Bigger wheels and tires = worse mpg?

[05silvergt4me]

Hello all. I just got a set of 18" Sparco DS1 with some Kuhmo ASX- 225-40-18. Since I've put them on my mileage seems to have dropped about 2 mpg. Im guessing heavier wheels and wider tires maybe?? Haven't noticed this on previous cars though...

[Shawn06SpecB]

yes.

 

they are heavier and harder for you car to turn, plus the weight is further from the hub making it even harder to turn. called the "moment of inertia" in any physics/engineering books, but i bet they look sweet!

[GTkansan]

The wider tire is likely a big factor, also, if the overall tire diameter went up (a 225-40- is taller than the stock isnt it), then your aero drag can change as well because of alteration of the flow characteristics (similar to lower riding springs increasing highway mileage).

[Maddie08]

Hmm... the larger circumference should translate as a higher final drive ratio thus increasing the mpg if not all the time certainly on the highway.

[05silvergt4me]

I was kind in a rut. I bought the car summer tires only. They were terrible in the rain. So I found these 18's for a good deal and they look sweet. It just suck about the gas mileage though.It's bad enough I average 22-23mpg, now around 20...ugh.

[JoeFromPA]

Maddie08 - That's mostly a myth. It CAN help, yes, but it truly depends on every application. Whereas going with a larger wheel/tire will push weight further away from the hub (if not weigh more as well). Also, the tire is usually the heaviest part. So the heaviest part is also moving away from the hub....just really increases the weight of that "gear".

 

225/40/18's have a total height of ~547mm

 

215/45/17 have a total height of ~529mm

 

So you've increased the height of the gear by ~28mm, a little over one inch. And made it heavier.

 

It can make, in effect, a taller final drive ratio. But it's pretty difficult to overcome everything else it does, unless it's a low-friction tire of low weight on a lightweight rim

 

Joe

[sactojesse]

Some of the perceived drop in mpg is likely due to the fact that your speedometer and odometer are now under-reporting your mph and miles traveled due to the larger than stock diameter. My calculations indicate that the difference in rolling diameter is approximately 2 percent with 225/40R18 at 25.1 inches and 215/45R17 at 24.6 inches.

[05silvergt4me]

I'll check my mileage with a calculator when I fill up.

[cfdrumr]

you get 20? I wish i got that

 

I get 14.7 when i drive like an old lady

[05silvergt4me]

well I just filled up, and some how the computer is still right. I got about 20.3mpg. And that is mostly driving like a grandma. Yeah I mess around when the roads are snowy....

We get that crappy reformulated gas here in winter though, which makes your mileage drop around 2mpg.