I'm pretty sure I could do better because at the moment I'm being lazy and using the adaptive cruise control virtually all the time. Although it uses EV mode some of the time I know a few places where I could get it into EV mode a bit earlier. Got my first long motorway drive next week so will be interesting to see how it does. My Jazz would get me over 60mpg on my trips to North Wales and that's going to be tough to beat - basically a fight between a 1.3 litre engine in Atkinson mode and a 1.8 litre in Atkinson mode. It does use the battery power occasionally at 60 mph and cruises at less than 1,500 rpm but I don't know if it will be enough to return a better figure.
My guess is that for sustained cruising at 60 mph or above your Corolla will pull ahead on mpg as it sits lower on the road and should be aerodynamically better than the Jazz by a big enough margin to offset any deadweight due to the hybrid system. Cruising at 50 or below probably sees the Jazz pull ahead. Don't forget the Land's End to John O'Groats run by the Mk 3 Jazz - very impressive mpg at a sustained 40 mph. Hybrids best deliver their benefit in variable speed motoring.
Yeah, I have driven a few relatively short sections of MWay/DC at 60mph and it was interesting to see how 'active' the system was in mixing the two power sources. A lot of people think that the hybrid stuff is all or nothing but actually it's forever swapping between sources and often combines them.
I can just about see that if it can cruise at 60mph at <1,500 rpm and it comes to a slight incline it might make sense to utilise the battery rather than increase the RPMs. Of course the battery energy is not free(*) so eventually it has to increase the engine RPMs above what's needed for cruising in order to recharge. But perhaps (and these are made up numbers) it goes something like:
Climbing an incline with battery+engine@1500 followed by engine@2000 temporarily to recharge saves more fuel than climbing the incline at 2500rpm.
And of course most inclines are followed by a decline which will help improve the advantage further.
One thing I've noticed is the performance difference. From stationary and at low speed the Corolla would leave a Jazz in the dust. It's actually quite 'scary' just how much get up and go you get when it's an electric motor doing the work. At higher speeds there's little to nothing in it and in fact I think that in 'normal' mode at least the Jazz might have the edge. In 'power' mode possibly not but it seems that a 1.3 l engine operating on the Otto cycle outguns a 1,8 l engine operating on the Atkinson cycle and the battery can only help overcome that for a while.
(*)In fact several experts say that trying to run in EV mode as much as possible is actually not the best strategy. The system is around 80% efficient so they say that the car should be left to mix/match as it sees fit rather than trying to play the 'I'm an electric car' game.