MPG drop

[HoosierJeeper]

Usually around this time of year I'd be getting 13.5 or so, that's with a ski rack, rock rails, skids, no highway driving, all city with all stop and go. This year in the same driving, I'm getting 12.5. The only thing that is different is that I put 85w140 in the diffs during the summer. Is that the difference? The ******, tcase, coolant, spark plugs get changed according to schedule. The spark plugs are Autolite double plats, about 20k on them. Should I try a fuel system cleaner with the next tank? Any ideas?

I don't expect much MPG, more than anything I want to make sure nothing is wrong with the KJ. The WK, also with a 3.7 but no ski rack is getting around 17 in similar conditions.

 

[LibertyTC]

If you are just doing short trips HJ it takes a while for the thicker gear lube to warm up especially if it is cold out. Also the winter gas may also lessen mileage.
Check your tire psi as well, what is it currently at? Often colder air offers less psi in them shoes!
Overall it sounds like nothing to worry about ..carry on!

 

[HoosierJeeper]

PSI wise I'm at the 32-33 range.

 

[LibertyTC]

Forget the door sticker. Take a look at the tires themselves and see what psi range is stated on them.
I am fairly confident you can bump them up to 38 psi, and find less rolling resistance.
That may give you back the 1 mpg right there.

 

[ol-flattop]

Forget the door sticker. Take a look at the tires themselves and see what psi range is stated on them.
I am fairly confident you can bump them up to 38 psi, and find less rolling resistance.
That may give you back the 1 mpg right there.

You're right that the door sticker isn't the authoritative source for best tire pressure to run (except on OEM tires, I'd argue), but carelessly adding more air is an invitation to a couple problems.

Sure, it'll decrease rolling resistance, but it will also change the contact patch. This may adversely affect tread wear, causing the tires to wear out sooner and require replacement, effectively ruining the attempt at greater efficiency.

Considering we're talking about a Wisconsin winter here, I'd say the more important problem will be decreased traction. A smaller contact patch is the opposite of what we want in low-traction conditions, especially on all-season (i.e. too hard to perform optimally in cold temperatures) tires! In fact, a lot of people purposely air down a couple psi in the winter for this reason, sacrificing a small amount of tread life to gain a small amount of traction.

There are several supposed methods for fine-tuning your tire pressure, including simple trial and error; but just stuffing more air in because the tire says it can handle it isn't the best way to go. The tire can handle the pressure and will return better fuel mileage, but won't wear evenly or provide the best traction if you inflate too high above the sweet spot. Everything's a tradeoff.

Anyway, HJ isn't just trying to get that 1 mpg back - it's about what's different now from previous years that's manifesting itself in a 7% drop in fuel mileage. Unless this KJ's tires were at 38 psi (for example) the last several winters, inflating them more now won't address the actual issue, whatever it may be.

Ok, I'm done. Carry on.:icon_mrgreen:

 

[LibertyTC]

I would be real curious to see on snow, how much if any difference there would be in tire patch between 33psi & 38.

 

[ol-flattop]

I would be real curious to see on snow, how much if any difference there would be in tire patch between 33psi & 38.

Just a rough calculation to illustrate. The relationship isn't totally linear, and we'll have to disregard specific tire types and characteristics for the sake of a simple comparison. Let's say curb weight is 4000 lbs (a bit low, but close).

That's 1000 lbs per tire (again, disregarding the obvious weight bias to the front tires for the sake of simplicity).

1000 lbs supported by a tire at 33 psi will have a contact patch of 30.3 square inches. 1000 lbs / 33 (lbs/sq.in.) = 30.3 sq.in.

1000 lbs supported by a tire at 38 psi will have a contact patch of 26.3 square inches. 1000 lbs / 38 (lbs/sq.in.) = 26.3 sq.in.

30.3 sq.in. - 26.3 sq.in. = 4 sq.in.

4 sq.in. / 30.3 sq.in = 13.2%

Essentially, adding those 5ps will subtract about 13% from your contact patch. And that, my friend, is significant.

 

[HoosierJeeper]

I think the PSI is fine, ran it with the last set of Terras and got good wear. I think I'm going to try a fuel system cleaner with the next tank of gas. Any suggestions?

 

[Dave]

I think the PSI is fine, ran it with the last set of Terras and got good wear. I think I'm going to try a fuel system cleaner with the next tank of gas. Any suggestions?

