about tuning. Your MPG probably could get a little better. BUT- choosing to drive a 455 car should mean that MPG concerns should be #52 on the top 10 reasons you drive an Olds with a 455 in it. I never look at gas mileage as a goal. Try 3.91 with a TH400 (no OD). Then you really shouldn't care about MPG.

Obviously, you want to ensure you're at a good tune because bad tuning can net you bad things like wasted gas and eventual faster engine wear, so you have to glance at how fast your 455 is eating fuel, but generally, you know when it's running well and running like crap.

 

Are you comparing an old tech V8 to a newer EFI’d V8?

 

MPG depends on your engine, how well it's tuned, and how you drive it. I have several cars ranging from 575 HP (Olds 455) to a supercharged newer Corvette (700 HP) and likely get 10-12 max MPG from all of them but I DRIVE them. Lots of WOTs, burning the tires, and quick starts to enjoy the cars, the HP, and the experience. I built the cars to drive and enjoy and don't track the MPG at all.

 

thanks. I have a Holley 750 with a edelbrock performer intake
the engine specs are above with build sheet in my previous comments

timing when I checked was retarded at idle
I hooked up the vaccum advance and i had about 20 degrees advance timing

I have a crane cams hi-6 ignition box with a Hei distributor

my total timing is about 30 degrees

 

Sounds like you could use some timing. Total should be up around 34-36. You could see as high as 40 with vacuum advance hooked up. My experience has been that lower compression likes timing, provided your fuel will tolerate it.

 

Yup, start with timing. With a typical HEI, I try to get initial timing around 20* or a tad more, mech advance to total out 34-36*, usually on ported vac, all in by ~3000 rpm, then 10* of vac advance. This is not necessarily easy to do with factory style HEI, usually need an out of the box performance version or a mech advance curve kit and vac advance kit. Once the ignition is in order, it’s time for the carb.

I am a devotee to wideband fuel:air gauges. Where were they all my life? I know some old schoolers scoff but there is good accuracy and feedback, in real time and optionally reviewable data to be had. You’ll need an O2 sensor bung in collector or a bit downstream with manifolds. If it generally runs well, see what the gauge shows that it’s doing. Chip away at it circuit by circuit. Find the sweet spot between butt dyno, drivability and A:F readings. Assuming it’s a double pumper, you’ll never get optimal mileage with it but it can likely be improved. Efficiency = Drivability = Performance = Economy, with economy following its own “theory of relativity” and the realization that an old school, 2 valve, carbureted, large displacement engine is only going to return so much driving economy. Dial in what you have as is reasonable and practical and enjoy it!

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I get about 60 smiles per gallon with my W-30.

 

Back in the day I had a '74 Torino, Husky and Starch style, with a 400 in it. I kept it tuned regularly, replaced the timing set because it had the same type cam gear as the Olds did. It would do about 10 mpg around town no matter how I drove it. Being young and dumb at the time, I didn't mind, but I kept having too much month at the end of my money so I learned to combine reasons for driving around. I didn't really drive any less, I just felt better about it.

Tune it and drive it when you can. Enjoy it when you do.

 

My similarly built 455 gets between 10 and 12mpg depending on how i drive it,323 rear gears . Actually the same as my modern f250 ford truck ! Go figure

 

Great response! I feel the same every time I drive mine! My wife laughed last night when I told her about this posting and that people were attempting to quantify the MPG of their old muscle cars. We got a good laugh out of it.

 

My og. 136k 455 in my D88 ragtop with 2.73 gears got almost 12 mpg around town with #1 cylinder bad, 120 psi compression, more then 50% leak down, bad exhaust valve.
All other cyl. are 155-160

 

Good lord, either you guys have some wildly inaccurate speedometers/odometers, fuel leaks, or a badly tuned engines. I can get 15mpg pretty easy with either my 81 455 powered 81 gmc, or the 69.

Before doing anything, verify the speedo is correct. You can download a speedo app on your phone, or use the mile markers on the interstate. You may find the mileage is better than you calculated.

Once the odometer is accurate, or you know how to accurately determine mileage, then you can start trying to improve it.

Fill the tank, record the current odometer reading, drive the car until it the tank is close to empty and then refill. Note how many gallons it took to fill the tank. Divide the miles driven by the gallons, that’s your current mpg.

Does your distributor have vaccum advance? Is it functional? If not, you are missing out on some economy. How is the tune on the carb? Most aftermarket carbs are calibrated pig rich, more than likely you will need to play around with the carb to get it dialed in.

 

I do have vaccum advance hooked up

carb probably hasn’t been turned in a long time

I was watching a few videos today on just doing a base tuning on the Holley double pumpers 750
Might try that