What's the widest tire I can put on a 78 2+2? It has the snowflake wheels. What are those, 6 inches wide? It came with 205/65-15's. Can I go to 60's or 55's?

 

you are refering to the aspect ratio of the tire, which is the height of the tire, not the width. The "205" is the width. You could probably go with 215's or maybe 225's

 

I was actually asking two questions. First, about the maximum width. Secondly, whether low profile tires would cause any problems. I could have worded it better.

 

A lower aspect will make your speedo read wrong, but I think that's about it. Might affect your fuel economy a little since your engine will be running a little faster, but it should make it a little quicker off the line. I bought my Z with aftermarket wheels on it already, so I'm not sure how wide a tire you can put on the stock rims. I'm sure somebody on here will help ya out.
Good luck!

 

With a +10mm offset (moves center of tire toward center of car) you can run a 225 width front and rear - maybe even a 235 in the rear. I am currently running 225/50-16's on all four corners. With coil-overs and some work on the rear fender well lip you can get to 235's in the front and 255's in the rear. If you stick with the stock '0' offset you really shouldn't go wider than 215's. If you lower the aspect ration you'll screw up the speedometer/odometer unless you change wheel diameter at the same time.
IE: 195/70-14 = 205/60-15 = 215/50-16 = 225/40-17 when it comes to diameter/circumference. The lower the aspect ratio the better your cornering characteristics, the harder your ride, and worse for drag racing. The higher the aspect ration the worse your handling, the softer your ride, and the better for drag racing.