My experience is Running a19 front wheel gives optimistic speedo data. Re 18 inch wheels low profile tires are a must. example 110/80/18 on the rear.I think my Series C with a 19" front was approx. 10 mph fast at a true 70 mph. My 21" is spot on, based on a bicycle speedo I jury-rigged.
If the bicycle unit was right, a 19" reads 77 and an 18" reads 81 at a true 70, assuming similar tire aspect ratios, using circumference ratios.
The upside for me is bragging rights to fast riding speeds without the risk.