What is the benefit of going with a 7.5" rim over a 7"? Just more tire on the ground? Is that necessarily better?

Does going with 7.5" affect auto-x classes?

I should preface that the end objective would be handling and acceleration as opposed to aesthetics.

 

tire width. more than a 215 and a 7" doesn't support the sidewalls properly.

 

MINI stock rims are 17x7 only +48mm offset.
If you autocross in stock classes then you are limited to this size, No 17x8 or 17x7.5" rims. Offset needs to be within plus or minus 1/4" of stock.

If you autocross in other modified classes the rules will be different for each class whether Street Touring, Street Performance, or Street Modified.

Read the rules posted at SCCA.com or go to the Autocross forums on NAM for more information, it is all there.

17x7 is a very common rim size and usually owners will put tires that are about 205 or 215mm wide on them.

The same tires can be mounted on rims that are slightly wider like 7.5 or 8".

As the rims get wider there is less clearance inside and out for the suspension parts and the wheel inner arches. The same tire that fit with a stock sized rim is now wider due to the rim being wider. Once you drop your suspension you have much less clearance or wider rims are closer to rubbing.

1/2" wider is not usually a problem so most owners will use 17x7.5 and not have any problems. 17x8 will be more of concern depending on the wheel offset. +35mm means the offset is less and the tire is moved more to the outer side of the car and might rub the inner wheel arch plastic while +48mm with wider 17x8 wheel will be moved closer inside (and out) so rubbing the rear trailing arm or aftermarket swaybar is at risk.

Given the same 215/45-17 tire mounted on a wider rim you get-
Usually more weight from the wider rim- not good
Wider tread section as the tire is stretched on a wider rim- might increase contact patch a little with the right tire pressure
Stress and stretch the tire if it is at it's limits for fitment on that rim width-
Might affect handling or wear a little if driven hard using tires that are too narrow for wider rims- always match the tire and rim to the right sizes.

Some examples (mostly BF Goodrich g-Force KDW2)
205/40-17 fits 7-8" rim, perfect for 7.5" rim, 8.1" section width for 7.5" rim
205/45-17 fits 6.5-7.5" rim, perfect for 7" rim, 8.1" section width
215/40-17 fits 7-8.5" rim, perfect for 7.5" rim, 8.7" section width
215/45-17 fits 7-8" rim, perfect for 7" rim, 8.5" section width
225/45-17 fits 7-8.5" rim, perfect for 7.5" rim, 8.9" section width
225/40-17 fits 7.5-9" rim, perfect for 8" rim, 9.4" section width (race tire)
235/40-17 fits 8-9.5" rim, perfect for 8.5" rim, 9.4" section width

Section width-
http://www.tirerack.com/tires/tirete...p#sectionwidth
For every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".
So when using a wider rim than perfect fit width you get 0.2" wider section width for every 0.5" wider rim width.

Tread width-
Although a better measure I didn't have all the numbers for all the sizes.
Couple of examples
215/45-17 7" rim, 8.5" section width, tread width 7.6"
225/45-17 7.5" rim, 8.9" section width, tread width 7.9"

Handling/Acceleration-
A little better for any tire that matches the wider rim given the same sidewall height and tire model.
More weight is generally not good for acceleration/braking or handling but if you can find a wider rim for the same or less weight then that is good but usually more expensive.

mpg-
Wider tires and larger contact patch will help with handling, accelerating and braking but increase rolling resistance and could reduce mpg by a little maybe 2 mpg. Higher tire pressures or at least correct tire pressure will help to keep mpg as high as possible without making the ride quality compromised.

 

Dayum...YOU RULE!!!!

Just one question. Was the info regarding section width backwards for the 215/40 vs 215/45? Why would the 215/45 be better for the 7 and 215/40 for the 7.5?

 

Awesome post, minihune. What size tire comes stock on an 07 MCS?

 

For MCS of all years, for 17x7 rims you get 205/45-17 tires.

No the information for that size is correct due to 40 vs 45 series sidewall and given 215mm width.

They are both close. The 215mm and 40 series tire can take the 7.5mm rim better but will fit the 7" rim as well. 40 series offers better handling but not as much comfort for a given tire. The tire you choose can make a big difference in ride comfort and noise. Note the 215/40 tire is likely to be lighter than the 215/45-17 tire by about 2-3 lbs.