318 Magnum 70 Dart Gas mileage

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wes beem

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I recently purchased a 1970 dodge dart with a 318 magnum out of a 98 durango and 727 auto trans Its been converted to a 4bbl Edelbrock carb with long pipe headers dual 2 1/2" exhaust with flowmaster mufflers. 4:10 gears with posi trac. 14" Wheels. Wondering what mods i need to do to the car to get decent gas mileage so i can afford to actually drive her around. This is my first classic car so alot of this is new to me.

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It's not the engine but the gears from what I see. My LA 318 (but it is a 2 barrel not much smaller than the secondaries of a Edelbrock) gets almost 20 mpg. highway with a 3:23 gear and a 255-15 tire. Tall tire I know but.....what kind of mileage are you getting with those 4:10's? Even if it is a stocker, it probably burns tires for a city block! lol
 
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I recently purchased a 1970 dodge dart with a 318 magnum out of a 98 durango and 727 auto trans Its been converted to a 4bbl Edelbrock carb with long pipe headers dual 2 1/2" exhaust with flowmaster mufflers. 4:10 gears with posi trac. 14" Wheels. Wondering what mods i need to do to the car to get decent gas mileage so i can afford to actually drive her around. This is my first classic car so alot of this is new to me.

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Easy and cheap stuff to do to make your car a more pleasant daily driver. I'm guessing you need a new heater core since it isn't connected, so get this https://www.summitracing.com/parts/sgt-94503 and this https://www.summitracing.com/parts/cel-65074.

3.23 or 3.55 gears. I have a 2.76 pegleg and I HATE it. My commute to work is less than 10 miles.

And this so you don't get wet feet. Gasket Kits

And WELCOME! =)
 
I have a similar setup, 98 5.2 magnum in a 71 dart, stock longblock, cam and all. M1 dual plane, thermoquad, Doug's headers, summit 2.5 exhaust, 904 and a 3:23 sure grip. Gets almost 22 mpg and can cruise cross country with no issues
 
Get rid of the 4:10 gears, I'd suggest 2:94 or 3:23, tune the carb, make sure you have a good timing curve in the distributor, and definitely use vacuum advance, that should net 20s pretty easy
 
I got 17 with a 318-4 barrel and 904 trans.
25 or so with the 5.9 and Dakota OD.
 
thanks for the replies guys, so i deff need smaller gears. Id be happy with like 18mpg in town (if thats even possible) i put 7 gallons of fuel in the car when i got it and ive driven maybe 10 miles and its very very low on fuel already
 
thanks for the replies guys, so i deff need smaller gears. Id be happy with like 18mpg in town (if thats even possible) i put 7 gallons of fuel in the car when i got it and ive driven maybe 10 miles and its very very low on fuel already

did you check to see if your gas gauge is reading correctly?

Full 9.64 Ohms +/- 1 Ohms
HALF 23 Ohms
Empty 73 Ohms +/- 12 Ohms
 
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i have not, but im afraid to keep driving it and find out in traffic when it dies haha
 
Put a vacuum gauge on the intake after the engine is warmed up.In neutral, rev it up slowly and steadily, and watch the gauge. When the vacuum stops rising, record the rpm.Slow it down until the gauge has lost 1 inch vacuum.Record the Rpm. Drop the Rs back to idle.
The rpm of highest vacuum is the preferred minimum hiway rpm.It will get you the best MPGs
The second number should be considered the absolute lowest rpm for efficient running.
The numbers will change slightly with mechanical timing, but the actual rpm will only vary a bit.
It is very important to have a well worked out timing curve and a fully functioning vacuum advance system.
Typically, the vacuum will peak just off idle for a stock teener, to 2400 with a 360 and a big a street cam. 2000 to 2200 is a good number, but a cam this big is gonna suck gas around town, depending on your driving style.

Around town;
the 4.10s will not affect your city mileage much.Much more will your driving style than anything. 2200rpm will get you about 40/45 mph in top gear.Early upshifting with a big cam is not very effective as the engine is forced to operate in a very low efficiency zone. The headers can really upset things.
Of course the thing that will really burn up the gas is a poorly tuned low-speed circuit in the carb, and an over-compensating accelerator pump.

On the hiway, keeping the revs down is priority one. But There is very little to be gained by reducing rpms below the rpm of maximum vacuum.
Example; if your engine vacuum peaks at 2200rpm, then she will want to cruise not much less than that.With 26"tires,2200@ 65 takes 2.63s. 2.76s are the nearest, and they will cruise at 2200=62mph.
Of course 2.76s will be really soggy off-the-line.
Unless you have a near stock cam.

So the first thing I would do is a compression test, to see what we have to work with. With small number gears, cylinder pressure is a really big deal.
After that is to get the tune right.
After that is to change your driving style.
And finally after that is to swap out the rear gears IF hiway cruising concerns you.

