35 to 38 mph is rather slow not? Is there a difference between off-road and road tires? Like I have Michelin S1 110/80R10 and 130/70R10 and we cruise at 47mph...
47mph is ~78km/h, no stock CT70 of the 6V era is likely to reach that speed, even going downhill with a gale-force tailwind. 42mph is quite healthy for a bone-stocker.
That said, the STs have some differences from the US-spec CTs. One such difference is a higher peak hp number, at 9,000rpm (the CT version reached peak hp at 8,000rpm). Even though both models were fitted with the same carburetor, the ST spec's a larger main jet. This leads me to believe that they also used a different camshaft, such as the SL70 grind ( sold here as the "fast road cam"). Increasing the peak hp rpm by 1000, which a cam change can do, would account for both the hp & mph (km/h) differences.