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.

My old stock 70 340 4spd A66 Challenger with 3.23's, a good working vac. advance system and a real sweet 800 TQ was getting 18 US MPG/22 Imp at 65-70. Before the TQ, I had an AFB 625 that got 2-3 MPG less. Bring on the Thermobog bashers! I sold the car and kept the TQ (#1966 rods)

Unless the combustion chambers on the X heads are smaller or the heads are ported to flow better than the 1.88 heads you will gain nothing with the switch. In fact you could go the other way and loose slight low end performance unless you are going to run in the higher RPM range. (like 6000+)

With your current setup, try out a TQ I've got a new rebuilt one that is on the way now, and I've run one before on a mild 360. The fuel mileage versus other carbs is quite noticeable, especially when compared to any Holley that I've run

i used to daily drive a BONE stock '70 340 duster. the only change it had was a thermoquad over the factory afb. it had 3.23 gears and 205/70/14's that car would get me 18mpg on the hiway at 65mph. very respectable for what it was really.

A stock 71 340 with a correctly setup Thermoquad carb(800 or 850) and distributor can achieve at 20 mpg highway with 3:23's. To be honest, mid twenties wouldn't surprise me. The 284 MP cam will knock that down quite a bit.

Under ideal conditions, warm day, flat road, continous driving at 60mph i have gotten an honest 20US mpg, closer to 26 imp mpg. I have the 600 eddy carb and it is tuned lean at cruise, cam is a fairly small 268* and 3.23 gears with an auto and stock convertor with stock exhaust setup. It's not a dog either cause it wil run a 13.7 in the 1/4. I think the secret to good mpg is a fairly small overlap cam and lots of compression. That and a properly tuned carb.