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Author
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Topic: Fuel economy question
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IIGood
Moderator Posts: 2542
From: Arnold, MD, USA
Registered: Jun 99
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posted 08-26-2001 02:32 PM
   
Actually, maybe 2 questions.After I upgraded to the 17" wheels, I noticed my gas mileage dropped slightly. When the 15" wheels were on, I was getting about 290 miles for every ~13 gallons. Once the 17s went on, it went down sometimes to 260 miles for about the same amount. Could the wheels be a cause? Also, the mileage has fluctuated. Note I said it went down *sometimes.* Though I've never seen the 290 mark since then (except for long trips, then I'd see more), I have seen some good numbers...today when I filled up, I put 13.1 gallons in after 283 miles. The mileage has gone up and down all summer. I don't think I drive it any harder than I did with the 15s, but could it also be due to the air conditioner being ran? I've noticed the lower mileage has come through the summer months...usually on the hotter weeks. Any thoughts? ------------------
Frank S.----MCA Member 40390
'99 Mustang coupe
'77 Ghia--"II Good"
'76 Cobra--project
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JoeC
Journeyman Posts: 91
From: Springfield, MA.
Registered: Jul 2000
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posted 08-26-2001 08:00 PM
I would think that with larger, wider, and stickier rubber you'd have more rolling resistance and thus your mileage would go down? When you went to the larger rims, did you adjust the apect ratio of the tires to correspond to the increase in rim size? I'm not sure what the formula is but I know I've read that if you don't properly account for both it will offset your speedometer readings (perhaps another M&M'er can comment from a point of authority on this?) which could also impact mileage as well. Using the A/C will definitely cause your mileage to go down too. Hope this helps.......... ------------------
Joe C.
2000 V6 Convertible [This message has been edited by JoeC (edited 08-26-2001).]
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kcode
Gearhead Posts: 2559
From: alvaton,ky,usa Suburb of Bowling Green, M&M #79, MCA #29208
Registered: Jun 99
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posted 08-26-2001 11:32 PM
Frank,
What is the overall height of each tire, 15 vs 17. Calculate the diameter or circumference and get the revolutions per mile to get a comparison to see how close your odometer is.Mike
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tempo1993
Gearhead Posts: 583
From: CT, the home of high taxes
Registered: Jul 2000
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posted 08-27-2001 06:38 AM
The aspect ratio forumla is for every inch of rim size you add, subtract 10 off the aspect ratio, re: 195/70/15 would now be 195/50/17  ~Scott~
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IIGood
Moderator Posts: 2542
From: Arnold, MD, USA
Registered: Jun 99
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posted 08-27-2001 04:01 PM
Steve calculated the whole speedometer/tire size thing out for me when I originally mentioned the upgrade a couple months ago. He said the tires would only throw off my speed reading by half a mile per hour. ie, if the speedo reads 55, I'm *actually* going 54.6xx mph.I'm guessing the A/C has something to do with it, like Joe said. I guess I'll really see in the weeks to come, as it starts to cool off around here. ------------------
Frank S.----MCA Member 40390
'99 Mustang coupe
'77 Ghia--"II Good"
'76 Cobra--project
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71ragtop
Gearhead Posts: 2459
From: Atlanta, GA
Registered: May 99
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posted 08-27-2001 09:33 PM
I would also have guessed that it was the wheel change. if you are actually traveling slower than the speedo says, would that mean that your mileage is more or less than actual? Sounds like a test question to me.Mike
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