On Zero's website they describe the formula for the 70mph commuting number. it is

S and DS 70 mph Highway Commuting Range = 1 / [0.5/(70 mph steady-state range) + 0.5/(EPA UDDS range)]

since they give you the UDDS range and the 70mph commuting range, you can then determine what their 70mph rage is. here is an example for the S ZF9

1 / (((1 / 63) - (0.5 / 114)) / 0.5) = 43.5272727

where 63 is the commuting range, and 114 is the UDDS range for the S ZF9. So from that the range at 70mph at a nice and constant speed, probably in a tuck without a headwind, should be around 43Miles.

43miles at 70mph is 0.61hours

the nominal capacity of the battery is 7.3kWh (also on their spec page on their website)

So estimated power draw at 70mph is 7.3 / 0.61 = 12kW which is 16hp which seems about right to me.

I believe goes up with the square of speed, so at 75mph you would need about

(12 * (75^2)) / (70^2) = 13.7755102

about 14kW which would then reduce your range to

7.3/14 * 75 = 39miles.

But you also have to remeber that as power to the motor increases, the efficiency probably decreases, so 39miles at 75mph is probably a little higher than you should actually get (at constant speed, in a tuck, on a flat road, without a head wind).

-ryan