Just got my 2018 Jazz Sport 1.5L 6 speed manual about a week ago .
On a 70MPH dual carriageway run , driving very gently , the trip computer reckons about 48MPG
Driving around town and "testing" how "perky" its engine is the trip computer reckoned about 34MPG, but this was with me being a bit heavy, so when I start acting my 50 years of age again, it'll probably be a bit better !
I've never owned or driven a 1.3 Jazz, so I can't give any meaningful comparison.
Its more perky than my outgoing 2.0L petrol Mazda6 at low speed and off the line, but that's a much bigger car carrying around another 380Kg of weight, for an 155BHP engine. But the Mazda has a bit more in gear aceleration at lower RPM.