Effect of 7.5 width all the way around?
 

So I wanted to know if anyone knows the effect of running 7.5" width all around with maybe a spacer in the rear? Can't remember if its the tire size or width that will effect the handling.

I would imagine this would make the car very tail happy, and I do not recommend this.

So it's the width of the wheel that keeps the back end more stable?

The rear tires wear faster on these and 911s because of engine Trans power delivery
Less tire isn't gong to improve things except mpg and drifting. Look at the BRZ with its pryis tires
I believe the correct square setup is to put same rear width up front

Yes, along with the front and rear sway bar thickness.

Reducing the rear tire width would reduce rear grip and tend towards more oversteer. Since the stock setup tends towards understeer, a small decrease in rear tire width would tune out the understeer - much in the same way that going to a wider front (like a 225) does the same thing.

Most Boxster race cars run with the rear tires/wheels also in the front (square set up since all tires are the same size all around), this gives the front a lot more grip but then the rear wants to slide around a LOT. Adjustable racing sway bars are used to adjust the car back to neutral handling (or whatever degree of understeer/oversteer the driver prefers).

I would not recommend running any square set up (same size fronts or rears all around) unless the car has adjustable sway bars. Now, don't get me wrong, you are probably not going to die - at least not driving around town or on the freeway at normal speeds. But the car will be very tail happy near and at the limit and the stock sways do not have any means to adjust the handling back to something more neutral. The car could be be difficult to control if the rear end starts to step out in sporting driving or an emergency situation unless you have experience driving a car with such a setup.

Thanks for all the input

A slightly different viewpoint:

No problem at all if you size your tires properly. The original 986 came with 16" wheels that were 6.5 and 7.5 width if I remember right. Running a daily driver on 205/225 tires works just fine and is pretty economical. It is also an excellent choice for rain or snow tires because a narrower tire will be less prone to hydroplane.

1st Gen 987 Boxsters and Caymans also were available with these wheel widths in 17" running 205/235 tires.

Im glad this came up.
Why does it matter if your wheel is 7.5 vs 8 since you are going to put the proper tire on anyway?
for example if I were running a 235/17 tire on the rear, the tire is the same but the wheel is .5 wider.
shouldnt the tire have the same amount of contact on the ground?

Please school me, I need to know

Certain tire sizes work best on a few different rim widths but it is limited. From this chart a RE-11 235/40/17 is best suited for an 8-9.5 rim width. A narrower rim might work but you won't get the best performance from the tire. A 225/45/17 fits great on a 7.5 wide rim. That is what I would choose to run on a 7.5" rim width.

A 245/45/17 will also fit on a 7.5" wide rim because it is a taller tire with more sidewall.

Bridgestone****Potenza RE-11

This isn't about smaller wheels on the rear but a great article about tire/wheel sizing.

How to PROPERLY select and size TIRES for PERFORMANCE > MotoIQ - Automotive Tech, Project Cars, Performance & Motorsports