fuel mileage question

bigmaho · **Posted:** Sun Sep 14, 2008 10:58 pm

One thing I can't figure out. Before I unplugged the MAF my tach would show approx 2200-2300 rpm at my normal hwy speed (67 mph). Today, after the change, My tach was showing 1900 at the same speed. Where did those extra revs go? I didn't grow an extra gear (or did I?)

nursecosmo · **Posted:** Sun Sep 14, 2008 11:20 pm

Great news on the MPG! 2200-2300 RPM at 67 MPH would be about where 4th gear would be spinning. Maybe you were not getting into 5th for some reason.

Threeweight · **Joined:** Mon May 14, 2007 5:57 pm **Posts:** 679

One caveat on tire pressure... if you use your Jeep off road with regularity, high pressure can cause some issues. Most obvious is rough ride, less so is poorer traction on loose surfaces (due to the smaller footprint). I just had a split develop in the sidewall of one of my Firestone Destination AT's after driving 40-50 miles on very rough dirt roads, with the tire at 42 PSI. It isn't a rock cut, a flat out split in the outer layer. Likely a combination of a cheap tire/sidewall and high pressure, but just something to consider.

geordi · **Posted:** Mon Sep 15, 2008 5:04 pm

That sort of thing is MUCH less likely to happen when you aren't running Firestone Blowout 500's and are using a tire with a STEEL sidewall instead.

Were those tires even LT rated? If so, I'd be interested in knowing the date code. If they weren't older than 5 years, you obviously got a bad batch. Lastly, what was the sidewall max pressure? 42 sounds high for most tires, unless you are getting into the higher load ranges of LT or full-on truck shoes.

KJ 119 · **Posted:** Mon Sep 15, 2008 6:43 pm

I've been running my tires at 28psi for a year now and the ride was smooth also off-road driving was great,but mpg was awefull.
Boy was i a dumazz.Jacked the psi up to 39 before evacuating from Gustav,on my way to Florida,and gained 5mpg!!!!!
Never believed it till I tried it.Went wheeling at Eglin AFB while there and off-road performance went way down(loose sand was tougher with higher psi).

Moral of the story: Keep higher psi on the road(for mpg)
Lower psi off-road(for traction) and dont forget to air back up

Wobbly · **Posted:** Mon Sep 15, 2008 6:44 pm

[quote=

However - point of physics - the contact patch on the road will match vehicle weight and tire pressure.
i.e. if you have 40psi - you have 1 sq inch of rubber for every 40 lbs of vehicle if you have 33psi - you will have 1 sq inch for every 33 lbs of vehicle. The shape of that contact patch is controlled by the tire design - but not the size - that is weight and pressure only.

[/quote]

http://www.performancesimulations.com/f ... ires-1.htm

Threeweight · **Joined:** Mon May 14, 2007 5:57 pm **Posts:** 679

geordi wrote:

That sort of thing is MUCH less likely to happen when you aren't running Firestone Blowout 500's and are using a tire with a STEEL sidewall instead.

Were those tires even LT rated? If so, I'd be interested in knowing the date code. If they weren't older than 5 years, you obviously got a bad batch. Lastly, what was the sidewall max pressure? 42 sounds high for most tires, unless you are getting into the higher load ranges of LT or full-on truck shoes.

Destination AT's are light truck all terrain tires.

http://www.tirerack.com/tires/tires.jsp ... tion+A%2FT

I got them last summer because they were very highly rated on Tire Rack and on other sites (including the tires forum on Lost KJ). 44 PSI is the max pressure mine are rated for. I am otherwise very happy with the tire. Could be that I just got a bad one. I'd concur with folks running higher pressures on-road to reduce rolling resistance and improve fuel economy. However, if you do much off roading you will want to run a lower pressure. Airing down and airing up works great, but I'm too lazy. My tires are at 36 PSI now (looking for the happy compromise).

nursecosmo · **Posted:** Mon Sep 15, 2008 9:28 pm

Wobbly wrote:

[quote=

However - point of physics - the contact patch on the road will match vehicle weight and tire pressure.
i.e. if you have 40psi - you have 1 sq inch of rubber for every 40 lbs of vehicle if you have 33psi - you will have 1 sq inch for every 33 lbs of vehicle. The shape of that contact patch is controlled by the tire design - but not the size - that is weight and pressure only.

http://www.performancesimulations.com/f ... ires-1.htm[/quote]

Wobbly. Did you even read the article in the link that you posted? As is shown irrefutably in the tables that AVON tire provided, contact patch is in no way linearly correlated to PSI. If you wish to use physics you must understand the principles involved. PSI and contact area is nonlinear in a complex toroidal shape like a tire.

geordi · **Posted:** Mon Sep 15, 2008 11:17 pm

Threeweight wrote:

geordi wrote:

That sort of thing is MUCH less likely to happen when you aren't running Firestone Blowout 500's and are using a tire with a STEEL sidewall instead.

