Werm9

I just got a set of 19" wheels a few months ago and should I get the tire size changed in the computer for optimal performance? Could it be the reason why I am only getting 21MPG (250 miles on a 13gal tank) on my 2.4L 4 cyl engline?

__________________

2001 Grand AM 2.4L 15,000 Miles

drdisque

what size tires do you have?

the rim size is largely useless.

__________________
Dr. Disque - 2007 Mazda 6-S 5 Door

Werm9

i went from a 215/70/15 to a 225/40/18

__________________

2001 Grand AM 2.4L 15,000 Miles

drdisque

well, your car shouldn't have had 215/70/15's on it, the correct 15" size is 215/60/15 (which is probably what you meant)

Your new tires are 4.1% smaller than your 215/60/15's. That means your speedo reads 4.1% faster than you're actually going and makes you get slightly worse gas mileage since you turn higher RPM on the freeway. However, your recent downturn in gas mileage is probably due to it being winter, cars get worse gas mileage in the winter and winter fuel that the stations sell is less efficient.

Going up to 225/45/18 on your next set of tires will eliminate this discrepancy, if you want to.

__________________
Dr. Disque - 2007 Mazda 6-S 5 Door

Werm9

thank you. considering i just bought the tires i have know i'll have to sufferr getting only 20 mpg/ em.

__________________

2001 Grand AM 2.4L 15,000 Miles

sub006

Often the bigger wheels are much heavier than the stock alloys they replace. Although the new tires are smaller, it doesn't make as much difference because rubber and fabric are relatively lighter.

Weigh one of your old wheel/tire combos (if you still have one, how about the spare as long as it's the same metal and full-size). Then remove and weigh one of the new ones. Even five pounds of increased rotating, unsprung weight per wheel can have a noticeable negative effect on acceleration, braking and fuel economy.

Even though they use incredibly light alloy and even carbon fiber wheels, this is one reason Indy cars still use 15" rims and Formula 1 cars use 13"!