What is the Fuel Economy possible from a Carbed V-8?

The best I have ever seen was:

1983 Dodge Diplomat
Weight 3395 lb (Base Curb) / Real World with Options, Driver, and Fuel 4000 lb
5.2L 318ci V-8 2-BBL ( 130 bhp @ 4000 rpm / 240 lb-ft @ 2000 rpm)
Wide Ratio Auto Trans (2.74 Low) with lock-up converter.
2.45 Axle Ratio
27.1" Tires

City - 20 Mpg
Highway - 31 Mpg (60 Mph)

Several people on this board have stated that 20 mpg highway is impossible in a 4000 lb Truck because of Aerodynamic Drag. I really believe if a 4000 lb Diplomat can get 20 Mpg City a Truck should be capable of 20 Mpg Highway.

BTW my 2001 Dodge Ram Club Cab 5.9L (360) EFI at 5400 lb (2 People + Luggage) got 19 mpg at 70 mph on a trip a few years back. No reason with proper tuning a 1000 lb lighter truck with a Carbed 302 cant do better.

 

I don't know Gene K. Your numbers sound a little high. Those early 80's smog v-8's were not known for having great fuel economy. Those are the same numbers I got with both my dodge omni and chevy cavalier. Those were EFI 4 cylinders. I just dont see a carbed v-8 in a big sedan getting 20-31 miles per gallon. Esp. with 2.45 gears out back. You've got to give it that much more pedal to get her movin.......... Did you compute those numbers yourself or did someone tell you that?

 

I'm not sure how a 2.45 rear end and 27-inch tires qualifies as "real world," but the mileage numbers MIGHT be possible. On the other hand, what effect did those 27" tires have on the odometer? Was the odometer properly calibrated to those tires?

FWIW, I average 22.3 with the 4.6 in a '96 Town Car, commuting back and forth to work (about 4200 lbs. with driver).

 

I think it's possible. In 1988 I went on a 2 car road trip with a buddy of mine. I was driving a stripped down 1982 Dodge D-50 (mitsubishi) 121CID 4 cyl, 4Spd pickup. No power accessories (windows, steering, locks etc), vinyl interior, tonneau cover on bed. He was driving a 1988 Crown Vic 4 door, injected 302, power everything, cushy interior. We both filled up at the same gas station at the start. At 200 miles we both topped off and figured mileage. Mine was 27mpg. His was 31mpg. I was really upset.

 

Aerodynamic drag is a big deal at highway speeds and cannot be dismissed. Drag goes up with the square of velocity. The best mileage I ever got with a carbed truck is 19 highway on a 70mph run. The stock, but worn, 250ci I-6 1bbl only got 13!

1971 Chevy C10
283 V-8 (1964 engine)
2bbl Rochester carb
High-lift RV camshaft
Exhaust headers and dual exhaust, Cadillac mufflers
Three-on-the-tree transmission
3.73 gears
Bias ply tires

This thing probably didn't weigh much more than that Diplomat, as it was a shortbed 2WD "fleet" model with no options, not even power steering or brakes. I think the "trick" is to get the cruise rpm near the torque peak of the engine for maximum efficiency, especially with a carbed engine. I added both a tach and vacuum gauge.

I cant see why my 1994 F-150, with a sequential multiport injected 302 and overdrive, has never done better than 16mpg, with 12 being the norm.

Jim

 

Those are computed ie miles divided by gallons. The Diplomat was my Grandmothers old car (Last She Owned) and she also had a '78 that got 30 MPG Highway. Most owners of Diplomat/Fifth Ave will tell you these are real numbers.

205/75R15 (27.1") was the stock tire size.

2.45 Rear Axle (Factory Stock) is equivalent to a modern 0.70 OD Trans with 3.50 Rear Axle Ratio except it should get slightly better mileage due to less friction from not having to turn a gear set in high (1.0:1) and lower churning losses in the rear axle.

Producing Max Torque at 2000 rpm and with a 12% Lower first gear ( 2.74 vs 2.45 ) than a standard ratio automatic the 318 Diplomat got going just fine with a 2.45 Axle.

 

I can fathom it with a fuel injected 302. That's less cubes, computer controlled, and the Vic IS the epitomy of a highway cruiser. I love those cars.

 

The best I ever got was 19 mpg at 60 mph.
1974 Chevy LWB (4400 lb Loaded)
1971 400SBC 2BBL (255 bhp)
TH350 Auto
3.31 Rear Axle
28.3" Tires
2.5" Dual Exhaust w/ Crossover and Sonic Turbo Mufflers
Truck ran 9.90 1/8 mi with 26" McCreary Road Star 60 (Dirt Compound).

Part of it is the fact that modern EPA NOX requirements mean that Cruise F/A Ratio is 12.7-14.1:1 when the best fuel mileage would occur at around 16.5:1

 

Back in the early to mid 90's I had a 78 caprice. It had a 305 with a 2 barrel carb. I took it on a trip to dayton, ohio and it got around 25-26 miles to the gallon on the highway. We were following a friend out so I kept the foot out of the gas, that's probably why I did pretty well.

I also had a 96 caprice that got 27 on the highway... and that was with the LT1 corvette motor under the hood... as long as you kept your foot out of it (that's hard to do), you did pretty well.

 

The older dodges used what was called a "lean burn" motor. They were leaned way out, had hot spark plugs, and advanced timing from the factory. They were known to get 30+mpg from the 318. It was the way it was set up. We had an '82 Delta 88 with a 307 that got 27 or 28 on the higway and about 16 or 17 around town.

 

In general carbed V8's even FI ones dont get good fuel economy, it's not the nature of the beast so to speak.

My friend had a 79 Capri, 302, 2bbl, headers, 5speed the car got close to 26 city and thats with a few burnouts here and there. My dad had an 86 GMC 1500 longbox 2wd, 350, non cc q-jet, TH350, 3.08 gears and it was awsome how much you could drive on a tank of gas. I avrged 20 MPG city in it. When I told my I was amazed how good it was on gas he accused me of romping on it or I would have gotten better. Of course I did lay it to the floor a few times.

 

Well what kind of milage should i get in my 83 f-150 4x4 Auto 302 with a 4bbl Edelbroke Carb?

 

How about a 4 door full size sedan with a 335 CI flat six that would do 120 mph. It was built in 1948 and was known for going 250,000 miles with just general maintenance. It got around 30+ mpgs hiway. It was the then, state of the art, Tucker. Far too advanced for its time. Of course it didn't have fuel robbing emmissions systems either.

 

It's now up to 22.8 mpg. Not bad for an old fart's car!

 

I had a 1982 chrysler cordoba ( purchased new ) with the 318 2BBL carb. and it always got 25mpg on a trip. The first fuel injected car I ever owned was a 1987 mercury grand marquis with the 302 engine and it never got over 18mpg on a trip.