SuperCleanLGT Posted March 25, 2009 Share Posted March 25, 2009 After taking my stock wheels/snow tires off this week, I put on some 18s. Since then the MPGs have declined. Has anyone else experienced this before? I have had different wheels/tires on every other car/truck I've owned and haven't had this problem. Thanks for the insight!!! Link to comment Share on other sites More sharing options...
Jerboa113 Posted March 25, 2009 Share Posted March 25, 2009 could it be that your tires make your speedometer and odometer read differently thus changing what your car considers a mile for its calculation? could also be rotating mass/weight of the rims+tires. "The penalty good men pay for indifference to public affairs is to be ruled by evil men." - Plato Link to comment Share on other sites More sharing options...
Underdog Posted March 25, 2009 Share Posted March 25, 2009 How much has it dropped and how many tanks have you gone through? The Crimson Dynamo Link to comment Share on other sites More sharing options...
SuperCleanLGT Posted March 25, 2009 Author Share Posted March 25, 2009 The OD on the tires is the same as the winter set up. I believe the weight of the new 18s is less than the stock 17s, but I have yet to find concrete documentation. Thank you, Judd Link to comment Share on other sites More sharing options...
SuperCleanLGT Posted March 25, 2009 Author Share Posted March 25, 2009 I've been though a tank and change, and go to the same gas station (Hess crap 93). I noticed a 3mpg decrease (since the wheel/tire change). I drive very conservatively 90% of the time and obtain the same mpg every tank within 3/10 of 1 mpg. Link to comment Share on other sites More sharing options...
mwiener2 Posted March 25, 2009 Share Posted March 25, 2009 maybe they switched gas mix on you My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons Link to comment Share on other sites More sharing options...
05GT Guru Posted March 25, 2009 Share Posted March 25, 2009 So you are calculating your own mpg or are you just going off the display in the car? Link to comment Share on other sites More sharing options...
Underdog Posted March 25, 2009 Share Posted March 25, 2009 I've been though a tank and change, and go to the same gas station (Hess crap 93). I noticed a 3mpg decrease (since the wheel/tire change). I drive very conservatively 90% of the time and obtain the same mpg every tank within 3/10 of 1 mpg. Hmmm... that is a very tight deviation.. +/- .3mpg if I'm reading you correctly. I fill up from the same pump and drive the exact same route for my daily commute, and usually ~120mi of highway on the weekend, and I see a variation of +/- 2.x mpg over the last 20,000 miles. The Crimson Dynamo Link to comment Share on other sites More sharing options...
SuperCleanLGT Posted March 25, 2009 Author Share Posted March 25, 2009 I calculate my own, and log what the cpu says. They usually differ by 1mpg. It is weird but it is very consistant tank by tank like I said usually about 1/3 mpg. Link to comment Share on other sites More sharing options...
Scooter Posted March 25, 2009 Share Posted March 25, 2009 Are your 18's inflated properly? Not really sure why you would see that big of a difference if the OD of the tires are the same. I'm sure your 18's have a wider contact patch and probably have more rolling resistance than the 17" snows, but that shouldn't account for a 3mpg difference. Seek first to understand, then to be understood. In other words: SEARCH before you post! Link to comment Share on other sites More sharing options...
adidasoutback Posted March 25, 2009 Share Posted March 25, 2009 I put 18's on the catch is the width, the 16's I had where OEM which where only 6.5" wide the 18 are 8" wide. They have more contact area with the road and hence more friction on the road and more power required to move them. Id say its normal Link to comment Share on other sites More sharing options...
SuperCleanLGT Posted March 25, 2009 Author Share Posted March 25, 2009 The 18's are slightly wider. They were extremely over inflated when I recieved them (48 PSI per tire). I adjusted them to 45 fr 43 r. Snows were 42 fr 40 r Link to comment Share on other sites More sharing options...
mwiener2 Posted March 25, 2009 Share Posted March 25, 2009 i wouldn't call 48 from 45 extremely overinflated My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons Link to comment Share on other sites More sharing options...
