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Old 07-27-2010, 01:06 PM   #1
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Gas Mileage question

i got a 1998 4 cylinder with 31's and ive been clockin my mileage with a full tank and have been gettin roughly 200-220 miles from the 19 gallon tank. i figure it out to be around 11-12 mpg...is this normal and if not any suggestions of what i could do?

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Old 07-27-2010, 01:15 PM   #2
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i got a 1998 4 cylinder with 31's and ive been clockin my mileage with a full tank and have been gettin roughly 200-220 miles from the 19 gallon tank. i figure it out to be around 11-12 mpg...is this normal and if not any suggestions of what i could do?
forgot to mention its automatic...im new here so if this is a dumb question...step offff haha

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Old 07-27-2010, 01:17 PM   #3
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I'm squeezing a little more than that with my 6cyl and 31's. You actually have quite a bit of gas left in the tank after the light comes on.
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Old 07-27-2010, 01:21 PM   #4
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I'm squeezing a little more than that with my 6cyl and 31's. You actually have quite a bit of gas left in the tank after the light comes on.
yea ive heared that u can get close to 50-60 after the light but what was weird is before i shut it off i was right above the red section with no light on and then i started it up and then i was clearly below the red line with the light on
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Old 07-27-2010, 01:26 PM   #5
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Don't go by the gas gauge or low fuel light at all, both are notoriously inaccurate. Most say they are empty when there is still 3-5 gallons left. Heck, mine shows it is half-full within 50 miles after I fill the tank, and it shows nearly empty when the tank still has ten gallons in it. Go by your odometer instead and note the number of gallons your Jeep takes after a fill-up. That way you'll know your true range and mpg.
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Old 07-27-2010, 01:29 PM   #6
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Don't go by the gauge or low fuel light at all, both notoriously inaccurate. Most say they are empty when there is still 3-5 gallons left. Heck, mine shows it is half-full within 50 miles after I fill the tank, and it shows nearly empty when the tank still has ten gallons in it. Go by your odometer instead and note the number of gallons your Jeep takes after a fill-up. That way you'll know your true range and mpg.
yea thats a good point i completly forgot the gauge is really inacurate...because after all this i went and filled up and got roughly 14-15 gallons when it was displaying i was below red...thank you for your input
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Old 07-27-2010, 02:05 PM   #7
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Originally Posted by Jerry Bransford View Post
Don't go by the gas gauge or low fuel light at all, both are notoriously inaccurate. Most say they are empty when there is still 3-5 gallons left. Heck, mine shows it is half-full within 50 miles after I fill the tank, and it shows nearly empty when the tank still has ten gallons in it. Go by your odometer instead and note the number of gallons your Jeep takes after a fill-up. That way you'll know your true range and mpg.
This is the only way to go. estimating your fuel mileage by any other means is horribly inaccurate
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Old 07-28-2010, 09:07 PM   #8
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I have a 98 and it's in need of a fuel pump/sending unit. No owner's manual, can someone tell me what size tank is stock on the 98's? 4 cyl, 5 sp. Thanks for your help!
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Old 07-28-2010, 09:43 PM   #9
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I get average of 18mpg in a 6cyc manual. highest 22mpg. Stock wheels/tires. city driving.

in 1999 they made the 19 gallon tank standard. I think the gas tank size was an option before then
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Old 07-28-2010, 09:51 PM   #10
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Jeep + Gas Mileage =

Just drive it and try not to think about how bad it is. If you get below a 1/4 tank or worse, head towards the nearest gas station and top off.
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Old 07-28-2010, 11:29 PM   #11
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2004 12MPG Calif. Emissions

