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  #13  
Old 12-01-2012, 07:51 PM
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Re: Break-in MPG?

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Originally Posted by JTS97Z28 View Post
Uhhh, you filled your tires to 50psi?

Please put them down to the recommended 36psi for the 20" wheels. DO NOT run at the max designed PSI rating...for the your safety and others. That means when your tires warm up your pushing probably 53+ psi. Not good!
If the average consumer goes to the gas station to air tires, isn't it typical to always fill up your tires up slightly below max psi? Tires generally don't say what the "recommended" pressure is on the side of the tire...nor does anyone inform you of the recommended psi.

Also, the EVIC center says pressure is "okay" now at around 49psi per tire. You would think there would be some warning, recommendation, etc.
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Old 12-01-2012, 08:10 PM
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Re: Break-in MPG?

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Originally Posted by mufugger View Post
If the average consumer goes to the gas station to air tires, isn't it typical to always fill up your tires up slightly below max psi? Tires generally don't say what the "recommended" pressure is on the side of the tire...nor does anyone inform you of the recommended psi.

Also, the EVIC center says pressure is "okay" now at around 49psi per tire. You would think there would be some warning, recommendation, etc.


"If the average consumer goes to the gas station to air tires, isn't it typical to always fill up your tires up slightly below max psi?"


No it is not typical. I understand im probably not your typical person when it comes to cars, but as long as I have owned a car I know to follow the vehicles manufacturer recommended tire pressure that is typically posted in the drivers door jam. Open your door and read the sticker that clearly states cold tire pressure 36 PSI. Im not poking fun at anyone because im sure this happens on a regular basis, but for example I know my sister who is a typical driver and she knows where the pressure should be set.



"Tires generally don't say what the "recommended" pressure is on the side of the tire...nor does anyone inform you of the recommended psi."

Your right they do not as that's up to the manufacturer for the specific vehicle and is posted in the owners manual and on a sticker that's visible everytime you open your driver door. The tires state the PSI that should never be exceeded for safety and structural reasons.



"Also, the EVIC center says pressure is "okay" now at around 49psi per tire. You would think there would be some warning, recommendation, etc
."

I think the EVIC in the Jeep (and most other vehicles) will only give a warning for low pressure not high. I could be wrong though as I have never had any reason to overfill my tires by that much.


At any rate, I can only recommend that you put your tires back down to the recommended 36psi cold. Hope this helps.

James
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Old 12-01-2012, 09:08 PM
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Re: Break-in MPG?

Quote:
Originally Posted by mufugger View Post
If the average consumer goes to the gas station to air tires, isn't it typical to always fill up your tires up slightly below max psi? Tires generally don't say what the "recommended" pressure is on the side of the tire...nor does anyone inform you of the recommended psi.

Also, the EVIC center says pressure is "okay" now at around 49psi per tire. You would think there would be some warning, recommendation, etc.
The manual, and the sticker on the inside of driver's door sill list the recommended pressures. This sticker is generally present somewhere on the bodywork around the driver's door opening on any car. As I said, I have found the Jeep's factory recommended pressures to be a bit low - but filling to near the max PSI is the opposite extreme.

The Grand Cherokee is not heavy enough to require such high tire pressures. While the tire can mechanically withstand the pressure, it will cause the middle of the tire to bulge out because the GC is not heavy enough to flatten it. You will get great fuel economy because of the smaller contact patch, but the tread in the middle will wear away quicker, probably canceling the financial gains in fuel economy. It will diminish your traction and emergency braking performance as well.

If the factory suggestion for your vehicle is 36, 40-42 is probably the highest I'd go. And I'd keep a close eye on my tires for any signs of asymmetric wear in the middle vs. the outside tread.
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Old 12-03-2012, 01:12 AM
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Re: Break-in MPG?

I am happy to be getting 21.5 mpg with 31.5s on my new 3.6L 4X4. I was only getting 16.5 mpg with my previous 3.7L 4X4 KK with 31s armored in 2wd daily mode. My older WJ with a ARB bullbar, winch and armor on 31s was at 16 mpg. The air pressure of my 265/65/18 SilentArmors seems to be good at 38 psi. I am loving the better gas milage and more room inside my new Jeep, not to mention feeling more secure with all four wheels tracking on the wet mountain roads. Skids and rockrails armor to be added soon
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  #17  
Old 12-03-2012, 06:03 PM
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Re: Break-in MPG?

