340 Gets Terrible Gas Mileage

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Sport340

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Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
 
:welcome: aboard!

That’s not bad, anything over five you’re doing good.

Just remember, it’s not miles per gallon, it’s smiles per gallon.
 
I had a 460 ford and got 7mpg in the city and 13 on the highway.

6 does seem a bit low, i would expect 11 to 13 in the city.

BUT... There are way too many variables here.

Is your spedometer correct?
Do you have a high numerical rear gear?
Is the engine runnng rich?
Do you have a fuel leak?
Do you have a misfire?
Do you gun it from stoplight to stop light?
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
Click to expand...

What is the "performance cam?"
What rear gear?
What rear tire size?

Be honest do you keep your "foot out of it?"

And what kind of driving we talking about, suburban fair number of stops/ lights, or highway, or what, and what cruise speed?

What does the tach turn for XX mph, say 55, 65, 70 whatever?
 
Lean it out! Guarantee your rich. If you can smell it, It's wasted power. Fatten up the rods....easiest first step on the edelbrocks. I had a 351C in a 57 Fairlane. Erson Hi-Flow 2 cam, headers, Offy 360 intake, 3.5x 9 inch and a Predator PXP (?) Carb. I clocked 5 in town. That carb sucked on the street as it didn't have the street idle circuit of the PXi model. Put my Carter 600 back on and instant low end again and about 50% better mileage 7.5!
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
Click to expand...
Driving like you have an egg under your foot should bump you up to 8 MPG. Have fun with that. lol You should be able to do better. Make sure your odometer is accurate.
 
Hey guys thanks for the quick reply. I will answer some of the questions that I can right now.

I believe the speedometer and odometer is correct.
Not sure what rear gear is in the car. How can I tell what I have?
There is no fuel leak.
Could it be running rich, maybe. After I drive it a bit and come back to start it, I find I need to put pedal to floor while I crank it to start.
Do I keep my foot out of it, mostly but I will gun it from time to time. Guy has to have a little fun right?
I don't know what cam is in it. The place I bought it from did not have that info either.
Rear tires 255/45 R18
Suburban driving cruising 45 mph.
The question about a misfire, it lopes at idle but I believe that to be due to the cam.

Any additional comments would be great because I would like to drive it on a regular basis but 6 mpg might limit this especially if I'm using 93 octane.

Thanks

Dave
 
Make sure your vacuum advance works, better yet check your whole ignition system. And read your plugs.
 
Car Nut here is a picture of her. B5 Blue she is very pretty!


IMG_1034.JPG
 
Sport340 said:
Not sure what rear gear is in the car. How can I tell what I have
Click to expand...
There might be a tag on the diff.

If it is an open diff.

Put trans in neutral, raise one rear wheel. Rotate rear wheel 1 full turn and count rotations of the drive shaft.
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
Click to expand...
What's the timing curve look like?
What cam?
15 is possible if the cam isnt too hairy and the timing and curve is right.
Look for around 20 btdc initial & 34 mechanical, 48-52 total with vac advance hooked up.
 
You should get somewhere around 8-10 with that. However, the gear ratio can have a huge affect on mileage. Most 73 340 cars came with 3.21 or 3.55 gear.
Tire height will also hurt mileage. Shorter tire = more RPM and less fuel mileage.
 
chalk the driveshaft and tire, lay next to it and have your mate pull forward one tire rotation slowly while you count the driveshaft revolutions.
 
How does the oil smell? Put you hand by the tail pipe and then smell your hand...

Search the forum for oil threads as you need to run an oil for classics. I like Penn Grade Oil, some use Amzoil, VR1 and the such. These have an additive package for old motors, unlike the modern oils for new motors.
 
Welcome to the forum. There should be a metal tag bolted to the rear axle that tells the gear ratio. If it's not there, than the gear has most likely been changed
 
If you can't figure the rear gear any other way, get a GPS and take accurate speedo and tach readings. There are online calculators. Another way is jack up one rear tire with front safely blocked, put trans in N. See if you can turn wheel, and make certain you have the parking brake "off.

If you can turn the wheel you do NOT have limited slip, and do this: Turn the wheel exactly 2 turns while counting driveshaft turns. About 3 1/4 turns is 3.23, etc. If you can NOT turn the tire, jack up BOTH rear tires

Now, turn one tire only 1 turn exactly and again, count driveshaft turns. Same deal as before
 
Thanks for the suggestions I will get to work on some of this and report back. I appreciate everyone's comments and assistance. Keep 'em coming.
 
Back to basics. Dragging brakes can cause high fuel consumption. So lots of things to check. Start with the most obvious, the carb. Make sure the met rod pistons are free to move up & down. With met rod piston cover moved to one side, but still containing the piston, make sure the piston is down at idle & not moving. If it moves, it means the piston spring is too strong.

Check float level [ 7/16"], & that n/seats are sealing. Those carbs have plugs in the base that block passages that have been drilled during manufacture. Check that none of them are leaking. No gaskets are needed if you are careful.
 
Get an engine vacuum reading. You may not have enough initial timing. That's a very common problem when people put a non stock cam in one. Start with the basics. Make sure the engine is in good tune and go from there. Read the plugs. See what they look like.
 
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