So, I finally switched from the OEM 16" S-Winders with the runflats to 17" Rota RBs with General's UHP 215/45/17.

Handling is much better, but my - the MPG is down about 10% !!!

The combo weighs about the same, the height of the car is almost the same (OEM tires are 195/55/16) - is that only because of the width of the tires ???

Thanks

 

Maybe overall height is shorter on the new wheel and tire combo . This could cause a drop in mileage . Same as changing gear ratios .

 

According to this tire calculator
http://www.discounttiredirect.com/di...foTireMath.jsp
your new wheel/tire combo is .18in larger than OEM.
I'm not exactly sure how that would equate as far as gas mileage goes.

 

thanks, don't think that .18 inch should make any difference...

 

What pressures are you running in old tires and in new tires?

 

Are the run flats all season? Switching from AS to UHP tire and adding another 20mm to tread width could have a lot to do with it. More grip generally = worse gas mileage.

Also, what do the RBs wind up weighing in that size? I would imagine any difference in wheel weight would be offset by the lighter tires, but...

Definitely check you tire pressure as already recommended as well.

 

the width def. will account for that.

worth it in my opinion tho!

 

Tire diameter really not that different.

Better handling usually means more fun to drive and less gentle with the cornering and throttle which usually means lower mpg.

Tire pressures about 37-38 psi is good for mpg.

215/45-17 is a size with more rolling resistance vs stock 16 tire, slightly higher tire pressure helps but not too high or comfort will be a little less.

Be gentle on the brakes and throttle and recheck mpg later.

 

you're probably just doing more spirited driving to break in the new wheels

 

The tire size is 1% larger, so you'll have 1% fewer miles on the same tank of gas.

As for the other 9%:

The 17" wheels move the rotational mass of the wheel further out. Even if it weighs the same as a 16" wheel, it takes more effort/hp/gas to spin it. Maybe not much, but it's a fact of physics for wheels that have the same design.

The wider tires also likely weigh more than the OEM's (even though they were runflats). You'd have to get the exact specs to be sure. Same effect as listed above.

Higher rolling resistance/friction (as others have mentioned) from the grippier and wider tires. There's a reason those hybrids have skinny, rock hard tires to help get better mpg figures.

So you have a few factors working against you in the mpg department. But you also have the benefit of better looks, and much better handling. I think it's a fair tradeoff.

If you drive 12,000 miles a year, at 25 mpg, you'll use 480 gallons. At 27 mpg, 444 gallons. 36 gallon difference x $3.50 per gallon = an extra $126 per year in fuel costs, or about $.34 per day. Not so bad when you look at it that way I hope.

 

everybody, thanks for the advice !

I'm running 38-39 PSI on the tires and they are pretty light, I remember picking them on tirerack from that reason too (18 lbs just like the wheels as far as I remember).

I'm trying not to be throttle-happy , but yes, you are right - the handling and the looks are definitely worth it, just didn't expect that much of mpg decrease, maybe after they break in it will go up again...

 

I suffered the same 10% MPG loss on my Passat when I went from the OE 16" wheels to 17s. Revs/mile were the same, or close enough to not impact speedo accuracy. So, I'm not surprised that you'd suffer the same going to 17s on your MC. Question for the assembled group: Has anyone checked to see the penalty for going from the OE 15s to 16s?

 

Measured gas mileage won't change at all regardless of tire diameter... because the odometer doesn't know you've changed tires and the odometer is how you measure miles. Now, if you measure true miles (mileage markers, whatever) then your gas mileage should improve by 1%...except basic driving inconsistencies between tanks (# of stoplights, % of miles in 6th gear, etc...) vary way more than that so I doubt you'd ever see a measurable difference...even you could measure true miles.

 

Some observed mpg data. Has been repeatable for me.

Stock 15" holey rims and stock all season 175/65-15 Conti tires: 29 mpg

Aftermarket light 17x7 rims with Max Performance 215/45-17 tires: 26 mpg

Not so easy to drive the 17" wheel/tire combo easy to preserve good mpg, requires effort.

These are the best mpg numbers I can generate under best traffic conditions.

