Hello Winfried,
welcome at Stridaforum 
Out of curiosity; could you please tell me where did you find this information?
Of course can an 18 inch EVO go up to 45 km/h, maybe even up to 60 km/h - very similar to a 16" wheeled EVO…
But perhaps YOU will get in trouble if you have to pedal the required ~ 129 revolutions of the crank per minute?
For a 16 inch EVO you just need to reach a cadence (= revolutions of the crank per minute) of ~ 144.
It’s a simple calculation:
45 km/h are 45 000 m within one hour…
that means you have to travel 750 m per minute.
Now you will need the gear ratios of each bike, you can find them here:
Gear ratios information about different models of Strida
The last (or highest) gear will be of interest for high speed, please note also that the 2nd gear of the two speed Stridas (SD, MAS) is identical to the 3rd gear of the three speed Strida (the EVO).
The next step is to multiply the gear ratio with the tyre circumference - the result is the development in meters, that’s the achieved distance per one revolution of the crank;
Amuro Lee did that already for us - thanks once more!
The (rounded) numbers for the last (or highest) gear of Strida EVO, Mas or SD are
for 16 inch ~ 5,2 m and
for 18 inch ~ 5,8 m
(Disregarded are here different tyre circumferences due to different tyre widths.)
Last step is the division of the distance per minute through the achieved distance per crank revolution to get the number of the required revolutions for a certain speed (in this example 45 km/h).
So, you will need to pedal with a 16 inch EVO (750:5,2 =) ~ 144 crank revolutions per minute and with an 18 inch EVO (750:5,8 = ) ~ 129 rev/min, doing that for a whole hour you would in fact ride with a speed of 45 km/h.
Source: http://sheldonbrown.com/gloss_ca-g.html#cadence
Solving this theoretical problem from the vice-versa direction, it would look like that:
Assuming that you pedal with a (far more) realistic cadence of 90 revolutions per minute:
Development multiplied with the number of revolutions per minute multiplied with 60 (for one hour).
For 16 inch EVO: 5,2 m x 90 x 60 min = 28 080 m/h = 28 km/h
For 18 inch EVO: 5,8 m x 90 x 60 min = 31 320 m/h = 31 km/h
Said in different words: You will - at equal cadence - ride with an 18 inch Strida about 11,5 % faster than with an 16 inch wheeled one.
In theory…
Yeah, sure…
The 3-speed Strida
Regarding speed I couldn’t notice any difference to my 18 inch Schlumpf powered two-speed Stridas, there’s just one more gear between high and low gear of the speed drive.