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Question: I own an '83 Chevy pickup with 160,000 miles on a 305. It seems to still run well, but it gets terrible fuel mileage. The truck is lifted, with a winch bumper and 33-inch tires with a 3.73:1 gear ratio. Back when fuel was $1.50 a gallon or less, I could live with that, but now that the price has doubled and may stay that way, I am a lot more concerned. I am really thinking of dumping my old tired Quadrajet carburetor and replacing it with something more modern like Edelbrock's Performer carb and intake manifold-or maybe forget the manifold and put on an adapter. Right now I get 11 mpg and hope with the change that I can up it to the 15-17 mpg range. I have about 500 bucks to spend.
Answer: Let's start with the truck first. The combination of lift, winch bumper, and larger tires are part of what's hurting your fuel mileage right now. First, there is the matter of wind resistance and/or what I like to call "dirty air." That winch bumper and the exposed axle and transfer case cause a lot of air disturbance that prevents a smooth airflow under and around the truck. Have you noticed how all the late-model trucks now have wraparound bumpers and low-hanging air spoilers up front? You got it-they want the air to flow smoothly around the truck, not under it. However, even with these changes and electronic fuel-management systems, I get complaints about new 4x4 trucks only getting 14 mpg, or sometimes less. Let's face it-trucks are big ugly boxes that move a lot of air at highway speeds.
The tires add more rolling resistance along with a higher gear ratio, which just may be putting your engine out of its proper rpm range.
As to your carburetor and manifold changes: if you really want to do it right, I would suggest that you hit the wrecking yard and go with a GM throttle-body fuel injection, manifold, and matching distributor, and one of the aftermarket wiring harnesses. A Painless harness is about $300; the wrecking-yard pieces are most likely under $200.
You have a couple of choices in Edelbrock carburetors. While the Performer series works well on the highway and can be had for as little as about $225, I have found through testing that the Thunder series AVS version may be better off-pavement-its price is in the $350 range. A Performer manifold and the gaskets will run you about $185, so you're right into your budget. I am not a real fan of carburetor-to-manifold adapters.
Another carburetor I would consider would be a Holley Off Road Avenger. I have used it with great success, but it's not the best for fuel mileage, and again you would need a manifold change.
Overall, instead of a carb/manifold change, I would consider either having your present carburetor professionally rebuilt, or buying a quality aftermarket rebuild. Jeg's and Summit, for instance, have some remanufactured Quadrajets in the $300 range-right within your budget, with money left over for a nice open-element air filter. Quadrajets work great off-pavement and get probably the best fuel mileage of any carb, due to their small primary venturis. It is something that I would recommend.
The bottom line is I don't think that you will ever reach that 15 to 17 mpg range, but hopefully I am wrong. I ran some figures through my spread sheet and came up with gas at $3 a gallon and a $500 expenditure. To jump from 11 to 13 mpg will take about 12,000 miles before you reach the break-even point, money-wise.