Four-Cylinder vs. Six-Cylinder Wrangler
2.5L vs. 4.0L differences when buying an older Jeep Wrangler.
Jeep prices have gotten pretty crazy lately. Not everybody has tons of money to throw at a newer JK or JL Jeep Wrangler. But if you still want to go Jeeping, there are plenty of older YJ and TJ Jeep Wranglers out there for somewhat more realistic money. The most common engine choices when shopping for an older Jeep Wrangler will be the 2.5L four-cylinder and 4.0L six-cylinder. Yes, the early YJ six-cylinder engines were a carbureted 4.2L, which was supplanted by the 4.0L for the '91 model year, and the later model TJ went to the 2.4L Power Tech four-cylinder in 2003, but for most Wranglers you'll be shopping for, the choice will come down to 2.5L and 4.0L. Here's some relevant information that'll help you decide which you can and can't live with.
Jeep Wrangler 2.5L vs. 4.0L Horsepower and Torque
The '87-'90 Jeep 2.5L had a TBI fuel injection system and made 117 hp and 135 lb-ft of torque. In 1991 the injection was upgraded from throttle body injection to multiport injection, and power increased to 120 hp and 140 lb-ft. The 4.0L six-cylinder replaced the 4.2L for the '91 Wrangler and came to the party already sporting MPI injection and making 181 hp and 222 lb-ft. In 2000 the power increased an additional 9 hp for a total of 190 horsepower until TJ Wrangler production ended with the 2006 model year.
Jeep Wrangler 2.5L vs 4.0L Transmission Differences
Both 2.5L and 4.0L Wranglers were offered with either an automatic or manual transmission. For the automatics, a three-speed TF999 without overdrive was the only option for the YJ. When the TJ came along in 1997, the Chrysler 32RH became the three-speed auto, which—in a slightly inaccurate, dumbed-down description—could be thought of as an electronically controlled TF999 with a locking torque converter. In 2003 the Chrysler 42RLE four-speed auto with overdrive became the automatic transmission, finally offering automatic Wranglers an overdrive gear. Automatics take power to spin their internals, so the additional parasitic loss coupled with the fewer gears to help keep the engine in the powerband make the six-cylinder the preferred engine if you're shopping for an auto-equipped Wrangler. The four-cylinder Jeeps feel sluggish with an auto, especially once you add larger tires.
For manual transmissions, aside for the short-lived AX4 four-speed offered in the '87-'90 2.5L Wranglers, your five-speed manual options are generally going to be the AX5 in the 2.5L Jeeps (although the '03-up 2.4L engines received the NV1500) and the AX15 or NV3550 in the six-cylinder Jeeps. All of these transmissions have gear ratios well matched to their respective engines. The AX5 has a somewhat accurate reputations as not being that durable, especially with larger wheels and tires, but overall they offer good service. The AX15 used in the '91-'00 4.0L Jeeps is one of our favorite five-speed transmissions, with sturdy internals and excellent manners. The NV3500 used in the '01-'04 Wranglers has a nice 4.04:1 First gear ratio, but in our experience it's a bit noisier and not as sturdy as the AX15. In 2005 the five-speed NV3500 was replaced with the six-speed NSG370 manual. At the time it was considered a big upgrade, but we've found this transmission to be a weak link, needing to be rebuilt with much greater frequency than its five-speed predecessors.
Jeep Wrangler 2.5L vs. 4.0L Gearing Differences
Factory Jeep Wrangler axle gearing between 1987 and 2006 ran the spectrum from eco-friendly 3.07s up to a reported 4.56 gearset available in the '03-'06 2.4L Wranglers. In factory trim, almost all are adequate, but we're talking about purchasing Jeep projects, so to cover the broader strokes of what to look for, consider that almost any 2.5L Wrangler will benefit from installing the deepest set of gears that you can, especially if you're considering a tire size of 33 inches or larger. We run 4.88s in our '89 2.5L Wrangler with 31s and 32s and would go to 5.38s if they were available for our axles. On the 4.0L Jeeps, the added torque and power from those extra two cylinders goes a long way in terms of delivering acceptable on-road performance with larger tires. This is especially true if your 4.0L Wrangler has 3.73s or Rubicon 4.10s. We'd run up to a 35-inch tire with either of these gearsets. Considering a full axle regear with parts and labor can run up to or over $2,000, purchasing a more expensive 4.0L Wrangler with the factory 3.73 or a Rubicon with 4.10 gears instead of a cheaper 2.5L vehicle might make better financial sense in the long run.
Jeep Wrangler 2.5L vs. 4.0L MPG Mileage Differences
Wranglers are as aerodynamic as a brick, so adding things like a lift and larger tires is going to greatly impact your freeway mpg numbers. In stock trim we've gotten about 19 mpg out of a dead-stock '95 YJ with the MPI 2.5L, AX5 manual, factory 4.10 axle gears, and small 225/75R15 tires. Similarly, we knocked down roughly 17-18 mpg with a dead-stock '97 TJ with the factory 4.0L, 32RH three-speed auto, 3.07 axle gears, and 30x9.50R15 tires. But add a lift and larger tires, and despite regearing the axles as optimally as possible, we generally get mpg numbers in the 12-14 mpg range on the highway no matter what. So unless you're leaving your YJ or TJ dead stock, you're going to burn almost the same amount of fuel to move your Wrangler from point A to point B whether you've got the 2.5L or 4.0L engine. In terms strictly of mpg, there's almost no difference.
Jeep Wrangler 2.5L vs. 4.0L Drivability Differences
OK, there's really no comparison in terms of drivability, with the clear advantage going to the 4.0L in almost all areas. Whether it's in-town drivability between stoplights, ability to pull hills and grades, and low-end grunt off-road, there's no replacement for displacement. A four-cylinder Jeep can be a fun vehicle in any terrain and forces you to slow down and smell the flowers, but if you just want to be able to merge into traffic or—dare we say it—effect a pass on the road, the 4.0L is the clear winner.