Manual transmission and no cruise control. All mpg calculated by filling the tank the same way and reading odometer without correction on tire size.

 

I'd be interested to know this too.

 

prepared to be geeked out.

a reason why your gas mileage might go down is due to the moment of inertia.

not for the faint of heart:

http://en.wikipedia.org/wiki/Moment_of_inertia

basically, inertia is the force required to spin an object such as your tire/wheel combo about an axis, like your wheel hub.

Inertia = k (a constant) x mass x Radius ^ 2.

Note that the radius (distance from axis to center of mass) is squared. This has far greater effect on the equation than relatively small changes in mass.

Example: Your 15" tire/wheel package weighs 25lbs. You replace it with 17's that weigh less, let's say 20lbs, and have the exact same OD. You think you did good right? Saved some weight. Here's the math:

We will assume that the center of mass of the tire/wheel package is about 15" from the center of the wheel. This is the distance from the spinning axis to where most of the weight sits. For the 17" package, we'll do the same.

Equation for 15":
Inertia (force required by engine to spin wheels) = k (same for both so we can ignore for this example) x 25lbs x 15" ^ 2

I = 5625

Equation for 17":
I = 20lbs x 17" ^ 2
I = 5780

As you can see, it will require more power from the engine to spin up that 17" set, even though it weighs 20% less. Obviously, I'm oversimplifying and making a few assumptions about the center of mass, but it demonstrates the principal. As the center of mass goes outward, the Inertia goes up quite a bit.

Think of that science experiment in gradeschool when you spin someone on an office chair while they are holding some dumbells to their chest. As soon as they push their arms out with the weights, the speed of the spinning decreases drastically. Imagine how much harder you'd have to spin the chair to maintain the initial spinning speed with weights fully extended.

-Michael

 

A few flaws with your thinking: the radius of a 15" wheel and tire is not 15", and it's not accurate to assume the center of mass is at the extreme outer edge of the wheel. Also need to consider a 2"larger wheel means 2" less tire sidewall, which in the runflats is substantial. So the inertia may not be significantly different if you go from heavy stock wheels and runflats to a better setup.

 

Well I did mention I oversimplified things. The center of mass of the tire is probably near it's center. The center of mass of the rim is probably just inside the inner 'lip' of the rim. Average them together and the outer edge of the wheel is a good guess.

The actual value of the numbers is not that significant since I applied the logic to both sizes.

Try plugging in two sizes with the same weights. Notice what happens to the inertia of the larger. It is not so insignificant.

In any case, it was just something to consider.

 

It's the width! going wider you loose, MPG that is. Hopefully the fun factor makes up for it!
I've own several Jeeps and getting bigger tires Is a must and the Jeep forums talk about this extensively. Running a skinny tire will net better MPG.

I have the 175/65-15 on my MINI and average 41 MPG so when it's time for replacement I will think smiles per gallon when the fat tires go on.

 

Nice explanation superc00per. While I understand the explanation, I'm sort of simplistic about all this, meaning I don't really need to understand why there's a loss in fuel economy, I just want to know how bad it is so I can decide if it's worth it.

One funny thing I noticed during my test drives was that the 17" optional wheel/tire combo rode much better than the OE 15s or 16s. Anybody notice this? I actually wondered if the 17s were underinflated, but the handling was fine.

Anybody got info on the 15-16 conversion?

 

This may just add to the confusion, but last month I swapped out my 15" stock holies with winter tires (195/60R-15s) to my summer set-up (205/50R-16 Hankook Ventus HRIIs on MB Drifters) and immediately noticed a 2-3 mpg improvement in gas mileage.

OD is almost identical and I know the holies are a bit lighter, so it's a bit puzzling. My driving has been considerably more spirited on the summer set-up. When it was cold out I didn't let the car sit at idle to warm up or do any other similarly fuel-unfriendly practice.

The only possibilities I can come up with are: (1) the snow tires must really be a lot more inefficient on dry pavement; (2) the warmer (10-20 deg) spring temps are helping; and/or (3) the local petrol providers just happened to switch to their summer blends about the same time as my wheel swap.