The best thing to boost gas mileage is to drive in a conservative manner. There isn't anything you can do to the engine that will make even half as much difference to the gas mileage as what you do with your right foot.
the thing about it though is to get somebody that's real experienced with it.you have to clearance the block for the longer stroke and it's real tight tolerances.But no doubt,stroking a motor is the way to go,they have stroker kits for 5.0 mustangs that punches them out to 347 cubes,that's a huge increase.I'm building a 355 for my Chevelle right now and if I would've had the dough I would've done a 383 stroker.
leave the gun,take the canoli's
As far as gas mileage, driving habits will help the most, but keeping your Jeep well maintained will help like new filters plugs and fluids and what not. You did not specify which 4.0 so if it has a distributor make sure you have a good set of wires too. A CAI will help a little and so will a free flowing exhaust but the MPG differences will be secondary to performance and it will all go back to how you drive her.
You will not have to change the quench height unless you go as far as the 4.9 or 5.0 stroker engine. You can build a 4.8L stroker with Chevy 350 pistons with the stock block. This is the route I have opted for. I have to have pros do all the machine work for me as I don't have the tools to do things like overboring the cylanders and regrounding the crank and such. So far all is going well and I have almost completed gathering all the parts that I will need.
I hear a stroker kit improves fuel economy by allowing you to keep it in a higher gear when cruising and accelerating. A hesco kit also is said to give you 225 whp and up to 280 lb-ft of torque, even more with a cam and performance head setup. There's also a turbocharger setup that's featured in the latest JP magazine that may suit your interests. If you stay in vacuum, you won't have any huge decrease in mileage, and a stroker kit will make that turbo give you all your power right at idle, which is what you need for wheeling. And it shouldn't be too bad at the stoplight against that lightning either.
Yeh so basically the best thing you can do for performance on the 4.0 is build a stroker. If you do don't waste money on a kit, do some research and build your own "kit". You will save yourself some cashfor other go-fast-goodies like nitrous or forced induction.
[B]1999 4.0 WJ Limited [/B] NX Wet EFI Kit;HESCO Head, Cam and Water Pump;Apexi SAFC2;Accel 24lb injectors;K&N FIPK; 2.5" Dynomax Ultraflow SS Exhaust; Dynatech Cat; Banks SS header;Custom 3" Head Pipe;E Cut-out;62mm TB & Spacer; MSD DIS-4; Jet 180°stat;H&R springs;Eibach Sway Bars;Brembo Rotors; Streetscene grill;E.L. Dash ; Autometer C2 EGT, N2O & Fuel Pressure;Edlebrock A:F Meter
My 4.0L has a cold air intake, bored tb, spacer, headers, hi flow cat, super 40 muffler, and a 2.5" pipe. With that and no interior at all, removed for my bedliner. It's FAST, it's a 2 door btw. Night before last I raced a Ford F-150 5.4L V8 auto and we raced up to about 100 and I beat him by 3 or 4 car lengths. It's pretty fast, I can spin my tires while I get up to speed all the way to 60! If you do that, which I would highly recomend before any major work, you'll definently notice a nice increase. Other things that aren't very expensive to get more power would be using an intake manifold from a H.O. and getting it ported. That won't be very significant but every little bit helps and if you do it yourself with junkyard parts it'll be very cheap. You can build a stroker out of it in a weekend if you know what you're doing. All you need is a 258 crank and connecting rods. To make it actually work well you'll want a new cam and lifters and a valve cover spacer to prevent and contact. All that for a stroker will be under $500 with junkyard parts and a new cam and lifter setup. Beyond that there's always nitrous, but you'll need intake and exhaust work first. And forced induction which is expensive, unless you get a turbo from a junkyard and do some custom fab work!!!
"Nitrous? So is that like viagra for your Jeep?" -mom