Folks,
 
I'm about to go to staggered tyres and have settled on 215/40-17 for the front on 7.5 inch rims but I can't decide between 235/40-17 or 245/40-17 (maybe even 245/35-17) on 8.5 inch rims at the back. The car is a 90 SW20 Turbo running at standard ride height and is a daily driver, not a track car.
 
My concern is that the 245 tyres at the rear will unbalance the handling by being too wide, because at the moment with 215 tyres all round there is already a touch of initial understeer when turning in and I think wider rears will increase this tendency. Once turned in with power on, the car settles and transitions smoothly but earlier than I'd like into oversteer.
 
I'd appreciate any views from people who have run with a setup like this to help me with my decision.

I have the same width wheels on my car and have run both tyre sizes. The 235/40 will be closer in rolling diameter to factory, therefore your speedo will be read more accurately than with a 245/40.
In saying that, I have a 245/40 at the moment, handling difference is probably not even noticeable between the 235 and 245, especially for daily use, but running staggered tyres should balance out the understeer/oversteer.
 
I'd suggest once you fit your new tyres, if you're not comfortable with the way it handles get a good wheel alignment at proper suspension specialist, talk to them about what you're feeling with the car and get the alignment adjusted to suit your preferences.

If you run a 215 & 245, your running the factory stagger. (Same stagger to the 195/225 on the 92+ Rims)
You shouldn't have an issue with that.
 
If you want to stay road legal, I'm pretty sure a 235 on the rear is your only option.

Is road legal going to matter with 17" rims? Didn't the first 90 model have 14" rims standard? Which means 16"x7"/16"x7.5" is the largest rim you can go legally.

Is that still the rule. 2" large dia 1 inch wider?

i think there is a new rule in place regarding that Joel.  I think (and i could be wrong) that it's now changed to the rolling diameter of the wheel - including tires.  So you can run a larger diameter rim as long at the total rolling diameter is the same diameter ie 17x8 215/40 +35 vs 18x8 215/35 +35.
 
Is this correct?
 

yeah you're right, there are new rules. 7% larger overall diameter than what is shown on your tyre placard. 
 
BUT it also says "Replacement of wheels where the rim width exceeds the largest wheel combination specified by the manufacturer by greater than 25 mm."
 
rim width 1" wider than the tyre placard. does a 90 have 6.5" rears?
 
http://www.rms.nsw.gov.au/registration/downloads/vsi/vsi6.pdf
 
Does that mean they no longer care about rim diameter, but now they only really care about rim width?

Possibly.  That's a good point.  I guess this is where offset comes into play - maybe.  
 

Thanks for the info guys. I think I'll play it safe and go with the 235 rears.
 
The width question is interesting, as these rims are still a bit on the narrow side (but OK) for these tyres. Looks like there may be a set of deliberately (or accidentally) conflicting requirements here.
 
I also wonder whether wider TRD rims that could be bought from Toyota at the time would count as being manufacturer options for setting the width baseline.
 

Looks like I'm fine here in Canberra. ACT works according to the National Code of Practice (NCOP11). Within that VSB14 covers modifications. For wheels and tyres it allows overall diameter no more than 15mm larger than standard, maximum tyre width no more than 30% wider than the widest standard tyre and rim width to match that 30% wider tyre. Track must not be increased by more than 25mm and it is stated that this means offsets can't change by more than 12.5mm. This makes offsets the real limiting factor.
 
Based on the stock 195 front/225 rear tyres mentioned above that would allow maximum front rim width 9.5 inches and rear width 11.0 inches, so I'm well within the limits. For track, my offsets are also OK.
 
Hope this helps others.

Hi,
Does anyone know what the legal tire / rim width for a 1990 mr2 sw20 is? Im running the stock 14"s (195 front and 205 rear) and I'm hoping to upgrade into some 17s going 205/40/17 front and 235/40/17 rear and Ive been struggling to figure out if the rears are legal, i know its a trivial question. i want to keep the tires/rims legal for the build I'm doing on the car.
Cheers