Try Chevron/Techron Fuel System Cleaner. I use it once a year or so. You could try that.


As for the mpg, mine drops 1-2 mpg easy with winter gas. Running heater/defroster all the time and extra ideling doesn't help mpg.


I run 35psi in my A/T's in the winter. (they're 245/75's).


The only other thing I could think of, is to take the ski-racks off the roof for a week to see if they perhaps create more drag than you think?


Dave

 

[ltd02]

I'm a big fan of that Techron too!

 

[Leeann]

I use Techron, too. I also run 35 psi in winter.

 

[LibertyTC]

Ya I like & use the Chevron Concentrate Plus as well a few times a year.
Best if you can find the 20 oz bottles. The jeep is close to 20 gallon tank.
So ignore the up to 15 gallon labels on the 10 oz bottles, use two bottles, and take the jeep for a good run with the cleaner in.
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[MarkJ]

And remember, I dare everyone to compare air pressure gauges and find two within .5 psi.

 

[Birdman330]

Lucas fuel injector cleaner, good stuff teats up to 24 gallons in one little bottle. Use it twice a year.

 

[HoosierJeeper]

I put in some of that Techron stuff in last night, hopefully this will help!

 

[Hedsic]

Lucas fuel injector cleaner, good stuff teats up to 24 gallons in one little bottle. Use it twice a year.

I got like a 4 gallon giant bottle of that stuff last year for like $10 at Walmart..

(it's not really 4 gallons but it's a pretty large bottle compared to normal large size they carry)

I put in some of that Techron stuff in last night, hopefully this will help!

Maybe check the air filter too? I've seen people pull some crazy stuff out of their filters that where clogging them up. Maybe a Chinchilla family crawled up in there and made a nest or something crazy

 

[TheBlueKJ]

Maybe check the air filter too? I've seen people pull some crazy stuff out of their filters that where clogging them up. Maybe a Chinchilla family crawled up in there and made a nest or something crazy

I've seen some down right disgusting filters to the point I was surprised the engine could breathe. Cabin air filters were even worse, some had mold on them and to think that's the air those people were breathing

 

[CactusJacked]

In reference to the tire's contact patch mentioned, wider tires will have more of a tendency to float on the snow, whereas a narrower tire will do better at digging down deeper to grab the pavement. If airing the tires up more narrows the contact patch, then he's doing himself a favor for better snow traction. Wider tires work better for dry pavement traction, not so much for rain and snow. With that said, I always inflate my tires more than the factory sticker says, and never have irregular tire wear. Keep in mind, those inflation numbers are a suggestion based on giving the consumer a balance between gas mileage and ride comfort. If the sticker listed the (higher) pressure you could really get away with, people then would complain to the manufacturer about the harsher ride.

 

[profdlp]

I think all of the suggestions are good - even the ones that contradict each other. (I mention the last because there are a lot of variables involved and what might be right for one is wrong for another.)

Just to add on, I went through something similar a time or two and found the following to work:

1) Might just be winter gas. Wait for spring.

2) Absent an overabundance of patience, try using premium Shell gas. Whatever they put in there it works wonders on my Liberty. A tankful or two and she's back to her old self. (I seem to have a "good" one; if I drop below 16mpg city-only I feel cheated.)

 

[ol-flattop]

In reference to the tire's contact patch mentioned, wider tires will have more of a tendency to float on the snow, whereas a narrower tire will do better at digging down deeper to grab the pavement. If airing the tires up more narrows the contact patch, then he's doing himself a favor for better snow traction. Wider tires work better for dry pavement traction, not so much for rain and snow. With that said, I always inflate my tires more than the factory sticker says, and never have irregular tire wear. Keep in mind, those inflation numbers are a suggestion based on giving the consumer a balance between gas mileage and ride comfort. If the sticker listed the (higher) pressure you could really get away with, people then would complain to the manufacturer about the harsher ride.

Meh. I guess I agree with pretty much very thing you've said, but you left some important concerns out.

First, while a narrower tire does better on snow, increasing inflation will only narrow the contact patch a small amount - most of the difference will be will be in its length.

On top of that, opinions are mixed on whether a wider or narrower tire does best on ice. Since sipes are about the only reason a tire has any traction at all on ice, I'm inclined to keep as many of them as is practical in the contact patch!

Additionally, note that no one said the factory sticker should be followed. I only said that it's foolish to select a value simply because the tire says it can handle it.