BAC5.2 Posted March 25, 2009 Share Posted March 25, 2009 Are you driving more aggressively now that you don't have to suffer snow tires anymore? [URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard Link to comment Share on other sites More sharing options...
slimjoe Posted March 25, 2009 Share Posted March 25, 2009 It probably has to do with the tires (compound and width) more than anything else. What did you go from to what? When I switch from 215/45/17 A/S (Contiextremes) to 225/45/17 (Dunlop Starspec Z1s), I notice an approximate 10% drop in fuel economy (about 2mpg). It happened last year and it happened again just now when I switched. Link to comment Share on other sites More sharing options...
sicklyscott Posted March 25, 2009 Share Posted March 25, 2009 I'd have to go with mass on this one, just because they weigh the same doesn't mean the leverage isn't different. On an 18 inch wheel and tire combo the weight is distributed further out to the circumference than on an equivalent 17. Because of that it takes more force to move the wheel. Link to comment Share on other sites More sharing options...
Gire Posted March 25, 2009 Share Posted March 25, 2009 Are you driving more aggressively now that you don't have to suffer snow tires anymore? This would be my guess. Putting your cool wheels back on makes you want to go faster! Link to comment Share on other sites More sharing options...
RJS5689 Posted March 26, 2009 Share Posted March 26, 2009 Are your 18's inflated properly? Not really sure why you would see that big of a difference if the OD of the tires are the same. I'm sure your 18's have a wider contact patch and probably have more rolling resistance than the 17" snows, but that shouldn't account for a 3mpg difference. X2, I went from heavy stock spec.b 18" with factory 215/45/18 tires to a fairly light set of 17" wheels with 205/55/17 snow tires. I didn't really notice much of an increase in fuel economy. I'll do another check when I switch back to summer wheels. Link to comment Share on other sites More sharing options...
NutBucket Posted March 26, 2009 Share Posted March 26, 2009 It probably has to do with the tires (compound and width) more than anything else. What did you go from to what? When I switch from 215/45/17 A/S (Contiextremes) to 225/45/17 (Dunlop Starspec Z1s), I notice an approximate 10% drop in fuel economy (about 2mpg). It happened last year and it happened again just now when I switched. This is my belief as well. Sticky tires = crappy mileage, generally speaking;) Link to comment Share on other sites More sharing options...
djsnoops Posted March 26, 2009 Share Posted March 26, 2009 This is my belief as well. Sticky tires = crappy mileage, generally speaking;) i hope not lol. i just purchased dunlops z1s star specs too, but not 18s. got a set of 235/40/17s Link to comment Share on other sites More sharing options...
TPLGT Posted March 26, 2009 Share Posted March 26, 2009 i wouldn't call 48 from 45 extremely overinflated Ummmm. What are you guys running then? I run 35/33.....now I am in FL with no snow but that is perfect for me. I would certainly not call them UNDER inflated. Rehab is for quitters. Link to comment Share on other sites More sharing options...
NutBucket Posted March 26, 2009 Share Posted March 26, 2009 I run 37 all around. Seems to work well. Link to comment Share on other sites More sharing options...
stanjk3 Posted March 26, 2009 Share Posted March 26, 2009 What tires do you have? Some are heavier than others. Friends don't let friends drink cheap beer. Link to comment Share on other sites More sharing options...
rtbrjason Posted March 26, 2009 Share Posted March 26, 2009 X2, I went from heavy stock spec.b 18" with factory 215/45/18 tires to a fairly light set of 17" wheels with 205/55/17 snow tires. I didn't really notice much of an increase in fuel economy. I'll do another check when I switch back to summer wheels. Did you notice any performance increase with the lighter wheels? I can't believe how heavy the stock 08 spec b wheels are. I'm wanting to get a fairly light set of 17s myself, and hope to see some improved braking, acceleration and maybe suspension wear. Link to comment Share on other sites More sharing options...
SuperCleanLGT Posted March 26, 2009 Author Share Posted March 26, 2009 I believe the drop in MPG has to be from a warmer weather compound and wider patch. I am actually driving conservatively (as usuall) to see what mpg difference is. The tires are Hankook Ventus Sport K104 225/35ZR-18. The snows are Hankook Ice Bears in 205/50/17. I was thinking about it and am not too concerned but was just currious about the mpg Link to comment Share on other sites More sharing options...
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