Around town with a lot of stops and starts,those 2.76s may possibly actually cost you mileage, and the reason is, that every time you take off, you will be into the powervalves. And when it shifts into second and you want to speed up, you will be deep into the mains.
here is something to try.
With 4.10s and second gear, the road ratio is 5.945.
With 2.76s and first gear, the road ratio is 6.762
The difference is 13.7%
So go start off in second gear and feel the laziness, keeping in mind that the 2.76s will be just 13.7% less lazy,lol. The2.76s are not for everyone.
Here's another;
take your car up to 32mph,in second gear. This should be about 2450rpm.
With 2.76s and first gear, 2450 should be about 28mph. So once up to speed, there is very little difference around town.You are just using a different gear.
Of course there are several other compromise gears to choose from, as earlier mentioned.
When it comes to gears, you are always trading take-off-power, for hiway cruising. If you don't spend any time on the hiway, I see very little benefit to ditch the 4.10s.
If you do, or want to,spend time on the hiway, then the go-to gear is 3.55s for a 318. Perhaps the Magnum can suffer 3.23s. The compression test will help determine that.
 
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Put a vacuum gauge on the intake after the engine is warmed up.In neutral, rev it up slowly and steadily, and watch the gauge. When the vacuum stops rising, record the rpm.Slow it down until the gauge has lost 1 inch vacuum.Record the Rpm. Drop the Rs back to idle.
The rpm of highest vacuum is the preferred minimum hiway rpm.It will get you the best MPGs
The second number should be considered the absolute lowest rpm for efficient running.
The numbers will change slightly with mechanical timing, but the actual rpm will only vary a bit.
It is very important to have a well worked out timing curve and a fully functioning vacuum advance system.
Typically, the vacuum will peak just off idle for a stock teener, to 2400 with a 360 and a big a street cam. 2000 to 2200 is a good number, but a cam this big is gonna suck gas around town, depending on your driving style.

Around town;
the 4.10s will not affect your city mileage much.Much more will your driving style than anything. 2200rpm will get you about 40/45 mph in top gear.Early upshifting with a big cam is not very effective as the engine is forced to operate in a very low efficiency zone. The headers can really upset things.
Of course the thing that will really burn up the gas is a poorly tuned low-speed circuit in the carb, and an over-compensating accelerator pump.

On the hiway, keeping the revs down is priority one. But There is very little to be gained by reducing rpms below the rpm of maximum vacuum.
Example; if your engine vacuum peaks at 2200rpm, then she will want to cruise not much less than that.With 26"tires,2200@ 65 takes 2.63s. 2.76s are the nearest, and they will cruise at 2200=62mph.
Of course 2.76s will be really soggy off-the-line.
Unless you have a near stock cam.

So the first thing I would do is a compression test, to see what we have to work with. With small number gears, cylinder pressure is a really big deal.
After that is to get the tune right.
After that is to change your driving style.
And finally after that is to swap out the rear gears IF hiway cruising concerns you.

Around town with a lot of stops and starts,those 2.76s may possibly actually cost you mileage, and the reason is, that every time you take off, you will be into the powervalves. And when it shifts into second and you want to speed up, you will be deep into the mains.
here is something to try.
With 4.10s and second gear, the road ratio is 5.945.
With 2.76s and first gear, the road ratio is 6.762
The difference is 13.7%
So go start off in second gear and feel the laziness, keeping in mind that the 2.76s will be just 13.7% less lazy,lol. The2.76s are not for everyone.
Here's another;
take your car up to 32mph,in second gear. This should be about 2450rpm.
With 2.76s and first gear, 2450 should be about 28mph. So once up to speed, there is very little difference around town.You are just using a different gear.
Of course there are several other compromise gears to choose from, as earlier mentioned.
When it comes to gears, you are always trading take-off-power, for hiway cruising. If you don't spend any time on the hiway, I see very little benefit to ditch the 4.10s.
If you do, or want to,spend time on the hiway, then the go-to gear is 3.55s for a 318. Perhaps the Magnum can suffer 3.23s. The compression test will help determine that.


Thanks for the info, yeah highway driving is my priority as i plan to be road-tripping in this car and i dont wanna have to stop for gas every 100 miles.
 
Thanks for the info, yeah highway driving is my priority as i plan to be road-tripping in this car and i dont wanna have to stop for gas every 100 miles.
Oh I hear ya there!
No matter what, an intake and exhaust system that flows really well will allow the engine to work easier, make more power. Tune the carb well combined with a good ignition system, it will do really well in the Hwy.

Overdrives really help mileage but, that can be another venue to chase later. There is a sticky thread here on "How
To...." an OD trans into the A body.

My '03 Quadcab 4X4 Dakota with the V6 did well for the heavy underpowered tube it was. The OD trans really kept the rpm's down at Hwy. speeds. It had 3.55's for gears. And a dang tall tire. (Stock for the truck but really tall next to a car tire.)
 
first thing ya do is get rid of those 4:10 gears... i like the 2.94 gears myself... then fine tune the *****..
 
Step one - verify the tire pressures are at the maximum recommended pressure.
Step two - Get an alignment by a qualified shop.
Step three - Properly tune the engine - timing and carburetor. Make sure you have a vacuum advance that functions too. You should be able to get mid teens easy, high teens being careful with the gearing it has.
 
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