[mannydantyla]

Here's all the tuning he did:
My goal is to get 25+ mpg from my 99 XJ with 30 x 9.5 R15 ATs - Page 221 - JeepForum.com

I used to get 15 to 16 mpg highway before I started trying to improve. Now I'm getting 24 mpg at 60 mph highway on a good day. On a bad day 21 mpg. Most days 22-23 mpg. I consider that darn good for an XJ with 1.25" lift, 30 x 9.5 R15 Cooper AT3 tires, and 200 lbs of skidplates. My XJ is super peppy because several of the mods improved torque and-or HP, as well as gas mileage.

Though it should be noted that I'm using 89 octane gas (see explanation of why I'm using 89 octane gas in the Kolak ignition section later in this post).
I've been driving it with these mods for 1.5 years. It's time to post an update to this thread.

I've learned which mods are helpful and offer the most bang for the buck, and which are not worth it.

I was thinking about this today when I was helping my cousin with his new-to-him old 98 XJ that needs fixing up, and his kids 89 Comanche that needs fixing up. They want to know which of my mods I recommend for them, and which they can or should skip. That's a valid question since several of my mods helped my gas mileage, torque, and/or HP, but some mods made no difference, and one or two mods reduced my gas mileage (and I had to undo those mods).

Here are the mods that gave me the most bang per buck by improving gas mileage, and some of these mods also improved torque, HP, easier startup, and smoother idle.

1) 205F Stant Thermostat (part # 13352) is great for Northern climates year round. Cost $5. It helps efficiency because gas burns more efficiently when engine is fully warmed-up to full op temp. Very nice side benefits are a warmer heater and defroster for more comfort and faster defrosting in Winter.

If you've ever driven in a really cold damp blizzard, you may have experienced that your defroster can't keep windshield thawed out, defrosted, and defogged. The solution is a 205F thermostat. Your defroster (and heater) will easily keep your windows thawed, defrosted, and defogged in any weather.

This is a great improvement (warmer engine) in colder climates, especially in Winter, but also in Spring and Fall. In Summer it won't hurt anything because it still runs the same temp as with the 195F thermostat that I used to have. With either stat (195F or 205F thermostat) my XJ water temp is 210F to 212F in Summer (which is perfect for efficiency) and 208F to 210F in Winter (near perfect). A fully warmed up engine runs cleaner and more efficient (as long as it's not warm enough to engage the mechanical fan clutch or electric fan). My fans engage-activate around 214F or 215F (so running at 208F to 212F does not engage-activate my fans).

For hot Southern climates and year-round use, I don't know whether a 195F or 205F thermostat is best. I'll leave that to Southerners to decide what they think is best. If you choose a 195F thermostat, then I suggest a Stant Superstat because it does a great job of maintaining a steady engine temp, especially during warm-up, and Superstats are capable of very high flow rates on hot Summer days.

2) Adjusted auto transmission cable for better shifting at appropriate rpm. My engine was lugging and not shifting when it should. After I adjusted the transmission cable, the shifting and engine efficiency improved noticeably. I adjusted it myself the first time, with some success, but still not as good as I wanted it to be. I wanted it to shift at a slightly higher rpm so my engine could have more torque for my 30" tires on hilly highways. So then I had my local transmission shop adjust it and they got it shifting really nice at the perfect rpm and they didn't even charge me for it.

3) I changed my rear diff oil from 75w140 synthetic (tow package rear diff oil) to 75w90 synthetic. This reduced my rolling resistance noticeably (especially on cold days) and helped my gas mileage enough to be a worthwhile mod, especially in cold weather. Cost $20 for two bottles of 75w90 synthetic gear oil. I'm using Valvoline Synpower 75w90 and like the results. According to its spec sheet it has a better viscosity index than other brands of 75w90 (thinner when cold, but just as thick when hot - compared to Lucas, Mobil One, and other brands I compared). Newtons recommended this to me earlier in this thread and I've found it helps.

4) I removed my stock roof rack. That probably helped highway gas mileage a little, but I don't know how much since I didn't do a before and after test for this mod.

5) I removed my winch and winchplate to reduce weight and wind drag. This helped city and highway gas mileage and saved my nerves. My winch plate used to whistle at highway speeds and drove me insane. Glad to be rid of it. A tow hook is good enough.

6) I removed my AC compressor and replaced it with an idler pulley. Cost $20 for idler pulley of Dorman brand from Oreilly's. My AC didn't work anyway, and I live in a cool enough climate to not need AC except once every 2 years or so. The AC system is very inefficient in a cold wet climate like mine. The AC compressor keep cycling on and off when I use the defroster to defog my windshield, and each time the AC compressor cycles it causes the electric radiator fan to come on too. Removing the AC compresor saved a lot of weight under the hood and it means my compressor and electric fan no longer cycle on and off when using the defroster. This increases my Winter-time efficiency. My windows still defog and defrost as fast as ever. Faster actually (thanks to my 205F thermostat).

