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I just noticed that my gas mileage has dropped substantially (I'm talking about 4-5 mpg) the last couple of tanks, yet the only thing I've done recently is get new tires installed. I know they're inflated proberly because I checked it myself. Could the fact that they're 235/60 R15 vs 215/70/15 do it? Or could it be something else altogether. I've done a search and I couldn't find anything correlating tire size to fuel economy so I figured I'd throw it out here and see if anybody had any ideas. Thanks!
 

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I wouldn't think it would be THAT big of a difference, but wider tires would negatifely affect your mileage due to more friction. Also if they're stickier tires they're going to have more friction (and thus loss of mileage) too.
 

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I'm having the same issue with my car. I don't have new tires but ever since it's gotten colder out my mileage has dropped by about 3 to 4 mpg's, any ideas?

--Dan
 

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The Parts Guy
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Do either of you guys let your cars idle to warm up in the mornings? Sitting still with the engine idling will eat up some of your gas mileage.

-Rod
 

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Arcadia, check your tire pressure when it's cold. When the temperature drops, the air will contract and the pressure will go down. That make the tires no longer properly inflated.

Rod's right too. If you let the car idle to warm up in the mornings, it will eat some of your gas too, depending on how long you let it sit. Or if you do like me when I leave work and let it warm up for about 30 seconds to a minute, then drive it really hard so that the engine warms up to get the heat on faster.
 

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I am having the same problem too. I changed my 225-60-16 tires with 235-55-16s and I check my air pressure monthly and always ok. My mpg seems to have dropped 4-5mpg. I have been told that I should change the o2 sensors, but at about $65 each I want to be sure. I have 86k orig miles on my o2 sensors.

W.Cup
 

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Last year when it got really really cold, my 93 T-bird dropped gas mileage big time!

Tire change will have some effect on gas mileage, but not 4-5 mpg. Shouldn't be more than 1 mpg at most I would think going from a 215/70 to a 235/60. Remember your mileage will also be off a little, so you will need to compensate for that too.
 

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Confirmed V6 Nut
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The gasoline companies change their formulas and additives for cold weather. I always get a big drop in milage during the winter but it comes back in spring when the formulas are changed for summer driving.
 

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Discussion Starter #9
Thanks for the replies. I don't usually let the car idle to warm it up since, I do live in SF and it's not really that cold. I also don't really want to change the O2 senors if I don't have to. How long is the life on them usually? I have 113k miles on the car and the O2 sensors have never been changed. Can I test them myself?
 

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Id say think bout changing ure front O2s sometime because i kno their more important.... i had gas mileage problems b4 then i changed my front 02s and completed alota maintenance work and the gas m improved alot. just make sure ure car is totally set in other areas like fuel filter air filter MAF etc... before u jump to the costly 02 sensors.
 

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Remember that an engine runs at its best and as efficient as possible when its nice and toasty. When you are driving during winter, that cold *** air tends to cool down your engine and trans. Idling before driving, tire pressure, and the fact its cold out explains the 4-5 mpg off.
 

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My mpg dropped off about 6 or 8 months ago. My car gets just as bad mileage now that it did in june or july.

W.Cup
 

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Thanks for the replies, i just replaced both my O2's today, cleaned the MAF and adjusted the tire pressure. I'll post again to let you know of any improvements
 

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Discussion Starter #14
Thanks! I figure I'll clean my air filter and MAF this weekend (I just changed my fuel filter a few months ago unless it can become clogged that fast...) and run a tank of gas and see if there's any difference. If not I'll look into changing my O2 sensors.
 

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I've been expieriencing the same thing over the last couple tanks, I usually use Amoco Gold because that's what runs the best with my Apten chip. I had been getting between 290 and 330 miles per tank mixed hwy/city, now it's down to about 240 to 260. Also have any of you guys with the 4.6 been noticing a surge of about 400 rpm every 10 seconds or so.
 

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No, I really can't say I have. Well, no, my car DOES do it when I first turn it on cold. But I've also got a pump somewhere going bad. My power sterring pump I think. It makes this AWFUL groaning noise for a minute or so until it warms up slightly. Sometimes a bit longer. I noticed the other day that it was really hard to steer while that was happening. The car also REALLY wants to get up and go. With my brakes being crap, I have to really push my foot down to keep the car steady. Anyways, if I let it idle, after a short bit at around 1200, it keeps dropping down to around 700-800, then back up to around 1200 and back down. Vrooom Vroom Vroom. You get the idea.
 

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I checked on the Super Coupe web site and ran the tire calculator. It shows a difference of about .75 inches in diameter between your new tires and your old tires with your new tires being smaller. If your checking your millage against your odometer it probably won't make much difference but if your checking against distance between places, then it will show a drop. Note: your speedo is now off, but the super coupe site has a calulator to figure out the right gear to correct it.
 

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my did the opposite, i got better mileage in the cold than i did in the summer, that could be becase i had a new fuel pump, filter and a new EEC installed, and put air in a tire that was low on air pressure.
 

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I know even done here in Florida my gas has dropped in mpg's, it used to be 300-350 now it's about 200-250 and it's been cold here too, if you call 45-50 cold. But hey it's florida.
 
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