Built 360 Gets Lousy Mileage
I would like to ask a professional about the mileage I am getting. What I have is a '90 Jeep Wrangler. I decided to buy a rusted-out '89 Grand Wagoneer as a donor vehicle. I rebuilt the 360 motor, the TF727 transmission, and the Dana 44 axles and put them all into my Wrangler. I knew the 360 was not known for its fuel economy, but I did think that 12-15 mpg was not asking too much. Currently, the still-new rebuild (600 miles on the ticker) is getting a solid 8 mpg (average). The guys at my local speed shop say that this is an acceptable mpg, but I want a second opinion.
I'll start with the details on the engine and work on down to the rubber. The 360 was relatively low mileage, but I had it bored 0.030 over. The same machine shop replaced the valves and installed new pistons on my old rods. When I asked them about the new compression ratio, they said that it was beyond them to say for fact, but that everything they did was to keep the compression relatively the same as stock. I installed a Comp Cam Xtreme Energy cam; this has intake duration of 256 degrees and 0.477-inch lift; exhaust is 268 degrees and 0.484-inch lift. A true double-roller timing set helps with the torque load. Outside, I used Edelbrock shorty headers that run two 2-inch pipes into one Flowmaster muffler with a 3-inch output. I installed an Edelbrock Air Gap manifold and topped that with a Holley 2-d Pro-Jection TBI. Spark is managed by a D.U.I. distributor, and I had a trusted mechanic set the timing. An electric fan also relieves some of the engine's responsibilities. You can see that the engine is rebuilt more than it is built up.
The TF727 was rebuilt to stock specification, and I added a B&M auxiliary cooler. I know that a three-speed is not the answer for fuel economy, but I just don't see many miles faster than 45 mph, and the budget didn't allow for a different tranny.
For the axles, I went with 4.88:1 gears and ARB lockers. The Grand Wagoneer had been a full-time four-wheel drive, but I kept my Wrangler's NP231 transfer case in the mix, so I installed Warn manual hubs in the front.
When I bought the Wrangler, it had a 3-inch body lift, and I installed a 4-inch leaf-spring lift. During the current build, I left the suspension as is, that includes keeping the spring-under design. If not for the tires, I would have actually lost some ground clearance due to the larger tubes of the Dana 44. For tires, I went with Interco IROK 36x13.5 radials. I have them inflated to 45 psi for pavement purposes.
As far as calculating my mileage is concerned, I always use the same gas station and usually the same pump. I can't swear that my odometer is accurate, but what I can say is that every time I pass one of those radar zones that post your speed, the sign is always exactly the same as my speedometer. Apparently, my tire and axle gear combo are right on target.
I know that the first response is that big tires, too few gears, and way too much open space under the vehicle are killing my mileage. And who could resist stomping on the gas pedal when you have a big V-8 in your stripped-down Jeep, but I have been trying to keep it civil since it hasn't been broken in fully.
I can say that the vehicle runs very well. It is harder to start than I would like, and it is grumpy until it comes up to operating temperatures, but after that, it idles smoothly, accelerates cleanly, and rumbles down the road like a champ. I have not found any fuel leaks, but it does smell of raw fuel, especially at start-up. I have been playing with the choke controls on the Holley, thinking that I was losing too much fuel during warm-up, but it seems to run best right where it is. While checking the specs of my cam to prepare this letter, I noticed a disclaimer saying that this cam needs 0.150-inch-longer pushrods. Could this be part of my problem?
Am I getting realistic mileage for my vehicle, or is there something that I can work at to improve that mileage?