255/35/18 vs. 275/40/17 ..Which covers more gap?

I'm an idiot when it comes to wheels and tire..My car currently has an Eibach spring kit and I will ge getting new wheels and tires soon (i have some cobra 17 right now but they are in bad shape so will either get a new set of cobras or saleens). I want a combination that will make the gap smaller. I know the 18" wheels are bigger but the tires are smaller then the 17" ones so I'm not sure.
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To be honest, the difference is not enough to worry about. Just get the size you perfer.

I had a 265-70-17 tire next to a 275-60-20 tire and they were almost exactly the same.