The FR in Firestone's model FR721 is just part of its model name. The tires were introduced in the 1970s to replace the Firestone 500 model that was recalled for massive failures in the form of tread separation (seems history repeats itself). When Firestone revised its internal nomenclature about 15 years ago, most of their radials sprouted names beginning with FR, presumably for "Firestone Radial," but it's not part of the tire size or anything universal like that. The "721" part, for those who don't remember the advertising, stood for the way the steel cords that make up the belt were wrapped... seven [strands]... around two... wrapped by one. I honestly don't know if they are still made that way today. FYI, the FR721s in the 235/75R15 size that fit most '64-on Imperials (and some earlier ones) all the way to the last NYBs actually have a 1-5/8" whitewall, not inch-and-a-half. Not a big difference, but it's a pretty substantial whitewall that looks quite nice on these cars. This size is also exclusively sold as an "XL" (extra load) tire which is quite suitably to our heavy cars. The 721 was the factory-installed tire on NYBs when you ordered the optional "wider white wall" larger-size tire. I still have an original 721 from my '78 NYB Salon (in the garage for posterity... don't think I'd run a car on 23-year-old tires). But I use the FR721s on both my '67 Crown and '78 NYB and have been entirely happy with the results. The ones on the Imp are approaching 10 years old and I will probably replace them soon just to be safe, despite their having well under 10,000 miles on them and showing no signs of wear or sidewall cracking. You can see a picture of the tire on the IML website's wheel page... just scroll down to 1978 and that's my Salon's wheel and tire! It's located at http://imperialclub.com/Repair/Wheels/covers.htm if you wanna look. Chris in LA Dieter wrote: > I forgot to ask you what are the letters FR stand for . Do you > know? The R could be radial? The F for the manufacturer ? I am just > guessing here. Sorry I did not think of it sooner, right after I hit the > reply key did I think of it .