new tires with worse mpg??? - Page 2 - Fuelly Forums

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Old 02-17-2010, 07:50 PM   #11
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I don't agree that wider necessarily means worse fuel economy. In fact, the issue is rather unclear. Different theories support both wider and narrower as being better, and data is severely lacking.

The OP went from 215mm to 225mm wide. If they're not the same brand they could very well be exactly the same width; 10mm is probably well within the margin of difference between one manufacturer and the next. Even if they are the same brand and model and it's actually 10mm wider, that's still only 5% additional width...nowhere near enough to make a measurable effect on FE.

The width, even if wider is less efficient, is nowhere near responsible for the fuel economy loss.
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Old 02-18-2010, 07:44 AM   #12
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i'll have to start a gas log...

the only difference in size is 215 to 225. Both are 45zr/17 91W

I went a little wider for more dry weather traction and feel. As stated, the tires do that very well, but with a 4-5 decrease in mpg. Similar applications on other wrx's have not had the same result. The only plausible theories i have so far are;

1. check tire pressure for proper inflation

2. wait at least 500-1000 miles for break in


beyond that there seems to be conflicting opinions???
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Old 02-18-2010, 08:05 AM   #13
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A 225/45R17 is 10mm wider and 8mm taller than a 215/45R17. Speedometer/odometer change is 1.4%.

Break-in might help, but the big help will be comparing them when they're worn to your old tires when they were worn, or comparing them now to your old tires when they were new.

Beyond that it's almost certainly just a matter of the model of tires you bought generally have more rolling resistance than the model you had before.
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Old 02-18-2010, 10:36 AM   #14
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When did you get the new tires? Did you get them in the fall? If so, have you considered the colder weather, higher ethanol content in the gas (most stations have a "winter mix"), possibly snow/road conditions not being the same?

That seems like a VERY significant loss to just be tire related.
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Old 02-18-2010, 01:34 PM   #15
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Quote:
Originally Posted by ben98gs View Post
When did you get the new tires? Did you get them in the fall? If so, have you considered the colder weather, higher ethanol content in the gas (most stations have a "winter mix"), possibly snow/road conditions not being the same?

That seems like a VERY significant loss to just be tire related.
they were purchased and installed about a month ago...

i find it hard to believe a tire's resistance can produce 20% decrease in fe as well, which is why i started asking ?'s in the first place... this is puzzling as well because i've driven wrx's since 2003 and never had such a dramatic difference with tires used... oh well
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Old 02-18-2010, 01:36 PM   #16
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If it helps...I had Bridgestone RE92 tyres (205/65-15size )on my car (not a WRX though) as OEM and they were very hard and lasted forever so I am guessing the rolling resistance was low but had no real data to back that up.

The new tyres may well be intended more for grip not LRR hence the difference in fuel used.
Also did you have an alignment done when the new tyres were fitted?
It may have given you a different set up from the previous alignment.

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Old 02-19-2010, 09:56 AM   #17
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Also keep in mind that new tires, of the same size, will have greater diameter. This will clearly throw off your odometer. Your real mileage may not be changing as much as you think....
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Old 02-21-2010, 11:25 AM   #18
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That is true, but will 1.4% or so mess up MPG's that much? I wouldnt think that that would be the cause, maybe that with the ethenol in gas and other factors maybe?
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