I am getting 21mpg hiway and 18.6 city in my 89 YJ with a 4.2L I am running a Nuttered Carter BBD carb, with a fresh set of Champion Copper plugs, new plug wires, Dizzy cap and rotor. I also have replaced every fluid in my YJ with synthetic. I am running a 235/75/15 tire so its not much smaller then a 31
I suggest starting with a tune up and doing the nutter bypass
I have not heard of intakes helping mpg or power.. Give it a tune up, make sure your o2 sensor is working change your air filter, proper tire pressures, proper gearing, . All come into play. I have a 90 with the 4.2 and I get about 10.. On the uncorrected speedo Sent from Peggy's office
Better air to the engine, the better it runs. Which means less work for the engine. I've read a few threads with guys getting better gas mileage with snorkels!
'87 w/ 4.2, AX-15 w/ 3.07 gears, 2" lift, Nuttered, Team Rush mods (including dizzy), MC2100 & 31's (with uncorrected speedo)....over 75 miles on 5 gallons (mixed driving, mostly city). So 15ish.....gets around 20 on long drives to the coast.
4.2 with 180k miles, Howell EFI, TF999, 4.88 gears, and 35s. I usually average around 12mpgs. I admit my YJ is on the heavy side with the 8.8, winch, full engine skid, and rockers. If I let my wife drive it she can squeak out around 14mpgs.
1990 Jeep YJ 4.2
Howell EFI, BDS 3.5" lift, M.O.R.E 5/8" Boomerang shackles, 1" Daystar BL, 1" JKS MML, TFI, Crager soft 8s w 35" Dunlop Mud Rovers, Ford 8.8, 4.88s, Dana 30 E-Locker, Terra Flex SYE, Tom Woods CV shaft, Warn M8000.