I have Lincoln continental 16" rims with 205-60r-16s and my revolutions per mile difference from the stock 650R15s is less than 10. No clearance problems on the front, but had to remove a chunk of spring seat on the back.

I am using Kumho Power stars (758?) on my older Volvos. 165-80R-15 on my PV444 and 185-80R-15 on my 220 wagon. The 185s are NLA, at least through Tire Barn, but I think you can still get the 165s. These are excellent tires for the money! The tires have been on the Amazon wagon for 2+ years and are wearing very well, they handle and brake well too.
The PV has been running on the Kumhos for over a year and again, for the price the wear and handling can't be faulted!
If you go with Coker you may want to sit down before getting the pricing.

Cheers!
OK

You didn't mention whether you have a sedan or wagon.

I have 195-75 X 15 on the original rims on my '68 122 wagon. I've compared the original spare which was used once - they are very close to the 6.85 X 15 that were original equipment.

Please note that this is for a wagon. I think the sedans had smaller tires (closer to the modern 165 X 15 already mentioned).

If you want to maintain a spedo/odometer that works you are limited to 165x80 on 4.5" or 5" wide rims or 185x70 on 5.5" rims.

--
Three 164's, Two 144's, One 142 & a partridge in a pear tree.

www.vulcantires.com still carries 2 different 165/80SR15 tires (Kuhmo Power Star 758 @$50.00 ea. and Federal 657 @$53.00 each).
If you have at least a 5.5J x 15 inch wheel, a good size would be 185/70R15 (very expensive however). A good alternate size would be a 195/65R15 which would only have a -1.6% odometer difference. The 185/65R15 would have a -3.6% diference and is close to 1/one inch shorter.
A good place to check on tire combinations and odometer differences, is to go to www.powerdog.com and click on the tire size calculator.

It sounds like your fronts are the original size. The link below will help you convert to modern sizes.

http://clubs.hemmings.com/clubsites/chevylist/tech/tire_size_conversion.htm

The equivalent should be P165/80R15, BUT, that's not that easy to find. The closest common size seems to be P185/65R15. It's 20mm wider, so there is some chance it could be too wide, depending on the actual width of your wheels.

So...that's the theoretical side. I'm also interested in hearing about the experiences of others, as I'm in the market for tires, too.

By the way, the link below is excellent for calculation speedometer error associated with changing tire sizes:

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