7) Motorvac cleaning of topend of engine. Motorvac is a trademark name for a professional cleaning process done by some auto repair shops. It's a similar idea as Seafoam, but I think it's more effective than Seafoam. Motorvac cleaning removes carbon deposits from the topend of an engine, and is a great place to start for a tuneup, IMO. I suggest changing motor oil afterwards (just in case Motorvac chemical contaminated the motor oil).

8) Modern 4 hole fuel injectors. Which injectors you should use depends on your model year of XJ (or TJ). See post 209. Cost $190 purchased from Motorman on Ebay, but that price might be out of date by now.

8) Kolak Ignition Kit (it's both a tuneup kit and an upgrade kit). See OP for more details on this. Includes new cap and rotor with brass contacts, new Autolite Single Platinum plugs, Magnacore upgrade racing plug wires, and an upgrade MSD coil. Gap plugs to 0.045" (for my 99 XJ model year anyway) for a longer, hotter spark for better ignition. Cost $230 last time I checked, but that price might be out of date by now.

Note: After installing Kolak ignition, my XJ now requires 89 octane gas to run well. It purrs beautifully on 89, but on 87 it runs so rough it shakes the whole car like an alcoholic who ran out of booze. So be aware that if you install a Kolak ignition you'll have to use 89 octane gas, which partially negates the financial benefit of gas mileage improvement from the Kolak ignition. So money-wise the Kolak ignition is a break-even deal, but is well worth it for the increase in low-end torque and easier cold starts.
10) Jeepers and Creepers 00 battery cables. These are very high quality, 00 thick, and give the engine improved grounding via the huge 00 ground wire, and a 00 thick positive wire as well. Cold starts are vastly improved, and I suspect these battery cables help me get the most from my Kolak ignition. These battery cables help my engine start easier, idle smoother, and might even improve my performance-torque-power (along with Kolak ignition). It may even help my gas mileage, but I don't know since I didn't test gas mileage right after installing these battery cables.

If you use flat washers as part of installing your battery cables and ground wire (to engine), use brass flat washers because this helps conductivity, IMO.

I don't know how much of my ignition improvement is from Jeepers-Creepers battery cables and how much from Kolak ignition. The combination of the two is great. I suggest doing both since IMO they work together as a team for easy starting in any weather, smooth idle, increased lowend torque, and a slightly more mid-rpm torque. Gas mileage improved slightly because of these electrical mods, but fuel cost savings of Kolak is partially offset-negated by having to use 89 octane gas.
11) Dynamax Cat Back and Dent-Free Downpipe help enough to be worthwhile, but I'd do these last since the prior mods help gas mileage and performance more per dollar spent, IMO. The Cat Back and Dent-Free downpipe work well together as a team. You can't fully benefit from either of these mods until you've done both these mods.

The best prices I've seen on Dynomax 2.25" Cat Back is at Summit Racing. I paid $150 with free shipping (but that price is probably out of date by now). Make sure you get the 2.25" diameter CB, not the 2.5" diameter CB. I tried both.

The 2.5" CB decreased my gas mileage and reduced lowend torque compared to stock exhaust. The 2.5" CB is recommended for strokers, but is terrible for a stock(ish) 4L engine, IME.

The 2.25" CB" improved gas mileage, retained torque, and improved my horse power compared to stock. The 2.25 CB is excellent with a 4L engine.
SummitRacing.com has the best prices on Dynomax cat backs and other Dynomax products.

I like the Dynomax muffler (included with CB) because it sounds slightly mean at idle, but quiet when cruising.

You can't buy a dent-free downpipe online. You have to make you own or have one made. My local muffler shop made me a dent-free downpipe and installed it at the same time they installed my Dynomax CB. Cost of my downpipe was $90 to make and install it.

12) Edited in Later: I should have listed this as number 1. I got my Jeep aligned and monitored my tires pressures carefully. Don't under estimate how much good alignment and tire pressure help gas mileage, and also help tires last longer and improve handling. Alignment and tire pressure are the first things we should do for our Jeeps to be more efficient.

In addition to the above mods, I did several other mods that didn't make much difference to gas mileage or performance. So I'm not even going to mention them here.

It's worth mentioning that during the 2 years of modifying and testing gas mileage, I added 225 lbs of skidplates under my XJ to protect it off road. This extra weight probably reduced my city gas mileage slightly, but I didn't test city gas mileage. I tested highway gas mileage. The skidplates did NOT change my highway gas mileage. The skidplates add weight, but make the underside of Jeep smoother (less wind drag). So IME skidplates don't affect highway gas mileage.

Note that he says at the end that the underbody skid plates did NOT help is fuel efficiency and may have hurt FE in the city due to their weight. Hmm....