i've got a 96 3.8l with stock rims. it had 205/70r15 on the back when i got it and 215/65r15 or 215/60r15's on front i don't remember which it is but my question is on the rear, i have some 225/70r15's sitting around at work will these fit the stock rim? they would be a little wider right?

 

Yes a little wider and also a little taller. because you are going from a 65 to a 70

 

that would give you about .8 inch more wideness,, BUT the aspect ratio "70" combined with the "225" would make your sidewall overall an inch taller. I myself would not do it. I hate talll sidewalls. But if that wideness is worth it to you then go for it. But then again those 225/70's almost seem like small truck tires.
and i have mach 1's OEM. SO they are 17x8's. Thats a stock size for the performance car. Im not sure for sixxer wheels. Id avoid those tires unless you REALLLLYYY HAVE TO! but just my opinion, sorry ive been beating around the bush.

 

The back ones are already 70's they are 205/70r15's. I kinda like the thicker taller tire look in the back. So the 225/70r15's would be the same hight just a little wider right?

 

they would be wider and taller. the 70 is an aspect ratio. Its the heighth but not a definite number. As it increases with the width. Where thats a 70, its height is 70% of the width or something like that. But it will be thicker, and it will be taller too. So you get a win win situation there. I dont think it would throw ur speedo off. But you like that thick tall look, so go for it! But as an example, my car has 245/45 r17. its just a TAD litttllle bit shorter than the stock size. but much wider.

 

The first number in the tire size is the width in millimeters. The second number is the aspect ratio, as mentioned, which is a percentage of the first number. The third of course is the rim diameter.

A 225 would be 22.5 centimeters, or 8.86 inches wide. Then, with a 70% aspect ratio, the sidewall would be 6.2 inches tall.

Compare that will the tire currently installed. 205 is 8.06 inches wide, with a 70% aspect ratio makes the sidewall 5.65 inches tall.

That is a lot of sidewall for my tastes but to each his own.