Boy and I felt bad I'm getting about 12 MPG with my 6cyl have 31X10.5X15 tires.Does anyone think the California emissions make MPG or horsepower less?
Have a manual trans
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Old 07-29-2010, 02:16 AM   #12
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I'm getting about 13 mpg with my 6cyl, manual. I also have 35's. I guess the only thing that might help me is my Jet Chip. So I guess I'm doing great.
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Old 07-29-2010, 09:15 AM   #13
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I guess the only thing that might help me is my Jet Chip.
I doubt that helps much at all.
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Old 07-29-2010, 03:30 PM   #14
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Do not forget the odometer may be off a bit also with larger tires. No way of really accurate mileage checking in my book unless all has been recalibrated. Who cares, it is a Jeep.
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Old 08-07-2010, 12:00 AM   #15
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About 13 MPG, 2000 TJ with autotrans, stock suspension, 30" tires. When I check mileage, I usually drive 600-800 miles and record total miles driven and total gas put in the tank for 3-4 fills. I get slightly better MPG with 92-93 octane than with 87, but probably not worth the added cost of premium vs. regular. Usually get about 240 miles on 19 gallon tank. I think when it was new I got about 280 miles on a tank.
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Old 08-07-2010, 12:19 AM   #16
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in 1999 they made the 19 gallon tank standard. I think the gas tank size was an option before then
I've read this before but I have the factory build sheet (window sticker) for my '99 sport and it says I have a 15 gallon tank, which the pump seems to confirm. So the TJ gas tank puzzle continues...
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Old 08-07-2010, 12:57 AM   #17
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i have an 06 4cyl 6 speed, if i keep my foot out of it and actually get to 6th gear i get about 21mpg, i have 235/75-15s which i think are a tad smaller than 30's? so their not aweful big tires but bigger than stock, i always fill up when i touch the red line and that is normallly about 15 gallons and on those 15 gallons i can get anywhere from 260-300 miles, never ran it to the gas light so im not sure if it is really a 19 gallon tank or not?
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Old 08-07-2010, 01:17 AM   #18
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my '99 4 banger 5 speed only has a 15 gallon tank on it and i get around 200 miles, with a set of 31's, intake, and exhaust.
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Old 08-07-2010, 01:39 AM   #19
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Have you calibrated your speedometer? If not...I think the stock tire size would be roughly 27.5 inches in diameter. If you are running 31s without calibrating you are going about 55 mph when your odometer reads 50. If so you are going 110 miles for every 100 your odometer reads.
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Old 08-07-2010, 10:32 AM   #20
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With my 30" tires, my speedometer runs higher than actual speed. Checked against my Tomtom GPS, and also when I see one of those roadside portable radar speed displays the LEOs put out. So when my speedometer reads 25, I'm actually going 23. I think the difference scales with speed, so when my speedometer reads 50, my actual speed is about 46.
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Old 08-07-2010, 12:57 PM   #21
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I've been tracking my mileage for about a month. Religiously, every fill-up gets recorded in my excel sheet. I changed my plugs & wires a month ago, that's why I don't compare anything prior. I always run premium (personal preference), and I stay away from the low-budget gas stations.
Over the past month, my MPG ranges from 14.62 MPG to 17.83. The way I drive doesn't seem to impact my mileage. When I drive carefully and use the skinny pedal sparingly, I sometimes get better, sometimes worse mileage.
One of my better MPG's was on a 4wheeling trip. That confused the heck out of me.
My Tacoma would fluctuate, but it was directly related to how hard I pushed it. It would also fluctuate much more than 3MPG.
Is anyone else experiencing this same kind of pattern?
(99 Sport, 4.0, 4spd auto, unknown -believe stock- gearing, 31x10.50)
I'm still learning what I have. The more I read (and drive) the more I believe it does have some mods done. I have a 4spd auto, which was supposedly not available in 99, and I believe I have some budget lockers both front & rear - last camping trip I dug 4 holes, straight down.
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Old 08-07-2010, 02:05 PM   #22
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^ you could save some money by running regular gas. The 4.0's do fine on 87 octane, I think the manual even reccomends it.
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Old 08-07-2010, 02:13 PM   #23
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I get on average 12.5mpg with a 6 spd manual with overdrive running 35s. It's crazy, but Hwy or city it's the same.

