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I think you mean 255/60R15 and 225/70R15. The difference is mainly in the width. They are both near the same height. A 255 may fit alright on a 78. I'm sure someone with a 78 will chime in.
225/70R15 is standard on '78 Vettes. 255/60R15 is optional.
In general, the 70 series will ride better, the 60 series will handle better. While that's a generality, the real difference is in the make tire, tread design and tread compound. Tires are not created equal. Many, many threads on the subject abound on this site, with just as many opinions.
...the original tires were either the P225-70R-15 Goodyear PolySteel Radial in either blackwall or solid block white letter style. This tire was a one year design tire as late '78/early '79 the size of the lettering became larger. The P255-60R-15 original tire was an outline white letter tire designated as "Goodyear GT Radial".
There's a lot of good make tires out there. To be honest, the best tire technology today is put into the 16" and larger sizes. The sizes our Vettes use is essentially obsolete technology when compared to the newer stuff.
What it generally comes down to now for our cars is price and the look you want. The BFG Radial T/A seems to have the look most seem to want, but they're in short supply according to other threads here.
I have T/A's on my '78 and think they're excellent tires. On my '69 I have Coopers and think they're excellent as well.
Take your time, look at the tire manufacturer websites and check the ratings, the appearance, the availablilty, etc. Then you can make up for mind what to go after.
That tire isn't made anymore and I don't believe anyone is reproducing them either. I guess there's a possibility of finding an original set, but remember they're thirty years old if you do and who knows what kind of storage conditions they've endured. Even if you found a pristine NOS set, I would consider them show only and not for driving on.
You know what you do after you install new tires? you get your car alligned.
i did not. after 2 summers i Got 2 buy 4 new tires...but before i have to install a new control valve and a couple of tie-rods. live n learn.