Miles Per Gallon and other questions

How many miles per gallon does a 1971 340 plymouth duster get? Here are the specs.

650 cfm carb
1971 340 block
.30 over 10 1/2 to 1 compression pistons
stock steelcrank
edelbrock airgap intake
mopar purple cam shaft 484 lift
360 j heads (small valve)
727 trans with shift kit
7 1/4 non posi rear

Would i see a big performance increase if i but 340 x heads on this car with a 8 3/4 posi rear?

what gear?

I honestly dont know. The trans is automatic. When im doin 60-70 mph the tach says 2700-3000 rpms.

Some 340s do real well while others not so good. I've heard of over 20mpg and under 12mpg on the highway. State of tune, cam, and gears have a lot to do with it.

wow 4 1/4 rear be careful. my 72 swinger had a mild 340, 10:1 stock 340 cam w/ 1.6 rollers, headers rpm air gap and a 750 holley dp, 727 and 3.91's. I could pull 17mpg highway at 65-70.

you won't see a noticable power difference between the J and X heads.

you might see an appreciable performance improvement by going to a different gear ratio (say from a 3.23 to a 3.91), but you might sacrafice fuel economy and comfort (buzzing along at 3300-3500 at freeway speeds vs. 2700-2900)

that cam, you'll be lucky to get much better than ~13mpg....

With the RPM speed and the MPH given I would say that you have a 3.23 gear given the low and high RPM and low and high MPH.
I had a pure stock 71 that I bought new with a 3.91 factory installed and at 65/70 it would pull an honest 15-16 on the highway on a trip.

youve got the 284/484 cam in there. I ran that with 3.91's and a 3000 stall converter and got between 7 and 9 mpg. looks like you have 3.23's so you'll do a bit better. I'd say 8 to 10 mpg

I've got about the same motor with 323 gears but I'm running a 750 Edlebrock carb and it's not too good and I get around 10 MPG's out of it. The carb isn't good and it hasn't been since it was new and I've got a new Carter to put on it and I think it will do a lot better then. Good Luck! Ronnie in Okla.

with a 7¼ rear you don't have likely more than a 3.23...probably less. IIRC, the LSA on MP's hydraulic cams really sucks for mileage, but it's been awhile.

hotroddave40's mileage thread in the Projects section has some really good info for choosing parts with mileage in mind.

And changing your heads would be a colossal waste of money. Except in extreme (7K+ rpm/race) scenarios, you will not see enough improvement going to a 2.02 head from the 1.88's...likely none at all.

That cam is killing you. If there's one thing the 284/484 was not made for, it's fuel mileage. That's a big cam for a small block and it's got a narrow LSA. You'll never get good mileage with it.

I am pulling 16-18 mpg out of mine at 65 mph on the hwy, with 26.5" tall tires, 3.21 gearing, and a stock converter. It turns about 2700 rpm at that speed. The motor is a 73 318, with stock compression replacement pistons, stock heads, Comp XE268 cam, headers, 2.5" exhaust, and a holley 600 carb on a performer intake.

The trans is a 904. I am hoping the milage might make an uptick when I move up to a looser 2-2,200 stall to complement the performance gain.

I have a fairly large cam, but that 284 is even bigger.

The relatively stock 70 340 with J heads in my 340 Formula S 69 Cuda pulls about 15-16 with 3.23s and a 2000 stalled 727. The manifold is stock and the carb is a worn out Edelbrock performer 600 cfm.

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That cam is killing you. If there's one thing the 284/484 was not made for, it's fuel mileage. That's a big cam for a small block and it's got a narrow LSA. You'll never get good mileage with it.

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I think the 8 to 10 mpg estimates are pretty close. Still better than the 6 to 7 I get

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Would i see a big performance increase if i but 340 x heads on this car with a 8 3/4 posi rear?

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I doubt you'de see a perf gain from the heads, but it's hard to say. Has either set been ported>

The ring gear size won't gain you anything. It's the ratio. Stick with a sure grip instead of GM posi unit though.

An upgrade to an 8 3/4 is probably a necesity, simply from a strength standpoint. I had a friend with an aspen with a 340 in it. It broke a couple of those 7 1/4's in really short order.