When I had the 27" stock tires I got 18-20. Those days are long gone.
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Old 08-07-2010, 02:33 PM   #24
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I always run premium...
If you understood what "premium" really meant, you would no longer run it. Premium is just a term gasoline manufacturers use to entice people to think it is somehow better, has more power, more cleaning additives, helps the engine run better, etc. but none of those could be further from the reality of what it really is.

The only difference between 87, 89, and 91 octane fuels is that 87 is the easiest/fastest to ignite. 89 burns a little slower and is more resistant to higher compression before it self-detonates, and 91 burns a little slower and is a bit more resistant to higher engine compressions yet. That is it. All three have the same amount of additives, fuel injector cleaners, etc. The gasoline manufacturers describe their "premium" fuels in such a way as to make some think they include more/better additives but they don't in reality.

Octane is simply an additive that is added to slow the gasoline's burn rate down and to allow it to withstand greater compression levels without premature ignition from excessive compression.

Why do different octanes even exist? Because some engines have higher levels of compression to produce more power. Corvette, Lexus, Mercedes, Mustang 5.0L, etc. are high compression engines so they need fuels that can resist the higher compression levels without premature detonation.

But if your engine was not designed to produce such higher levels of compression, then there is absolutely zero benefit to using a higher octane fuel. Zero, nada, none. If your engine does not ping with 87 octane, then there is zero benefit to running 89 or 91 octane. None. All doing so helps with is by contributing to the profit margin of the gasoline manufacturers. It costs them about a penny per gallon to turn 87 into 89 and a penny more to turn it into 91. But they charge 10-30 cents more so it is far more profitable for them.

Energy-wise, a gallon of 87 octane has precisely the same energy content a gallon of 89 or 91 does. Running 89 or 91 or 104 octane fuel in an engine that was designed for 87 is a 100% waste of the extra money it costs. And no, the engine won't run any cleaner either. There are ZERO benefits for any of our Wrangler engines by running anything with more octane than 87.
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Old 08-07-2010, 06:50 PM   #25
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So if your engine IS pinging, then it's beneficial?
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Old 08-07-2010, 06:55 PM   #26
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So if your engine IS pinging, then it's beneficial?
If it is pinging all the time, then yes but that also indicates an engine problem like a carbon buildup. It should not ping all the time so if it does, your engine needs work... like having it de-carbonized.

Occasional pinging like just before you downshift to go up a steep hill is normal.
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Old 08-08-2010, 12:58 AM   #27
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If you understood what "premium" really meant, you would no longer run it.
Thanks for the info, Jerry

I guess I should have worded that differently. All of the month referred to I ran premium.

I use an excel sheet to track my mileage, and it also shows me how many cents per mile I spend on gas. (This month was 17-22 cents per mile, mostly in the 19-20 range with all premium gas)
In my last vehicle, the pennies per mile was the same across the board. So, spending more on premium simply meant I was going farther between fillups (handy for camping / wheeling!)

I'm still new to my Jeep and learning.
I plan to spend a month on each octane level to see which is the most cost effective to run. I figure a month will give me a decent average.

I have heard that the Wranglers do fine on regular, so I have decided that will be month 2

My concern was the lack of variation between my driving styles. I was expecting to see my mileage vary at least 7 mpg, maybe more.
But then, maybe I'm just not so heavy-footed as I used to be, lol!
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Old 08-08-2010, 07:04 AM   #28
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Nice write up Jerry Bransford.

Today's Mechanic Tip - Unless you are on a road trip (READ - grocery getter or work commute) Keep your fuel tank above the 1/2 full mark or what would equate to half a tank. The fuel pump being mounted in the fuel sending unit relies being submerged in the fuel to keep it cool. #1 cause of fuel pump failures is getting over heated.

Mileage is all in the right foot...baring modifications that add weight and aggressive tires giving more rolling resistance. Learn what type of driving get's you the best mileage (RPMs, shift points, accel rate...). Even a bad alignment will rob you of 2-3 mpg.
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Old 08-08-2010, 07:25 AM   #29
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*edit*
answered my own question

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