2001 Isuzu Rodeo Gas Mileage issue

About 10 months ago, I noticed a fairly precipitous drop in gas mileage in my 2001 6-cylinder Automatic Rodeo, from the 18-20 mpg range down to about 11-12 today. I'm not 100% sure if this was an
overnight thing, or something that happened over time (my wife is the primary driver of the vehicle). I've taken it to 2 mechanics, both of whom don't know the cause. Several people have mentioned the increased use of ethanol in gas today. I use Regular Gasoline all the time (87 octane). Couple of things I've tried:
- Only put 10 gallons of gas instead of a full tank - no effect. - Run the car without A/C (1 mechanic thought it may be some kind of drag the A/C system was putting on the engine(???) - no effect. - 1 tank of 93 octane gas - no effect.
I did notice that the car goes about .95 miles for every mile the odometer reads...so, that would do some damage to my perceived gas mileage, but it doesn't account for the roughly 40% drop I've noticed.
Has anyone else had this problem? Any ideas? Fuel additives, or more regular usage of higher octane gas?
Thanks!
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ErikwithaK wrote:

Might have it emissions tested that might tell something. Maybe a bad O2 sensor? Does it seem to run fine and have normal performance?
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I just got the annual inspection done 2 months ago, and had no emission problems.
I'm not much of a car guy, but I will mention the O2 sensor to my mechanic.
Haven't noticed really any other problems with the vehicle.
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I have an '01 Rode Sport that gets constantly about 16-18mpg. It dropped from about 20 a few months after I purchased it (brand new with 26 miles on it). Since then its been pretty steady.
One thing I did notice was my gas gauge started acting up funny a few months back...I ignored it and kept on driving the car as normal, keeping track of the millage. Recently the "Check Engine" light came on. I got it checked and it turns out my fuel sending unit is acting up. It doesn't affect performance, but it is a real pain since you don't really know how much gas exactly you have left in the tank. Once I get my tax return I'm gonna get it fixed, but that might be part of the problem, if not, I hope you find and post a resolution because i would like to get better gas millage. Heh.
BTW, my Rodeo Sport is a 3.2 4x4.
face=Arial size=2>...</FONT></DIV><FONT face=Arial size=2>&gt;I just got the annual inspection done 2 months ago, and had no<BR>&gt; emission problems.<BR>&gt; <BR>&gt; I'm not much of a car guy, but I will mention the O2 sensor to my<BR>&gt; mechanic.<BR>&gt; <BR>&gt; Haven't noticed really any other problems with the vehicle.<BR>&gt; <BR>&gt;</FONT></BODY></HTML> ------=
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Tire pressure does this to my 2002 Rodeo. Also, short trips drops my MPG.
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Well 1st thing you should know is that the Ethanol in your gas isn't going to change your mileage, Ethanol gives better power and burns cleaner than gas and is used in Top Fuel racing, they use 100% pure Ethanol that is about 115 octane.
Is this Rodeo a 4X4? What type of driving is being done, around town or freeway? With freeway driving you get better mileage than around town. Are the tires larger than the stock tires, if so it will mess with your odometer and mileage. Look at the filters; oil, fuel and air to see if they need to be changed... If none of this things are it then look at the spark plugs, how long has it been since you changed them???
Mark 97 Rodeo Automatic 4X4 94 Rodeo Manual 4X4

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Thanks for the info. The Rodeo is a 2-wheel drive. Most driving is around town. I just got new tires about 6 months ago (after I noticed the mpg problem), and I'm guessing they are slightly larger due to the .95 miles per mile calculation I did. Spark Plugs were just changed 2 months ago (with those damn 'boots' on them that jack up the price).
You are the first one who I've seen say Ethanol doesn't affect gas mileage. Both mechanics, and a family member were pointing an accusatory finger at it. I'll do a little research.
As to one other poster's comment about the fuel gauge problem - I have had my check engine light come on over the last 2-3 weeks, but it's intermittent - it turns off by itself, and comes back on a few days later. The mechanic couldn't pull any codes off of it, but he had it when the light was off. I didn't think the light was supposed to ever turn off by itself, so that in itself is a bit of a mystery to me.
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E85 you would see an aprox decrease of about 2% in milage (normal mileage 30mpg would be aprox 29.4mpg with E85), in the standard 10% blend we see you shouldn't see a drop.
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