The ZT is a heavy thirsty beast! I have heard claims of 28mpg on a motorway run, but frankly you will be very lucky to achieve this! I regularly get about 21mpg on a combination of fast motorway and 'A' road driving and I have never improved on 23mpg which I got when running in.
Having said that the V8 is very tight when new and it should give better MPG (and power) as it beds in which should take about 20,000 miles.
It is happy to drink normal unleaded as it is
a pretty simple unstressed engine.
However the 260 owners manual suggests 98RON will give the best results although it does not have knock sensors so any benefit is down to the fuel composition rahter than timing.
Several owners feel that 98RON adds extra
beans but not much. At some stage we will have to do a rolling
road before and after test to see if it really worth spending the extra
on high octane juice!