Just been doing some thinking. Hard to figure what the right +1 size for tires is. My Ninety Eight came stock with 205/70-15'*. With the Touring Sedan or the performance option, they offered two different sizes:
1) 215/60-16
2) 225/60-16

Now the stock tire has a diameter is 26.3 inches. if I upgrade to either of the other two, I end up with:
1) 215-60-16 - Diameter - 26.2 - Speedo reads .5% fast. basically a wash.
2) 225/60-16 - Diamteter - 26.6 - Speed reads 1.3% slow. Little worse.

The big tire option talked about a lot is 245/55-16.
Diameter 26.6 - Speedo reads 1.3% slow.

Played around a bit and looked at other options:

225/55-16 - Diameter 25.7 - Speedo reads 2.1% fast
235/60-16 - Diameter 27.1 - Speedo reads 3.1% slow.

Point of the exercise? Really doesn't seem to be one "right" size. Any of the options listed seemingly would work just fine. I seriously doubt that Olds made any differences in speedos, etc to make up for a 1.3% speed difference between the two stock sizes.

Done with navel gazing.

Are you actully considering upgrading? If so, I suggest going up to a 16" wheel. Let'* get more specifics from you before debating sizes.

You start with the wheel. What size do you want to focus on?

Have two sets of wheel already. Touring Sedan 16'* and Bonneville Crosslaces. Ran them last year. The TS wheels need restoration in a big way, while the crosslaces are very nice. Seem much lighter than the TS wheels too. Have to weight the TS wheels sometime.

E-mail tirerack.com with the same question and they will answer in an hour. I did, and they did.

Torque Stars are significantly heavier than crosslace.

Just go for whatever size gets you as close to 1% as possible. If it doesn't, play with the width and profile until it does. Wider will cost you fuel and gain you traction IN GENERAL.

Wider will also cost you more, especially when you get into odd sizes. My 245/55/16'* aren't very common or easy to find.

Now that your set on 16'*, it'* time to pick a tire and see if they even offer your odd size needs.

Here is a really cool site that lets you figure out your own plus one tire and wheel sizing.

http://www.1010tires.com/tiresizecalculator.asp

Just enter the numbers for your car'* stock tire size in the appropriate spaces and check 3 other tire sizes against them at one time. It'* free quick and easy.
And since you are in Canada, the prices for the tires they suggest will be close to accurate.

http://www.miata.net/garage/tirecalcold.html

That one lets you actually SEE the differences, either from the side or the rear.