This was actually a topic of conversation over the summer. There seem to be two schools of thought on the matter:
1) The stock-size replacement would be a 235/75 series radial, preferrably with a white-wall side. Any decent-name all-season modern radial should serve you well. Some people here will give you the names of what they use.
The other school (which I belong to) says:
2) You can't put too much rubber under an early Toro; therefore, 255/70 series tires, preferrably with raised white letters (RWL) are the largest tires that will fit under a stock Toro on the stock wheels, and are therefore a good bet. Two models that I know are happily in use are the General XP2000 II's (Sparky) and the Dunlop GT Qualifiers (me), though there are others brands out there.
There is also a lunatic fringe in the early Toro crowd, who says:
3) more More _MORE_! One guy has put 275/60 series radials on steel wheels that he had widened to 8" (I have a JPG of this car to share with those who would like to see a '66 Toro with an ATTITUDE!), but this required raising the car's suspension about 1". There is also talk of 17" alloy wheels, 50-series tires, etc., but so far that has been talk only. We'll have to see what is done with the Heretic... ;)
Submitted by: Bob Barry