MPG decrease with new wheels

Tires were the ones that came on the rims. Michelin x green 215/45/17

 

I think you are going to be dissapointed with the improvement just going to LRR tires in the 18 inch size as opposed to going back to the 15 inch OEM wheels. The larger diameter wheel puts the weight of the rim and tire on a longer moment arm (technical term for laws of physics) and by doing so it increases the torque required to rotate the wheel. Hence more energy is consumed to move the car, especially starting out from a stopped position. I dont know which issue is making the biggest impact on your fuel effeciency, but I would suspect thet the larger wheels are responsible for at least 50% of your loss.

If you want to understand better, pick up a weight, say 2 or 3 lbs and hold your arm out fully extended, next tie that same weight to a 1 foot long stick and put it in your hand and hold it out just like before. You will feel the difference.

 

I think the OD of your new wheel/tire combo is somewhat larger. I put it in my spreadsheet the other day, but then something bad happened... Anyway, I believe it was a bit over 1% increase. You can check it out with online tire calculators.

So, you're actually going a bit further than the car claims. Also, the larger OD is lowering your gearing a bit, I think, helping with mpg.

All in all, bigger/wider non-LRR tires are going to hurt. Our stock tire is 215/45R17, Michelin Pilot, and it's about neck-and-neck for mpg, comparing with the 195/65R15 Michelin X-Ice (snow tires) I put on through winter.

(edited the above: the 195/65R15's are Michelin X-Ice)

 

Not totally true, I graduated from Engineering school in 1971, so my memory isn't the best, but there are different energy vectors involved with a rotating mass. One is outward, or thechnically the Correolus (spelling?) component. It in itself requires more energy to maintain the rotation of the body and that component is non recoverable. I suspect this is part of the slow down you are experiencing when coasting because this Correlous component is not liner it is exponential. It's been a long time since I studied the physical aspects of mechanics in engineering school, but just reading in other forums on here about guys trying the 17 inch wheels even with LRR tires they still find a loss in MPG as compared to the OEM 15 inch size. Again it's exponential, so 1 more inch over the 17 inch wheels is a significant increase in resistance to rotation and loss of the ability to recover that portion of the energy used to achieve it. Also the energy necessary to maintain it once you get up to speed.

 

You can expect zero mpg with 14 inch wheels as they wouldn't clear the brake caliper holders.

SCH-I535

 

You're not going to see a huge improvement with a good LRR 18" tire so just run the ones you have down and replace them with LRR ones later. It doesn't make sense economically to toss perfectly good tires. It's inevitable. You are going to suffer mpg loss when you upsize your wheels. Trust me.

 

I liked the handling and the fuel economy but they were too rough for my taste. They felt MUCH harder than the 205/50/17 EP422 I tried. Fuel economy between the two didn't seem much different, the ride was a lot better but handling took a nose dive. Damn trade offs.

 

The OE Ecopia tires are Ecopia EP20. They are different than the aftermarket EP422 which is better but still sloppy. I liked the EP20 better than the OE Yokohama S33Ds but mainly from a comfort standpoint.

The EP422 has good snow traction as does the PureContact. Think comfort vs. performance and you can make a better decision. The PureContact is the safer tire, particularly in wet conditions where it sticks like gangbusters!

 

Probably about equal. The PureContact may last longer because of speed rating difference and such. The DWS is a great tire. I assume you read the TireRack test reviews on both tires? Not the consumer reviews.

 

If you can handle a lil rougher ride then go with the PureContact. It's the better and safer tire.

 

Are you going to save any money ditching your free tyres and buying 18" LRR which may only give you marginal improvement in fuel economy?

 

Be aware that the total diameter of 17" wheels is slightly less than the 15" wheels. thus slightly less economy.