Really good gas mileage

kidarkitect said:
My 89 231 disco and 145k on the clock gets(got) 26mpg regulary, I haven't been on any highway trips lately. 19-22 on daily driving. This is with 30's and 3.07's in a manual calculating for difference. I recently changed to 3.55's and added a heavy bumper so the math will make my brain hurt. Its still doing well so I'm not worried,

When I bought it, it got 14. Replaced O2 sensor- up to 20. Oiled Air filter-21. Downpipe back exhaust (2.25 dp 2.5 cat and back) 26. Also throughout all this I cleaned the ccv line out and played with stuff I shouldn't have :) Still have yet to replace my spark plugs and wires although thats coming. I wonder if my disco has helped with it at all, I don't know. Front skid as air foil-maybe I don't know.

Maybe get a front skid if you don't have one.
Take your roof rack off if you do have one.
Drafting is good.

Anyone know if it would be bad to coast in a manual on the highway? Downhills and what not.
Click to expand...

No problemo coasting, just don't shut off the engine like some A**hats do.
 
25mpg is uncommon, but not unheard of. Congrats :cheers: we're all jealous

My hunch is there is a tolerance range of correct readings on the O2 sensors installed from the factory, and some O2 sensors at the far end of the range(while still technically still"in spec") will allow the 4.0 to run lean and get really great economy in cruise/highway driving mode, while still not hurting the motor. This might be in combination with some other "almost out of spec" sensor or component. Somebody posted up here with one of the "high mileage", XJs a while back, bitching about the drop in mileage after changing his O2 sensor.

I get around 19-20 highway, longest tank was just over 420 miles, but I can't verify how "full" the tank was after each fill-up, so one or both pumps might have over/under-filled the tank. (wasn't the same pump, they were about 420 miles apart :D )
 
OP - I would look at getting some new injectors - the Ford style, 4 hole ones that lots of folks on here seem to like. That is about the only "upgrade" you might be able to do in order to get just a hair better mileage, other than what others have posted here...
 
Stupid internets...
 
kidarkitect said:
My 89 231 disco and 145k on the clock gets(got) 26mpg regulary, I haven't been on any highway trips lately. 19-22 on daily driving. This is with 30's and 3.07's in a manual calculating for difference. I recently changed to 3.55's and added a heavy bumper so the math will make my brain hurt. Its still doing well so I'm not worried,

When I bought it, it got 14. Replaced O2 sensor- up to 20. Oiled Air filter-21. Downpipe back exhaust (2.25 dp 2.5 cat and back) 26. Also throughout all this I cleaned the ccv line out and played with stuff I shouldn't have :) Still have yet to replace my spark plugs and wires although thats coming. I wonder if my disco has helped with it at all, I don't know. Front skid as air foil-maybe I don't know.

Maybe get a front skid if you don't have one.
Take your roof rack off if you do have one.
Drafting is good.

Anyone know if it would be bad to coast in a manual on the highway? Downhills and what not.
Click to expand...

What O2 sensor did you buy?
 
UltimateG said:
Congratulations on getting great mileage.



But.

DO NOT "run it dry" Doing so will damage your fuel pump, as it's lubricated by the gas.

I have a tough time believing the accuracy of your numbers... I suggest you need a slightly more scientific system for measuring fuel consumption. Are you sure your Odometer is accurate?
Also, the xj fuel gauge is wildly inaccurate. If you want to get a better idea of fuel consumption, start with a full tank. Record your mileage, run a few carefully measured tanks through, then fill it up again and record your mileage.
Click to expand...

The fuel pump is COOLED by liquid fuel, not lubed.
 
I'll chime in - when I bought my '96 5-speed (stock, with stock 225 tires), it had ~50K. On a road trip, if I kept it at the speed limit (i.e. no faster than 75) I'd routinely see 25+ MPG. I also never saw below 20 or 21 in all city - yes, I drive conservatively, but not as bad as a Prius owner on an MPG competition.
Now, with ~183K and 33s/4.56/5in lift, I see a city/hwy avg of about 18. My own completely unfounded theory is that some of these 4.0s were lucky enough to have been built using all nearly perfect parts, assembled on a day that the guys on the line were particularly sober, and paired with a manual transmission that was built by the most pride-filled, Demings-inspired Japanese fellas. End result is a fair number of XJs running around with few to no driveline failures, coupled with surprisingly high fuel efficiency.
 
You guys taking into account the fact that your odo is going to be off if you haven't regeared your diffs and/or your VSS? My mileage is down around 13 without including that, but it's actually more like 15 once I take into account the fact that a "mile" on my odo is 1.15 miles or so right now.

(I get around 250 "miles" to the tank now, generally 18.5 to 19 gallons)
 
I have taken my tires into account thats the sad part, with 30s i was getting around 16 before.
Im always throwing codes though for multiple cylinder misfire...runs just fine though so i dont worry about it, other than really bad mileage
 
I'd suggest checking your fuel injection and ignition systems and fixing that - it'll pay for itself via gas mileage and not doing so can damage your catalytic converter, resulting in more thrown codes and making it hard to pass inspection without buying a new one. These guys make catalytic converters and thus would make more money if you don't do this and have to buy a new one, so I think the fact that they're recommending that you fix it speaks for itself: http://www.millermuffler.com/catalyticconverter.html

Overheating
There are many problems that can cause a cat to overheat or fail. The most common cause is unburned fuel entering the cat. Also faulty spark plugs and leads will cause the engine to misfire and ruin the converter. The cat will also be ruined if the distributor timing is out.
Click to expand...

Spark plugs, cap and rotor, ignition wires are a good place to start.
 
If a PO installed a 180 degree stat instead of a 195, would that cause a significant drop in mileage? Mine only gets about 16mpg w/31's and a 2" lift, auto, 3.55 gears, 95 Country. It runs real strong and smooth, just lousy mileage.
 
There are a lot of variables that come in to play, weight of the truck, gearing vs tire size etc. An engine is a air pump and as a sweet spot where it is most efficient, in our case its 2450rpm where it runs the most efficient. I run 31" with 4.56 gear which puts me at 70 mph at about 2400 rpm and on the highway returns an average of 26 mpg in mixed driving about 20 mpg. Now some may say the rpm is a bit high but others more knowledgeble then I are of the opion that this engine likes to trun fast, and I would have to agree. Try it and see do your best to keep it at 2450 and see what happens, I have no regrets going with 4.56 gear in my DD with an auto box.
 
My son claims about 24-25 on his 96 5-speed. I've gotten close to that on my 99, but generally run in the low 20's in mixed, mostly rural non-freeway driving. My 87 auto got a fairly long term average of 23. My 95 5-speed has always been hard put to top 20. All good runners, all the same foot on the pedals, and varieties of tires. I've never figured out the discrepancy.
 
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