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post #1 of 16 (permalink) Old 12-15-2008, 08:52 AM Thread Starter
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Tire pressure confusion

I have the stock tires on my 4.7 Limited (Goodyear Fortera's), and I'm a little confused as to what the appropriate tire pressure should be. I'm getting dodgy gas mileage (around 10 average, mostly city driving), and I think my tires are to blame.

The door jamb sticker says they should be inflated to 35 psi, which is what the dealer set them at when we bought it two weeks ago. They get to around 37 psi when they're warm. When I looked at the tire sidewall, it indicates max psi is 44.

What do I believe? I've run a variety of tires on my other vehicles, and usually inflate to the psi on the tire sidewall. This is the first time I've run into a major discrepancy between the vehicle manufacturer's recommendation and the tire manufacturer. I don't want to inflate it up to 42 and have the tire come unseated from the rim if I hit a bump, but I feel like I could add a couple MPG if I do.
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post #2 of 16 (permalink) Old 12-15-2008, 09:04 AM
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Quote:
Originally Posted by CmdrKev View Post
I have the stock tires on my 4.7 Limited (Goodyear Fortera's), and I'm a little confused as to what the appropriate tire pressure should be. I'm getting dodgy gas mileage (around 10 average, mostly city driving), and I think my tires are to blame.

The door jamb sticker says they should be inflated to 35 psi, which is what the dealer set them at when we bought it two weeks ago. They get to around 37 psi when they're warm. When I looked at the tire sidewall, it indicates max psi is 44.

What do I believe? I've run a variety of tires on my other vehicles, and usually inflate to the psi on the tire sidewall. This is the first time I've run into a major discrepancy between the vehicle manufacturer's recommendation and the tire manufacturer. I don't want to inflate it up to 42 and have the tire come unseated from the rim if I hit a bump, but I feel like I could add a couple MPG if I do.
You have to factor in the fact that tire makers do not make tires specifically for one type of vehicle. The recommended setting of 'max psi' is solely based on the tire's design overall. Your vehicle was design/manufacturered by Jeep/Chrysler folks. They determined that 35psi provides the most efficient balance of road noise, tire wear, etc.

You also have to factor in that gas is formulated differently by regions, as well as climate issues. If you're in cold weather, it takes a while for an engine to warm up. Thus, it takes more fuel to warm the engine; they run most efficient when they're warm. Thus, experts say try to avoid short trips, especially in cold climates; otherwise, you burn more gas for each short stop you take.

What is your typical driving like? Stop/go? Drive-thru's? etc.

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post #3 of 16 (permalink) Old 12-15-2008, 09:06 AM
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You need to follow the manufacturer's label. Those pressure's are what they feel are the proper inflation, for proper handleing for the specific vehicle. If you over inflate the tires, you may get an error message from the tire pressure monitor system.
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post #4 of 16 (permalink) Old 12-15-2008, 09:18 AM Thread Starter
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Originally Posted by XKJeep06 View Post
What is your typical driving like? Stop/go? Drive-thru's? etc.
My wife drives it exclusively during the week, and doesn't usually do anything more than an 8 mile round trip to and from work, with about 12-14 traffic lights in the entire round trip. So yeah, stop and go, speed limit 35 mph maximum. I realize that has much to do with our MPG, but I fear the tires may be woefully under-inflated if the difference between Jeep and Goodyear's recommendations is 9 psi.
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post #5 of 16 (permalink) Old 12-15-2008, 10:07 AM
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Originally Posted by CmdrKev View Post
My wife drives it exclusively during the week, and doesn't usually do anything more than an 8 mile round trip to and from work, with about 12-14 traffic lights in the entire round trip. So yeah, stop and go, speed limit 35 mph maximum. I realize that has much to do with our MPG, but I fear the tires may be woefully under-inflated if the difference between Jeep and Goodyear's recommendations is 9 psi.
That many stoplights, within that distance... seeing mpg that low is not at all unusual. My wife works 3 miles from the house, ONE light... and yes, I've seen the EVIC display about 8-10mpg (esp with the HEMI). You have to remember that EVIC calculates based on amount of gas in the tank vs. the avg time the engine has been running since the tripdometer reset. True calculation is done based on driven mileage divided your recent fill-up's gas amount in gallons (down to the decimal) thus... that's your true miles-per-gallon measurement.

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post #6 of 16 (permalink) Old 12-15-2008, 10:17 AM
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Quote:
Originally Posted by Motorcity View Post
You need to follow the manufacturer's label. Those pressure's are what they feel are the proper inflation, for proper handleing for the specific vehicle. If you over inflate the tires, you may get an error message from the tire pressure monitor system.
This is true as long as you are running tires similar to those it is sold with as the manufacture designed the vehicle to operate best with those tires at the pressure in the door jam, If you run a specialty tire you will have to adjust your pressure once the perfect balance has been found and you will have to determine that balance based on MPG, ride and other factors.



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post #7 of 16 (permalink) Old 12-15-2008, 11:49 AM
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great advice
http://www.sportcompactcarweb.com/te...ide/index.html

i have 265/70/17 goodyear wrangler silent armor. i run them at 36 psi, for daily driving. when i go off road, i run 20 psi.

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post #8 of 16 (permalink) Old 12-15-2008, 02:18 PM Thread Starter
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That's a good article, but the formula is sort of bunk. If I followed their advice, I'd be running 50+ PSI.

We've only been using the VIC to measure fuel economy, so maybe I'll just start doing it the old fashioned way of distance since last fill-up/fuel consumed. If I do just some math based on what I think we've been getting, it seems like we're able to go around 250-260 miles on each 17 gallon tank. That would put us closer to 15 MPG.

Maybe the VIC average fuel economy reading is only there to guilt us into driving more conservatively.
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post #9 of 16 (permalink) Old 12-15-2008, 04:33 PM
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I keep my TP between 40-44lbs, the maximum pressure as the tire states

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post #10 of 16 (permalink) Old 12-15-2008, 10:50 PM
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That max pressure is cold inflation pressure. What is listed on the sticker in your car is also cold inflation pressure. Your tires will increase in pressure as they warm up (less if you use nitrogen to fill the tires...).

I typically gain about 2-3 psi according to the evic as the tires warm up. I see a gain of 5 on hot summer days or on the slicks during an autocross (different car). And all my vehicles I usually run about 3 psi over the manufacturer's recommendation. Doing this I typically get a little better mileage, if I drive "normal", and better handling at the expense of tire longevity.

Tires should be replaced every 3 or 4 years anyway, regardless of mileage or remaining tread depth.

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