Were those tires even LT rated? If so, I'd be interested in knowing the date code. If they weren't older than 5 years, you obviously got a bad batch. Lastly, what was the sidewall max pressure? 42 sounds high for most tires, unless you are getting into the higher load ranges of LT or full-on truck shoes.

Destination AT's are light truck all terrain tires.

http://www.tirerack.com/tires/tires.jsp ... tion+A%2FT

I got them last summer because they were very highly rated on Tire Rack and on other sites (including the tires forum on Lost KJ). 44 PSI is the max pressure mine are rated for. I am otherwise very happy with the tire. Could be that I just got a bad one. I'd concur with folks running higher pressures on-road to reduce rolling resistance and improve fuel economy. However, if you do much off roading you will want to run a lower pressure. Airing down and airing up works great, but I'm too lazy. My tires are at 36 PSI now (looking for the happy compromise).

Wow, it does sound like you got a bad one. Sorry about that. At one point a long time ago, I got to experience bubbles on 6 different Michelins, on 2 vehicles. The tires had all been bought within a week of each other, and obviously were from a bad batch. I didn't buy them again for about a decade due to that.

Anyway, the idea of airing-down for off-road performance is a good one, IF the surface you are heading onto isn't mud or snow/ice. Sand is where you really want to "float" on a fat contact patch, and I believe it also helps traction on rocks. But for mud, I find that it is better if the tire can "knife" through the upper layers to find more solid ground underneath.

I will admit that I haven't done a lot of mudding in my KJ, but I did use it to pull out a fully loaded 10k lb box truck from mud that was about 8" deep. Yuck. I aired down to about 30psi in my 215/85 pizza cutters, but they didn't really spread that much. Probably b/c of the steel sidewalls.

YMMV, of course! :lol:

Wobbly · **Posted:** Tue Sep 16, 2008 1:18 am

nursecosmo wrote:

Wobbly wrote:

[quote=

However - point of physics - the contact patch on the road will match vehicle weight and tire pressure.
i.e. if you have 40psi - you have 1 sq inch of rubber for every 40 lbs of vehicle if you have 33psi - you will have 1 sq inch for every 33 lbs of vehicle. The shape of that contact patch is controlled by the tire design - but not the size - that is weight and pressure only.

http://www.performancesimulations.com/f ... ires-1.htm

Wobbly. Did you even read the article in the link that you posted? As is shown irrefutably in the tables that AVON tire provided, contact patch is in no way linearly correlated to PSI. If you wish to use physics you must understand the principles involved. PSI and contact area is nonlinear in a complex toroidal shape like a tire.[/quote]

Read the thread again. You are confused as to who made the physics reference.

ATXKJ · **Posted:** Tue Sep 16, 2008 2:35 pm

Wobbly wrote:

nursecosmo wrote:

Wobbly wrote:

[quote=

However - point of physics - the contact patch on the road will match vehicle weight and tire pressure.
i.e. if you have 40psi - you have 1 sq inch of rubber for every 40 lbs of vehicle if you have 33psi - you will have 1 sq inch for every 33 lbs of vehicle. The shape of that contact patch is controlled by the tire design - but not the size - that is weight and pressure only.

http://www.performancesimulations.com/f ... ires-1.htm

Wobbly. Did you even read the article in the link that you posted? As is shown irrefutably in the tables that AVON tire provided, contact patch is in no way linearly correlated to PSI. If you wish to use physics you must understand the principles involved. PSI and contact area is nonlinear in a complex toroidal shape like a tire.

Read the thread again. You are confused as to who made the physics reference.[/quote]

I made the physics reference and I'd agree that it's not a linear relationship - but it's not a function of the toroidal shape - it is a function of the sidewall design and reinforcing layers.

Atlhough I'd like to see the original data - the link to Avon is dead - and Avon's website doesn't list it and he doesn't reference any technical papers or reports - so it's interesting, but it's not conclusive -as the article points out - they didn't measure contact area - it's calculated and they had less deflection at 18psi than they did at 28psi??

and Avon 3000's appear to be their Formula 1 tires, which explains the low pressures.
but until you can extablish more of a relationship between the variables - you don't know how well that translates to other tires.
I'd really like to see what Avon's engineers thought of the data.

(I send them an email - we'll see if they respond)

bigmaho · **Posted:** Wed Oct 01, 2008 10:34 pm

Me Again,
Well... after a couple of tank fulls I'm back to 22-23. Haven't cleaned the MAP sensor yet. Wish it was a stick....

