What MPG are you 340 guys getting?

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Some,
to a lot,
of fuel mileage gains
can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
It's all in the combo, and especially in the ignition timing.
A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.

Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..
 
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Back in the day, 71 340 Duster, 4 speed, 2.76 gears, 20-21 mpg highway. I did have other gears that did not do as well.
 
Back in the day 340 4 speed all stock except carb 3.23's 20 all the time average. Another car 340 4 speed modified (small) 4.10's in the city about 16-18.
 
my 340 mild build got 22+ mpg with 3:55 and a gv overdrive
 
Ahhhhhh, there’s that GV/OD unit again!
If these things are in your budget, there really cool.
Expensive stuff.
 
340Dust-her said:
I'm seeing whether my 340 duster would get some decent mpg since it's coming along together. Nothing crazy, other than a stock 68' 340 with edelbrock performer intake/600 carb, rebuilt 904 transmission. I honestly forgot the gearing in the rearend, but its in the vicinity of 3:4's. Since it is a 3 speed, think it's possible to get at least 18mpg on the highway?
Click to expand...
AHHH! My 67 LA 340 Barracuda gets double digit fuel mileage.
Like 9.2-9.8 mpg on the highway going down hill with a strong tail wind.
 
Last Dart cruise I put in 7.5 gallons, drove 70 miles and came back with the gauge back in the same place. 9.3MPG and I was being pretty gentle with it.. lol.
 
Anywhere from 4 to 16 mpg. 391 rear end and 240 at 50 camshaft oh yeah I run a over jetted six pack :lol:
 
AJ/FormS said:
Some,
to a lot,
of fuel mileage gains
can be found in the cruise timing. Sometimes (depending on the cam) it is possible to run the engine at too low an rpm, and to not be able to give her the ignition timing she desperately wants.
For instance, if you gear your car for 65=2000, and the engine wants 52* of timing...... how are you gonna meet that need? With factory parts, it is nearly impossible. What if your cam is big enough to still be in reversion at 2000? What if, because of that reversion, your Effective Dynamic Compression ratio is just 4 or 5/1?
It's all in the combo, and especially in the ignition timing.
A manual trans, traditionally, has been cited to make about 10% better fuel economy, cruising at the same rpm. Part of that is in the TC slip; and that is why the factories have gone to loc-up convertors. And another part is inside the autos themselves. And a third part is in the transmission ratios.

Another major factor in fuel economy is your running cylinder pressure, which is governed by your Dcr (Dynamic Compression Ratio), the Ica (intake closing angle), and the throttle opening. This runs in conjunction with your carb, and carb size. If you gear your cruise rpm too low, and your throttle opening becomes too large, then she will get up on the mainjets. But now, the airflow thru the venturies will be quite low, because the power-requirement is so low. And it won't take much error in MJ selection to totally negate, or worse, all the work you put into the low-rpm running.
I used to run hi-compression 340s for years, beginning in 1970, in hi-school. 340s were never known for fuel economy except to say it was lousy compared to any other Mopar offering. And part of the problem, perhaps most of it, was the factory 268/276/114 cam.
That's part of the reason, as a streeter, I switched to 360s years ago. And the biggest reason I installed alloy heads on mine, was to get the super-high cylinder pressure..
Click to expand...


AJ, kinda surprised about the 340 fuel mileage comment. My experience has been the low compression 360's were the gas suckers. I guess it is the overall combo. Do you think that a 360 scienced out combo would make significantly better mpgs over a 340 optimized combo?
 
340 11-06 o.JPG


GAS MILEAGE??? :rofl:

Oh that's right, just one 600 - maybe you'll get twice the mileage!
 
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