A lot of the mail that crosses my desk asks that age-old question: "How do I get better fuel mileage?" On some vehicles, no matter what you do to them, that there is very little increase, or so it seems. A 10-percent increase in fuel mileage is a 10-percent reduction in fuel cost-and the ability to go 10 percent farther on a tank of fuel.
The first question to ask is why do you want better fuel mileage? Is it for the cost savings, extended range, or for the overall ecological benefit? The latter is something that everyone should be concerned with.
The extended mileage per total capacity is nice, but can always be increased by a larger fuel tank. However, on a lot of 4x4 vehicles there isn't a practical application for either an extra or larger fuel tank. Besides, extra fuel means extra weight, the enemy of both performance and fuel mileage. We will get back to this in a minute.
It then comes down to cost savings. Just how many miles will you have to drive before the "increased mpg" improvements are paid for? Let's take a hypothetical example. A Wrangler YJ with the 4.0L I-6 generally gets 15 mpg in town and 16 to 18 on the highway. We installed a 14-inch electric fan (eliminating the engine-driven unit), updated the spark-plug wiring to some quality aftermarket stuff, as well as a new coil. We also changed all the lubes, from engine to driveline, to synthetic lubricants. Results were an increase in fuel mileage to 17 mpg city and 19 mpg highway. Then we tried slowing down. We dropped average freeway speed from 70 to 75 to the 60-mph range, which now takes us 14 more minutes to get to work. Well, we picked up another 2 mpg. Secondary benefits were that the fan helped warm the Jeep faster in winter and kept it at normal temperature in traffic on hot days.
Our next step should maybe be a freer-flowing air filter, like that offered by K&N, and some exhaust system work from the "cat" (catalytic converter) back. There are several other ways that we can increase our fuel mileage, but only two I can think of that aren't detrimental to off-road performance. One is to eliminate any and all possible unneeded gear. Unload the tire chains, the Hi-Lift jack, or the "extra parts" box if they're not needed. Do you need two full gas cans on the back? What about the fullsize spare tire? Will the stock tire and wheel get you home? You've got a locker in the back and need to match tire height. Spend some extra time and swap a front tire to the rear if necessary (I've done it more than once).
Not only should we be concerned with weight but with aerodynamics as well. Cab-mounted driving lights, for instance, offer a tremendous amount of resistance. You can't use them on the highway anyway, so put them on some type of quick-disconnect system. The same goes for a winch-it's non-aerodynamic and heavy. One time before I went over to the Oregon Dunes, I weighed everything I took out of my Jeep, as weight is a real enemy in the sand. The total came to 494 pounds!
The second general approach to increasing fuel economy is to eliminate rolling resistance. Part of that is in aerodynamics or air friction, which we just covered, but mechanical friction is a key player. Proper tire pressure is definitely a factor. Lower your air pressure to 10 psi and try pushing your vehicle by hand. Compare it to the effort needed when all tires have, say, 35 psi. You'll find the vehicle much easier to move when the tires are aired up. Enough said. No, not really. While it's a real pain to have to swap tires, do consider having a set of narrow street tires as well as a set of real off-road rubber. If you're like most of us, we put on a lot more street miles than dirt miles, and besides, those narrow street tires last longer and have a lot less rolling resistance.
I touched on it a bit earlier, but think about lubricants. Ever try to pour 140-weight gear oil on a 30-degree day, or even 75/90W? Keep in mind, the engine has to move gears through this stuff. It's important to match oils and other lubricants to their intended use and operating temperatures. Synthetic lubricants, in my opinion, are fantastic. Not only do they flow better in cold weather but offer a lot better stability in hot weather. Of course, the big drawback is their high cost, somewhere between two and four times more than petroleum-based lubricants. Increased mileage between changes is a definite bonus plus much better lubrication at the extreme ends of the temperature scale.
One thing to really think about before you make any modifications that involve a lot of money is the "payback time." Let's take, for instance, my own flatfender Jeep that is powered by a 383 Chevy small-block and uses an Edelbrock Quadrajet for a carburetor. The carb works great at all angles with never an issue, other than I would like some better fuel mileage. It gets between 10 and 13 mpg. Nothing really great. (And yes, I use synthetic lubes.) I have considered one of the aftermarket fuel-injection systems, in particular the Edelbrock unit which I think generally sells in the $2,400 range. Let's say that it will improve my fuel mileage 20 percent. That puts the new figures in the 12- and perhaps 16-mpg range. Seems like a reasonable increase. Let's average the before and after numbers at 11.5 and 14.
I have this great formula that I can use that somehow magically gives me the answers when I plug in the right numbers thanks to my son-in-law's adeptness with numbers and such. Assuming gas stays in the $4 a gallon range, I am going to have to drive the Jeep 29,571 miles before I start saving money. Because it has now taken a back seat to Project Ain't it Grand-er, it's only a play toy that gets driven about 3,000 miles or less each year. This means the payback time will be about 10 years! I may have sold it or destroyed it in that time period. So is it worth it? Maybe, especially if I wasn't happy with the way the carburetor performed, but that is not a problem. If I was just looking at ways to help cut expenses, it may not be a practical solution.
