Mileage 101 for a Jeep XJ

The 5KXJ (Sept. '03) has lived up to every expectation so far. It's gotten us back and forth from Moab, been our parts hauler, the trail warrior, the ladies' magnet, the Friday night cruiser, and done the daily 130-plus mile trek to work and back every day. It's been bashed onto rocks without any body protection, been aired out in the dunes, had its unibody twisted up on technical trails, and rally-raced through corners we had no business being in. And up until a few days ago when we blew a radiator, we'd never had a problem unless you count the time when we had to cut the exhaust off.

The only drawback to the little XJ is the mileage. With 33-inch tires, some extra weight, and 3.55 gears, the 4.0L engine really had to push at lower rpm. We admit having an extremely heavy foot and were averaging 14 mpg on our 405 Freeway commute. This was just not acceptable in a 3,700-pound rig, especially in an era of better fuel economics and expensive gasoline. At the time of this article's inception, gas in Los Angeles was between $2.29 and $2.49 per gallon.

Something had to be done, and though lower gears will surely help the engine get back in the proper torque curves in the future, we hoped to benefit more from making some efficiency upgrades to help out our mileage.So how do we increase fuel economy without driving more slowly, losing any weight, or changing the final drive ratio? We increase the efficiency of the burn.

Increasing the efficiency of the engine's combustion will not only improve fuel economy but will also give us more power. But more power takes more fuel, right? We've all heard this, but then how do you explain increased power from air intakes and such? The key here: efficiency. Many of us forget that the power and efficiency of the combustion come from three parts: gasoline, oxygen, and a spark to ignite it. If you don't have a perfectly amalgamated mixture of fuel and oxygen with enough spark to burn it entirely, then you have room to improve.

By improving the spark, the oxygen flow, or the uniformity of the gasoline and oxygen mixture, you can therefore make a more complete burn. Our plan was to improve all three parameters as we headed first over to Turbo City to bolt on its Stage 3 kit. The kit is made to not only improve mileage but also to increase engine power on our inline-six 4.0L Jeep engine.

The Stage 3 kit includes a bored-out throttle body, a throttle-body spacer, an air intake and filter, a MAP (manifold absolute pressure) adjuster, and a Turbo City bolt-on exhaust. The total install time for all the Turbo City products was only about two hours, and could be done by any backyard mechanic with a couple of sockets and a wrench.

Once we were done with the Turbo City kit, we headed back to the office to grab a Performance Distributors Firepower Ignition kit that we had ordered from 4WD Hardware. The Firepower kit comes with a distributor rotor and cap, a Screamin' Demon coil, and a set of Livewires spark-plug wires. The idea here was to create a hotter, more powerful spark that would create a better, more complete combustion, thus giving us better fuel economy and more power.

Upon completion of the economical buildup, we performed a number of test runs to see if in fact our theory was correct. We were more than pleased with our results. In speeds below 70 mph, we saw an average gain of over 4 mpg. And above 70 mph, we saw a gain of 3 mpg. To see the results, check the "Driven by Numbers" sidebar.

Driven by Numbers
We recorded fuel economy results of the vehicle in stock form, then with the Turbo City performance parts installed, and finally with the Performance Distributors kit and the Turbo City parts. Some of these mileage numbers may seem a bit skewed. For example, in stock form we drove 65 miles at the same average speed and got different mpg averages. And at one point we got more mpg in stock form than in modified form at similar speeds. All these measurements were taken during normal driving conditions. This means that sometimes the vehicle was empty, and sometimes it was packed to the gills. Sometimes we were on the flats in massive traffic, and other times we were ascending 2,500 feet in only a few miles. This is everyday driving, and we took our readings as such.

It seems that our mileage improvement was very apparent at lower speed, but was sort of washed away in full-throttle jaunts. The best mileage improvements were recorded on 60- to 70-mph runs. At these limited speeds, we were seeing as much as a 5-mpg improvement. At higher Southern California freeway speeds we were only seeing around a 2- to 3-mpg improvement. Nevertheless, we did see an improvement across the board and also picked up some extra oomph behind the gas pedal. Our conservative estimate is that we improved mileage by 10-15 hp (most likely more).