The optimal tire sizes for the rim sizes you have selected is:
18 X 8.5 : 235/40-18 which is only .08" taller than stock front
18X10 : 295/30-18 which is only .07" shorter than stock rear
You will get no closer to stock ride height than with the sizes I have listed and the tire sizes listed will fit nicely on your rims.

 

Can you confidently say all that without even knowing the offset of the wheels he has? Maybe you are familiar with his wheels. I'd agree as far as what tire size is good for those wheel widths, but if you want to know how they'll fit on a Supra you need more information.

For example, how would those tires fit on the Supra if the wheel offset was say +25mm?

Getting a 295/30 - 18 on back would require exactly the optimum offset or you'll have to mod fenders or wheel wells.

 

Man just get the 285/35/18 and be done with it. They will fit just fine. They fit perfect on mine and my buddies car.

The front you can even roll a 245/35/18 which will also be fine.

 

please check the mkiv faq section for Steve Jarvis's wheel/tire fitment guide.

and search.

 

go with 285/35/18 for the rears. and 245/40/18 for the fronts. given that you have correct offsets, those will be fine.

 

Man just get the 285/35/18 and be done with it. They will fit just fine. They fit perfect on mine and my buddies car.

The front you can even roll a 245/35/18 which will also be fine.

:agreed: i run those same sizes but in 19's, you will be fine with the right offset

 

The tire sizes I recommended to Celica91st are the sizes that will put him as close to the stock ride height as possible. Yes, his wheel offset in the rear must be right or he'll have to roll/trim his rear fender which is no big deal. As for using 285/35-18's, sure they'll fit, but are much taller than stock.