One way you might use to figure inflation for highway use is to take the weight on each axle (For a Laredo V6 4x2 that is 50/50 so figure 2235 lbs/axle or 1118 lbs/tire). I toss in another 50% for cornering so 1676 lbs (if autocrossing, I figure the entire axle load is liable to be on the outside tire).

Now multiply by the max inflation and divide by the max load (on tire sidewall) which for mine gives about 39 psi. It works for me.

BTW lower inflation pressures may give a softer ride at the expense of MPG.
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Old 12-03-2012, 06:23 PM
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Re: Break-in MPG?

break in??? - no such thing, drive it like you stole it from the factory.

tire pressure - I leave mine at 36-38 or so cold and when I go on a long highway trip, I up it to 40 cold. I like the feel of more air in the tires on the highway, less "floaty" feeling IMO. Around town, it's nice to have a little more suspension with the softer tires.
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Old 12-29-2012, 06:59 PM
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Re: Break-in MPG?

I'm right there with you...

We have the 2011 Jeep Grand Cherokee v6 4x4 Overland and took two separate hwy trips and barely made it over 400 miles on the tank. I'm glad to see others are having the same issue because I heard Hyundai owners began complaining and Hyundai sent out rebates to owners due to false mileage claims.


What about starting.. I'm also experiencing problems with our Jeep starting..It's always saying it can't detect the key. It's already had to be towed to the dealership one time and been in 3 times to figure out why it won't start. They think it has to do with the computer but haven't found a solution yet.
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Old 12-29-2012, 11:35 PM
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Re: Break-in MPG?

Everyone plays games with MPG. I find the rating pretty accurate but have a 2WD Laredo that is almost 500 lbs lighter than a 4x4 Overland (4470 vs 4920). It really astounds me that both (and a V6 Durango) can have the same highway rating.

Of course Chrysler knows how to play the game. How many have seen the 2013 Ram 1500 ad mentioning 25 mpg when only the specially equipped HFE model is rated that way.

ps it mentions in the user manual how to pop the start button off and use the fobik as a key or didn't that work either ?
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  #21  
Old 12-30-2012, 12:43 AM
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Re: Break-in MPG?

Heavy V-8's with 4wd are very throttle sensitive.

To see what your Jeep is capable of, air up tires a couple lbs, and run at a steady 60 mph with cruise control engaged. Bet you will be surprised.
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Old 12-30-2012, 06:10 AM
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Re: Break-in MPG?

I'm getting pretty consistent 23 mpg hwy and about 18-19 mix of city/hwy in my 3.6L 4x4 '13 WK2, that just turned over 1k miles.

As for tire pressure, what is posted on the sidewall is the max for that specific tire, not what is recommended for the vehicle. That tire could be used on many different vehicles, none of them likely need 51 psi in the tires. With the weight of the Grand Cherokee, 33 psi it what is recommended for maximum tire wear and maybe even grip. As you inflate the tire beyond the recommended pressure, the center of the tire will begin to bulge outward, unnoticeable to the eye, until you have a few thousand miles on those tires, and you begin to see the center of the tire wearing much faster and having less tread depth than the rest of the tire. If you were towing or had a heavier vehicle, the tire would require more pressure, moving closer to that 51 psi max point, so that as weight compresses the tire, more air pressure pushes the center of the tire back out so that it has an even contact patch with the increased weight. This is why you are getting increased mpg with 50psi in you tires. You have reduced the rolling resistance of the tire by decreasing the contact patch of the tire. Less of the tire is now contacting the road, and you'll get less tread life from the tire.

In my JK I ran slightly less than the recommended pressure for optimal ride. You could run slightly more than what's on the sticker for better mpg. I used the chalk test, and set the pressure to a point that a chalk line drawn across the span of the tread, wore evenly. On my 37" MT/R Kevlars, that number was 26 psi. Really comes down to personal preference. At $370 per tire I had mine set to get the most life out of them.
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  #23  
Old 12-30-2012, 06:40 AM
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Re: Break-in MPG?

I can't say I've noticed much difference in mileage, except in the past couple of months, when we switched to the winter gas formulation in the Chicago area. If you're in an area where a winter formulation is mandated by the EPA, you'll get about 2- 4 less mpg, in my experience.

Steady cruising with my tires inflated to 38 psi yielded about 19 - 20.5 mpg over the summer, running at steady highway speeds. The best cure for mpg anxiety is not to think about it.
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Old 12-31-2012, 05:31 AM
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Re: Break-in MPG?

I only have 700 miles on my 2013 GC and I am only getting 12.7 average on the EVIC. Sucks.
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