MPG drop

[whatevah]

I got 22mpg from my XJ once when it was stock... 67mph on highways from AL through NC. Following a semi for around 30 miles of that. Around 360 miles from that tank (16 gallons, filled up early, didn't want to be out of gas 700 miles from home).

Now I can get 23mpg doing around 70mph... with 31s and 3.55 gears. However, my backroads daily commute gets me 14mpg. winch, bumper, rack, gear, etc.

Glad you were able to get it taken care of. My MPG actually just went up about 2 mpg after they stopped using the winter fuel mix here. (and it dropped 2mpg when they started using it last fall)

 

[bwh0069]

What is the difference between summer and winter fuel mixtures?

 

[Redsnake]

OK... perhaps I need to reset my ECM. Your situation sounds very familiar to my own.

'96 XJ, ... added new tires went f/ 235/75R15 to 30x9.5R15 ... a little bigger, but not that much bigger. I also did a huge tune-up... had to replace the distributor and while at it, swapped in some new NGK plugs and Mopar Performance plug wires. My XJ runs good, but my mileage seemed to take a HUGE dump... averaged about 18-20mpg on my daily driving... about 40/60 hwy/city... to about 14-15mpg doing the same driving routine. Same speeds, same driving style, etc...

My upstream O2 was changed out about 2.5 years back and has about 60X miles give or take... so it may need swapped out again. I just hate to throw $60 for no change.

Unless my odometer is off due to the slightly larger tires, then I am calculating my mileage correctly. Simply fill up..... drive and then fill up again, take the miles traveled and divide by the gallons of gas... pretty simple as I see it.

So is the only way to clear your ECM to unplug the battery for several minutes? Can I simply plug my code-scanner in and reset it f/there? I hate to have to re-program my Alpine Head Unit again... :|

 

[Ecomike]

The most I have ever pumped into my tank was 18.1 gallons. I read in another thread here recently that the fill pipe is located in such a way that it is nearly impossible to get more than about 18 gallons into the tank, probably on purpose. They designed it to hold 1-2 gallons of vapor if filled on a cold day, thus leaving room for expansion if the tank and gasoline heats up a few days later (say it goes from 20 F to 80 F on drive south). Liquid does expand as it heats, and while the radiator has an oveflow bottle the gas tank does not.

300 miles per tank??? I'm doin good to get 250 and I get about 20mpg. That's cause my "20 gallon tank" only seems to hold 17 gallons max. That's mostly cause I get nervous when my gas gage goes to E lol. I could prolly get 300 if i wanted to.

 

[bwh0069]

xl8 I don't know how you drive but when I drive the speed limit 20mpg is the norm.Yes I figured it right.You are invited to come by and witness anytime just send a pm

 

[bwh0069]

Just switched out both 02 sensors maybe everything will be ok now.

 

[JNickel101]

yes, you do need to recalculate (or fix your speedo gear) when you regear and/or put on tires that are different from stock...

 

[bwh0069]

I drove 325.5 miles on 15.1 gallons of unleaded regular.This is the best I have done so far.By the way GPS said I had traveled 327.41.I don't know which is right but both are over 21.5 mpg.

 

[Redsnake]

BWH0069...

Which O2 Sensors did you use? I hate to spend $60+ on one new O2 and then still have the same mileage I currently do. However... if this change will put my mileage back up by ~ 4-6mpg then it will pay for itself many times over after the first month of driving. Thanks.

 

[bwh0069]

I used bosch from the auto warehouse. 108.00 plus 8.50 shipping

 

[bwh0069]

This was my last step.I installed a set of manual locking hubs.So far I have done plugs,coil packs 02 sensor,Injectors pressure cleaned and the hubs.

2000 Cherokee with 265/75/16 2" lift 24 mpg hwy 21.5 average.

 

[JNickel101]

did you change your speedo gear?

 

[Redsnake]

JNickel101...

If that ? is directed at me, then my answer is Nope, haven't changed out the speedo gear.

I honestly didn't think that there was that much of a difference in tire size between 235/75R15s and 30/9.5R15s... I know they're bigger, but didn't think it would effect my mileage based on my odometer and speedometer. Guess if it's off just enough that could be showing the 4-5mpg drop... but that's not something I thought of.

 

[bwh0069]

Speedo corrected @ certified test center

 

[bwh0069]

I drove around in mostly rural areas this weekend.My mileage averaged 21.3.I am thinking about putting a Banks exhaust system on to pick up a little power.

 

[UNCC_99XJ]

ALL of them get this kind of mileage, and the people that say they get 20+ FWY MPG are either eggaggerating or they are calculating incorrectly.

When mine was stock, I would average 19-20 on the highway, AC on or off didn't matter, and mine doesn't have cruise control. Around town, I would get around 16-17, again AC on or off..didn't matter.

Now, on 31's with stock gearing, on the highway I get around 17-18....every now and then i'll hit 19.5 or 20. Around town, especially in the university area where everything's real close, and you spend 30 minutes idleing in traffic to go 2 miles, it drops waaay down to 15. Back home on the country roads, and the shortest distance is about 8 miles one way, it goes back up to 16-17.

Aside from good plugs, o2 sensors, and keeping out of the skinny pedal, the quality gas you run makes a big difference.....i've always had better results from BP regular, always.

Seeing as how 99% of my driving is highway at distances of atleast 25 miles one way, 17-18 isn't bad at all, all things considered.

 

[dobbs571]

Hello I am getting about the same milage 18 to 21 on hwy and 16 to 18 around town .Mines a 92 lardo 4.o HO 5 speed 3.73 gears with 225/75/R15 opt factory tires has 152,000 miles on it. If I baby it can get about 2 more miles per gal on hwy with cruise on .

 

[99xjclassic]

oops...edit

 

[bwh0069]

Where is the best place to buy header and cat back system?

 

[smcdonaldaz]

I have a '93 sport with 235s and roof rack holding my spare. I was getting a consistant mid 15s during my 50 mile daily commute. After my last oil change(2 tanks ago) when I switched to the 15k mile Mobil1 synthetic I've gotten mid 16s. Saving approximately a gallon/tank. I don't plan on letting the oil go the full 15k(paid $4.49/qt) but will break even after 10 tanks or so. I'm getting a swing away tire carrier this weekend so that should help a lot.

Thanks to all the advice on here I think I'll swap my O2 and reset my computer too.