One of my friends are selling me a set of wheels. They come with F:225/40/18 and R:255/35/18 and have about 70% left. I have a '98 GS300.
Are these tire sizes any good, assuming I do not race and like some comfort, or should I buy new tires (which is very expensive )

35r and 40r tires are different. 35 is shorter from rim to the edge of tire than 40. Remeber that your suspension starts with your tires. The bigger you go the smoother the ride quality. If you are looking for a smooth ride a bigger tire is in order. However, if you are going for a look and a sporty stance 35 or 40 is perfect. Mixing the two means you will have either a smaller tire on front than in back or vice versa. If you arent picky roll them for now and when they wear buy all 4 the same.

Thanks for your reply.

However, I thought that 225/40/18 and 255/35/18 are one combination. Both the sidewalls are 3.5 inch.

I do agree that bigger tires should give better comfort due to larger sidewall.

I run 18 inch rims on my GS, Fronts are 245/40/18 and the rears are 275/35/18. They are more uncomfortable than the standard 17inch and 16inch wheels. I don't agree with the bigger the tyre the more comfortable the ride, I believe and know from running 16, 17 and 18's on a GS that the bigger you go the smaller the profile and loss of comfort.

Exactly, the bigger the tire, not the size of the wheel. You have to run a smaller profile tire if you are going with a big wheel and consequently you get a rougher ride. Personally I think 18" looks sweet and as long as you don't skimp on tire quality you should be satisfied.