Ahhhh okay - so you're basing the actual cruising speed on maintaining a solid engine speed? I guess with a manual and no slippage in the clutch that isn't such a bad idea!
As mine is auto that would work when locked up, but at lower speeds it would be an issue. My ECU doesn't respond to the analogue input unfortunately - so it needs the speed in on the individual wheel speed data it would seem. I was lead to believe though that it needed 'other' data - but I've yet to work out what that might be.
Need to get back to it really and test some things - I will report back :)