IMHO it is always good to have less unsprung weight to allow the suspension to react faster. In this scenario I believe what is going to be felt more is the increase in wheel diameter which is going to change your final drive ratio. I think you will definately notice the decrease in acceleration.
Steve

 

Do a power to weight ratio for your MINI and find out what your HP/LBS is. Once you do you will see how much weight you have to drop to get the feeling of one extra hp but, after that you have to recalulate because it changes. Less weight is always good for performance and gas mileage. Not comfort though because you start feeling everything on the road.

 

Anybody know what the new 17" Cross Spoke Wheels weight each?

 

A 215/40-17 Yokohama ADVAN Neova AD08 is only 21lbs and better gearing.

A 225/45-15 Hankook Ventus R-S3 is also 22lbs but you can compensate with lighter 15x8 wheels (lighter than any 17s), thats the best of both worlds ie: width, sticky tires, lighter weight and much better gearing...the holy graill!! LOL

 

Many of the weights of the OEM wheels are listed here:

http://www.mini2.com/forum/faq.php?f...factory_wheels

 

Given the significant weight advantage over 16X6.5" wheels, I assume that those who did so well in HS at SCCA Nationals last season ran on 15X5.5" wheels. Is that the case? And what size Hoosiers were they wearing?

This thread seems like a more likely source for an answer than the SCCA Solo H-Stock forum I posted the question on the other day. There have been 67 views there, but no responses.

 

If you're tracking the car, it'll make a difference. But unless you're a top driver, you and not the tires are going to be the biggest limiting factor. And if it's just a street car, don't worry about it. Go for the looks.

 

First, 3 pounds may be marginal, but if you have a logical choice w/out a trade off then of course go that way.

First, some points made here and there. The wheels are rotating mass, so it is generally understood the penalty is a multiple of the weight involved, particularly for acceleration. Here, all the extra weight is out at the tire (rather than in the wheel metal), so even worse. Since you are dealing with wheels and tires that in combo are in the 40 pound range, 3 pounds is getting material. If you use the oft quoted factors of 2 to 4x for rotating mass, at 4 wheels x 3pounds x 4, that is more than a full sized spare of weight. Anyone tracking would presumably remove any on board spare.

Second, it's unsprung weight as mentioned too. Why make the ride unnecessarily less responsive unless there is a benefit?

Third, more generally as to width, expect better braking. If you read car reviews in the magazines, a pretty consistent trend is wider tire options lead to shorter max stops. From personal experience an a 450HP car with 275's vs. 255 on stock wheels and fitments, I can confirm that directly too. Yes, run over many cycles, the rotors will be the likely limiting heat soak factor (assuming the fluid doesn't boil or the pads melt), but for stops 1-5, figure the stickiest tires with the biggest footprint will do it best. Might compromise acceleration as it gets heavy, but the brakes will have plenty of clamping power to stop it (in stops 1-5 anyway) as rapidly as it can stay stuck to the pavement. Since 5-10 feet can be the diff between accident and not, I'll take the stopping power trade off.

All that said, I prefer wider within stock wheel parameters. Thus on my 2010 S I got a few months ago, it was clear within a day or two the run flats had to go. I read this board up and down about max width and frankly found lots of confusion. My sense is it arises a lot from people not being clear on first gen vs. second gen fitment, and that second gen. apparently has a bit more room to play. Also, I can see from looking at TireRack some tires in the same nominal width are clearly wider at the mid-"bulge" point. So, I focused variously on tire weight, tire AND tread AND sidewall width, and found my answer. I settled at 225/45-17, though having noted I could have pulled off a pound or two if I went to 215's. Also disqualified a tire I am inclined to try (Conti Extreme) in that as I recall it was one of the few with a wider actual sidewall without any obvious tread side increase. Net, after circling around and around, I ended up back at tried and true Michelin PS2s in 225/45-17. And for anyone curious, that tire in Michelins has three sub choices. The Porsche fitment was lightest (and the mid priced one), so that's where I went.

For the record, ZERO rubbing or interference on a R56. Makes many of the posts I had read inconsistent with my practical experience, again suggesting close scrutiny on the factors I have mentioned above. No marks on the fender liners either over 1000+ miles now. Night and day difference from the big bucks, terrible feel OEM Dunlop run flats. AND, as I recall it was on the lower end of the spectrum for tire weights in the given size, which I now have on my list routinely to consider where there are choices. As expected, the (minor) upsize also cleaned up some of the speedo error. What was 3-4 miles optimistic at 60 is now about 2 per cell phone GPS.

 

For what it's worth, my Mini is an '06 cabriolet with every possible JCW option including the black spoke 18" wheels. I'm running Hankook Ventus V12 Evo's (absolutely great tires) in the 215/40-18 size. Absolutely no rubbing even when run hard with two people on board. (And I'm 6'1" and 265 lbs.)

They did rub, however, when I first installed them because I was running 15mm spacers. But after removing them, no problems.

 

The Continental ExtremeContact DW in a 215/45/17 are 20lbs. (1 lb more than the 205/45/17 Hankook Ventus V12 Evos) This might be a good compromise to the weight vs width issue. Although these Contis are a bit more money than the Hankooks ($30 per tire), they seem to have better overall ratings according to the Tirerack surveys.

 

I do not think that the weight of the tire is the issue but the overall diameter of the tire.

I went from the stock 205/45/17 tire size with the run flats back in November to the Yokohama Avid Envigors in 215/45/17 and increased the diameter of the wheel by .3 inches

What affects the performance and Mpgs is the fact that more mass has been moved away from the center of rotation. Its called the moment of Inertia, and it has been increased.

When I put the new tires on I immediately felt the difference, and it did feel slower

Since then I have noticed an astonishing DROP in mileage! I could not fathom what was going on, at first I also thought that it was the extra weight of the tires, they are also 3 lbs heavier than the OEM wheels. But after a little research I believe that it is the increased Moment of Inertia.

I did the exact same commute last winter on the OEM wheels with 16k miles on them and averaged 30-33 mpg. Where as this year with bran new tires I've been averaging 24-26 mpg
Around town its been closer to 20mpg

I really want to figure this out because with Gas going up in price again that average mpg isn't good enough anymore.

 

That's way too much of a drop to be from the change in weight or diameter.

 

It was literally from one day to the next. When I got the tires put on I was in Worcester and then drove to Boston all highway, 65 Mph cruise control at night and only got it up to 34mpg over that hour long ride.

Ive had this discussion with others and they have said the causes could be winter mix of gas, the cold temps, higher viscosity of the air due to the lower temps (aero) cold operating temps, ect...
but I don't understand how last year I did not have this issue.

Believe me I would have brought it up.

 

If you want real good mileage, get the skinniest tires and rims that will fit, and air them up rock hard. Your cornering will suffer, but then you're choosing between economy and performance, everything is a compromise.

Dave

 

peter314: I agree with cristo above. Although your point about rotational axis is correct, a .3" increase in tire diameter only moves the tread .15" farther out from the rotational axis. I don't see how that little movement could noticeably reduce gas mileage. Three additional pounds per tire would probably have some effect on mpg, but I think it would be smaller than you're experiencing.

Maybe the winter gas change-over is playing a role. Covering the same route at the same speed, I've been getting about 1.5 to 2.0 mpg less in two different cars since about mid-November.

Others: I'm still waiting for someone to answer my question (post #31 above) on whether SCCA HS autocross hot-shoes use 15" wheels.

 

I think it's quite unlikely that a 10mm wider tire would decrease mileage by 20%. There must be other factors for the drastic change in fuel efficiency.