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Old 02-16-2010, 08:34 PM   #1
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new tires with worse mpg???

hi all! first time posting here and im glad i found this site!

i was wondering if i could get some info about my recent tire swap. I have a 2007 subaru wrx. The OEM tires were bridgestone potenza re92 215/45/zr17 and I was getting around 340-390 miles per tank with them. I just replaced them with continental DWS 225/45/zr17 and have averaged around 290 since. The stock tires weighed 21 lbs while the conti's are in at 19.7 so i figured i was in good shape (less rotating mass and all)... not so! does anyone know why this may be??? corrections??? the only thing i know i have to do is check the tire pressure, but im pretty sure their at sidewall level indicators still...

thanks!

ps.. maybe im just hyper sensitive about mpg since my wife got a prius and is getting 50 plus!
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Old 02-16-2010, 08:49 PM   #2
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alot of people say the oem tires offer the greatest mpg's. it makes it easier for them to get a higher epa chart. id check the tire compounds. im willing to bet the contis are much softer. a good rule of thumb is to go by tread life. a 60k warrenty tire is usually gonna have a much harder compound than say a 36k mile tire.
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Old 02-17-2010, 12:17 AM   #3
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alot of people say the oem tires offer the greatest mpg's. it makes it easier for them to get a higher epa chart. id check the tire compounds. im willing to bet the contis are much softer. a good rule of thumb is to go by tread life. a 60k warrenty tire is usually gonna have a much harder compound than say a 36k mile tire.
This CAN be true to an extent but at the end of the day, when you buy tires, just attempt to figure out whether they're LRR tires or not because there are tires with a tread life in the 600 but are anything but LRR tires. OP, have you already tried inflating to sidewall?
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Old 02-17-2010, 03:17 AM   #4
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There is almost zero measured data about rolling resistance in tires. Buying tires is a crapshoot.

Your new tires are (if the numbering is accurate, which it may not be) slightly wider and slightly taller than your old ones. That would mean you're going slightly farther than a mile for every mile your car measures. That would offset your perceived MPG loss, but not significantly for the tire sizes in question...probably not even measurably.

It's unlikley that the tire installer inflated your tires to their sidewall maximum. They probably inflated them to the pressure recommended by the car manufacturer. Definitely check. Maximum inflation will make a big difference, if it rides/handles/wears ok (and if not then you need to back it off until it does).

Weight doesn't make that much difference, and that includes rotating weight. There are a lot of myths about that stuff. The effect of rolling resistance on fuel economy is many orders of magnitude larger than that of dead or rotating weight. Rolling resistance comes from inflation, tire construction/design, sidewall compound, tread compound, tread design, size, sidewall height, width, total diameter, and probably a few other things I forgot to mention.
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Old 02-17-2010, 10:58 AM   #5
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hmmm.. thanks everyone. I know i need to check the tire pressure, but weather in rhode island hasn't cooperated lately! im sure the dealership probably inflated to car specs and not tire.

i'm not sure how to check the compound of the tire, but the tread depth is 10/32 on either tire and they are both rated to 50,000 miles. Revs are 843 (oem) and 828 (conti) and like i said, the weight is 1-2 lbs lighter on the conti's as well. I thought i did my homework but am just hearing about lrr tires for the first time!

I still want to wait for few more tanks and nicer weather to be to judgmental though, and the conti's do perform MUCH better in all conditions. Thanks for all the advice and i look forward to learning more...
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Old 02-17-2010, 12:56 PM   #6
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get a gas log going. its the easiest way plus you can help others.
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Old 02-17-2010, 01:07 PM   #7
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What are the speed rating and load rating of the tires (new and old)?

If you changed ratings, that can affect sidewall stiffness (amongst other things).

(A "Z" rating is a category of ratings - W, Y, and one other which I forget. It should show on your sidewall, after the tire size.)

Continentals are frequently a more mushy, passenger-ride type of tire, but the speed and load ratings will help.

-BC
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Old 02-17-2010, 01:18 PM   #8
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The OEM tires were bridgestone potenza re92 215/45/zr17
I just replaced them with continental DWS 225/45/zr17
Hi Mightyatom,

I see two reasons for worser FE:
1) as Cow already said: your new tires are wider=bigger rolling resistance
2) and even IF same quality/rolling resistance than the old tires, the new ones with new thread will give worser FE because of the higher (newer) thread. This because there's more rubber to deform with every single rotation.
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Old 02-17-2010, 01:27 PM   #9
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It will generally get better the more you ride on the new tires. They will wear down and you will get a little better FE, but dont expect that number to be big. It might just be 1-2 MPG better by the end of the life of the tires.
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Old 02-17-2010, 07:09 PM   #10
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Hi Mightyatom,

I see two reasons for worser FE:
1) as Cow already said: your new tires are wider=bigger rolling resistance
2) and even IF same quality/rolling resistance than the old tires, the new ones with new thread will give worser FE because of the higher (newer) thread. This because there's more rubber to deform with every single rotation.
good point, WTH did the op put on wider tires!? That surely is the reason for his messed up his mpg.
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