The Jimny will do 40mpg on normal roads. That is the book rate and also the tried & tested mpg. 40mpg is very good for an old style 4x4 jeep. Most up until much more recent models from approx the year 2000 on would do quite a bit less than 30mpg. Also reflecting on this is that to get one of the much newer 4x4's that do better mpg, then you need to spend many thousands on the purchase price and then again on the running costs over the lifetime of the vehicle. the Jimny's cost very little to run. Beware though they are very uncomfortable in ride quality and seating comfort on normal roads. Offroad it would be an even more extreme version of feeling like being bounced around inside a washing machine. Very capable though but I wouldn't want to drive any great distance on normal roads and expect to get out the other end feeling relaxed. Positively tiring on motorways with the engine whining away at high revs and still a pretty slow pace.
If you drive short distances on road and want full on offroad capability, then they are a superb choice. For mid to long distance by road they are sadly lacking to the point I sold my otherwise super little Jimny as it was impractical pre SHTF. if you are only going to drive short distances then the fuel economy is not that important, so why not just get a full size 4x4 and reap the benefits. One final point, you can fit very little inside a Jimny besides the passengers. Take the rear seats out and it is the size of a large boot. Hauling a lot of kit or anything of length inside the machine is not possible unless you go the roof rack & trailer route but these decrease fuel economy and severely limit its primary ability which is offroad. Umm. Somewhat compromised in the real world for now before any event. Still it would be better than a micra. lol. ( but is even smaller inside
40mpg at petrol literage prices too - not diesel. That would possible equate to about 42mpg at diesel prices per litre. :-) ....