KJ79 · **Posted:** Thu Oct 02, 2008 2:22 am

When my CRD was brand new and not broken in I got 26 mpg. Now at 30,000 m the mileage has steadily gone down, now down to 14 mpg (calculated). That is for 50% city and 50% Hwy driving 70mph on the highway- city driving is more around and in town but no stop-and-go. So I am getting at the most 280 miles per tank. Also it reeks really strong like unburned diesel, to the point that I have to keep the windows rolled up when I am waiting at a traffic light - or when driving real slow. Son, driving behind me, noticed grey smoke blowing from exhaust when ascending 12% inclines (engine was at normal operating temp). Air filter is clean, fuel filter clean, MAP sensor is clean, don't seem to pull air at least it is hardly blowing air when bleeding the filter head. MAP sensor, EGR valve, and AFC valve replaced recently by my dealer. The dealer is sympathetic, but is at wits end, checked compression, Bosch pump pressure and all checks out. No CEL's. Doing the ORM does not make any difference in mileage. Tires at 45 psi. Brakes are not dragging at least the vehicle will coast in neutral down my driveway at the same rate as my Scion.
My dealer actually suggested I go see another dealer for a second opinion. With diesel still at $4.78 per gallon here - I would be better off driving my friend's Hemi powered Durango or full size Dodge/Cummins .

wolcott · **Joined:** Thu Nov 10, 2005 4:16 pm **Posts:** 108

I'm sorry to hear of KJ79's problems--it sounds like something is way off. Some possible things to check are:

Injector spray pattern. You may have one or more plugged diesel injectors. These can be taken out and spray tested individually.

Timing of the diesel injection. This can be checked to make sure the diesel fuel is being injected at the right time. Some things that might throw it off would be pump problems or possibly a stretched timing belt.

There are a lot of other possibilities. It sounds as though you need to take the pump to a dealer who is knowledgeable in this type of diesel. Possibly some of the other contributors to forum know of a good diesel shop in your area.

CRDMiller · **Posted:** Thu Oct 02, 2008 2:26 pm

KJ79, I was getting pretty crappy mpg in my crd until recently, (relatively new owner) I removed three fist's full of junk from my fcv valve. I went instantly from 19.5 to 25.7 mpg and it's holding gears longer on grades.

I'm also seeing higher boost pressures at particular rpm/acceleration levels on the same terrain w/o downshifting. For instance what was once 36 mph (a loosing battle) at 4-5 psi (or slip into a lower gear; i think third to high 2) is now 36 mph accelerating upwards at 10-12 psi. It kind of makes me wonder what kind of crap is in the cac. (30 k miles worth)

And for the record I can easily get around 10-15 pumps a week of air out of my fuel head (as in 5 pumps, bleed, close 5 pumps bleed close)

fastRob · **Posted:** Thu Oct 02, 2008 8:24 pm

CRDMiller wrote:

And for the record I can easily get around 10-15 pumps a week of air out of my fuel head (as in 5 pumps, bleed, close 5 pumps bleed close)

With a fuel lift pump, no pumping.
Try it you will like it.
Many different ways of doing it.

Getting about 24mpg in mixed driving, my kids drive it too.

warp2diesel · **Posted:** Thu Oct 02, 2008 9:57 pm

....spouting slogans, but, check your odometer error, mine it 10%.
Also, air bubbles in the fuel system will retard injection timing by delaying the injection.
This will decrease fuel economy and power.

tulsa · **Posted:** Thu Oct 02, 2008 10:00 pm

warp2diesel wrote:

....spouting slogans, but, check your odometer error, mine it 10%.

Funny you should mention this but, my error is about 2% meaning if I drive 100 miles my odometer shows 98.0 miles with stock tires.
Lewis

warp2diesel · **Posted:** Thu Oct 02, 2008 10:02 pm

tulsa wrote:

warp2diesel wrote:

....spouting slogans, but, check your odometer error, mine it 10%.

Funny you should mention this but, my error is about 2% meaning if I drive 100 miles my odometer shows 98.0 miles with stock tires.
Lewis

Yours is an 05 and mine is an 06, that may be what it is. Others with 06 CRDs have reported a 10% error.

Have you had your body module flashed?

tulsa · **Posted:** Thu Oct 02, 2008 10:53 pm

warp2diesel wrote:

tulsa wrote:

warp2diesel wrote:

....spouting slogans, but, check your odometer error, mine it 10%.

Funny you should mention this but, my error is about 2% meaning if I drive 100 miles my odometer shows 98.0 miles with stock tires.
Lewis

Yours is an 05 and mine is an 06, that may be what it is. Others with 06 CRDs have reported a 10% error.

Have you had your body module flashed?

Do not know about whether the BCM has been flashed, have had dreaded f37 though.
Lewis

fuel mileage question

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