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Will switch to dual exhaust help 1985 Dodge D150 mileage?

Trying to improve gas mileage on re-manufactured “318” V8 which started out at 6 mpg and now only getting 11 mpg.
Want to know from someone who is not trying to sell me merchandise or services an impartial answer as to whether or not going to dual exhaust from single will help improve gas mileage with all other conditions static.

Any improvement would likely be so small as to be unmeasurable.


What transmission and differential(s) ratio in the truck now and is it two or 4 wheel drive? I don;t think going to a dual exhaust would save enough to pay for itself. Changing the gearing would give the biggest improvement and there are kits for putting self tuning TBI fuel injection in place of the carb. . I don’t know if they are available for your 318.

Unless you are going to drive a lot of miles, you won’t save enough money to pay for what you spend. And will you be able to keep an 85 on the road long enough to get those miles?

The ideal solution would be a compatible late model overdrive transmission with more speeds but I don’t know if one exists. Maybe you can find a Dodge truck forum to ask.

Doing a silly Google search shows that your truck new only got 10 city and 14 highway . You are not using this thing as a daily driver are you ?

Awesome. Pretty much what my hunch was. Was curious though because new 4 barrel carb manufacturer recommends only NON-ethanol and here in Oregon I have to pay $5/gal and not many stations handle it.

Farm truck with Tommy lift.

Was somehow hoping to get close to 14 someday but it’s not on the road much. And if it is, it’s always hauling or towing.

In shop being completely repainted. Bought it just for the cost of the Tommy lift.

I agree that a dual exhaust won’t help much. Replacing the carb with a self tuning fuel injection system for about $1000 will help a lot and will use regular fuel with ethanol. Any performance shop can supply kits from FAST, Holley and others. It will help mileage and allow you to use E10 gas rather than ethanol free.

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That’s odd - all it takes is the right rubber and plastic to handle E10, I’d think.

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It’s a 34 year-old brick on wheels with a V8. I think 11 MPG is as good as it’s going to get. Even brand new in the time before there was ethanol in the fuel, it didn’t do much better.

As of today, the national average fuel price is $2.59. If fuel at this price is stressing your wallet, maybe it’s time to drive something more efficient.

Where can you find the right carburetor parts made of the right rubber for a 34 year-old truck?

My '71 Charger had a 318 V-8 with the standard 2-barrel carb. It got only 12-13 mpg in town, and it maxed-out at 16 mpg on long highway drives. However, it was more aerodynamic than the OP’s truck, and it likely weighed a bit less.

Edelbrock makes e10 compatible 4 barrel carbs based on the old Carter AFB. Holley makes e10 compatible carbs in 2 and 4 barrel that will work.


Back in the day I owned a Roadrunner and a Superbee; both with the 383 Magnum. Both of them would get about 17 on the open road which surprised me a lot.

In town though, it was a different story. Easy driving would get 8 MPG and with mildly aggressive about 4 to 5.

Adding an OD transmission or going way up on the rear axle ratio would help. There are kits widely available which allow the installation of a GM 700R4 transmission to Chrysler engines if it comes to that. The later 700R4s are pretty solid.

Testing the spark advance for proper operation and adjusting the timing for peak performance while avoiding detonation when under load might result in a meaningful improvement. But if that model has computerized timing control it is best to keep it at the factory specification. I have retrofitted earlier carburetors and distributors to eliminate failing Lean Burn systems and the customers were satisfied though.

Let’s say modification gains you 1 MPG, how long to pay for the modification?


Especially if you’re towing/hauling stuff with it. 11’s pretty good for that vehicle, actually.


Short trips, off road and/or towing or hauling most of the time? Your fuel economy is pretty darn good for those driving conditions.

I bought a rebuild kit for the Rochester 4gc in my 62 Caddy last year that had an ethanol safe needle and seat and accelerator pump.

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My introduction to ethanol fuel problems was a 11/2 ton commercial flat bed that would not accelerate well. The fuel line at the tank had deteriorated to the point that it wouldn’t flow enough fuel to rev the engine. Any effort to make an older vehicle reliable would require replacing all the rubber fuel line from the carburetor to the tank.

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You’re right. I was fortunate that the Caddy only has one short piece of rubber between the steel line from the tank and the fuel pump. The pump is another story. I’m having difficulty sourcing a pump with an ethanol safe diaphragm.