Well, I’ve been monitoring ours on an almost OCD basis for the past two and a half years on each tank fill, and mostly urban I get between 27 and 31 mpg, maybe early forties if I do a long run using cruise control, so I’d say they could be on the optimistic side, try the “brimming the tank” method and use your phone to snap the mileage on the dash, and always get a receipt for your fuel. “If it ain’t broke, don’t fix it” -where’s the fun in that?

My mpg figures align with those of Gremlin. Pretty consistent 32mpg until I do a long steady motorway run then high 30’s to low 40’s.

I did brim test a while ago and with regular 8 to 15 mile type runs it was about 33, which i still thought was good for a vehicle that size. My P38 does about 24 on the same type of journeys so a big improvement. ( but crankshafts don't snap on P38s). The Best Journey 52 MPG reading was on a 240 mile run but before my ownership so maybe Miss Daisy was driving.
As you say Gremlin I think the computer calculations are a tad optimistic.