cdnpont wrote:

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70's

Taller

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 x2 

60 are not tall enough

x3 on the 70's. What do you have on your car now?  Tire size calculator

https://tiresize.com/comparison/

It depends on what you want as well.  If you want the hot rod look and have wide rims then you probably want as wide a tire as your wheels and your car will take.  However if you want the original look then you probably want as close to factory stock as you can get.  Also you don't want tires that are too wide for your rims as that can cause other problems as the tire distorts to fit the rim width.

Remember, in modern tire sizes the first number represents the tire tread width in mm, the second number represents the ratio of sidewall to tire width, and of course the third is wheel bead diameter, in inches.

For example, a P245/60R15 breaks down as follows:

P = passenger car

245 means the tread width is 245 mm

60 means that the sidewall height is 60% of the tread width, which would be 147 mm

R = radial tire

15 = 15 inch bead diameter (often referred to as rim size)

I've noticed over the years that there is variance between manufacturers (or even tire series) in actual sizes compared to the number spec, like the 245 is probably not exactly 245 mm, could be more or less.  Same with the sidewall height.

A couple of things to consider is that lower sidewall height can lead to better handling but harsher ride, but unless you use a larger diameter wheel to maintain the same overall tire height (and thus overall tire circumference), it will throw your speedometer reading off (smaller diameter tire will cause it to read higher than actual speed) and will also probably not look right on your car either.  A tire size calculator will help you get close to the overall tire height that was specified by OEM.

Anyhow, I apologize if I went too basic in the explanation, I've just noticed in my job over the years (where I purchased many tires for a fleet of vehicles) that sometimes there are details that folks don't quite understand but are afraid to ask.  I'm certainly not an expert, and I don't have a big Pontiac, so the advice of the others who do have that experience will be more useful than mine.

Best of luck!

Wondering what happened with the purchase ?

 A little research showed that the Parisienne came stock with tires that were 27" in height. You could get 255/60/15 that would maintain the 27" height and give you the width you want or you could get 215/70/15 to maintain the 27" height. To my mind the 215/70/ 15 is too little tire for that car. Anyway if the 255s fit in the wheel well thats what I'd be getting. At least one poster says thats what he runs on the back of his B Body.  That 245/60/15 is 26.5 "in height and is too short.