A 255 80R22.5 tire would be similar to a n 8.25x20 in diameter but it will be wider and could cause clearance problems on the tie rods. A 9R22.5 is just about a direct replacement for an 8.25x20. A 235 80R22.5 is about the smallest you can get in a 22.5, but it will be smaller in diameter than an 8.25x20 and will cut down your speed. I don't recommend the metric low profile tires on an old vehicle. The tried and proven 9R22.5 and 10R22.5 tires they came with 60 years ago seem to work better. Trying to convert inches to mm in tires will really not produce any available tire sizes.