Didnt realise that tyres are available in a size 275/65/r15. at ebay site
http://www.ebay.com.au/itm/Brand-New-265-70R15-275-65R15-255-70R15-High-Quality-From-ETyreStore-/170729510834#
This will change the tyre circumference 8.5% eg adds 8.5 kph to 100kph at the same revs. Means at an actual 100kph you drop about 240 rpm down from 3100 to say 2860.
As you say Ian you need the power to keep you from slowing on the hills and when towing with tall a gear you will need to change up earlier. But if the Sc is installed this woldnt happen.
Just as a matter of interest....when new the trike had a 1916cc engine that, upon climbing a small mountain near us would slow to 60kph in 2nd gear (auto). After the EA81 NA engien was fitted it would get down to 80kph in second gear. When the SC12 draw thru was fitted it would romp up that mountain at 105 kph in top!! with all the low down power anf torque you wanted. Dyno showed 30% power and 33% torque increases. So taller gearing then should be quite ok IMO.
www.youtube.com/watch?v=HXwh4z8d66o
SUBARU ea81, VW auto, Detour camper
My ex girlfriend said "its me or a trike". Her big mistake.