Compare the most economical cars

Diesel is still king for longer journeys, although the best (non-plug-in) hybrids are catching up now. A Volkswagen Golf 2.0 TDI diesel will pretty easily do 60mpg on a gently-driven long-haul. If diesel prickles your conscience, then consider a Toyota Corolla hybrid — now that the Prius is no longer on sale, this is Toyota’s long-haul economy champion, and the updated 140hp hybrid should see you get close to 60mpg on long runs. 

There isn’t quite one magic figure as all cars are different, and different road and weather conditions will also play their part, but the simple answer here is around 50mph. That’s fast enough that the engine won’t be labouring in a low gear, but slow enough that you’re not building up too much aerodynamic resistance. You may be building up a queue of road-rage behind you, but that’s their problem…

For an average family car, you should be looking at getting at least 40-45mpg overall, including city and town driving (which is always the worst for economy). For higher performance models, or bigger, bulkier SUVs, slipping into the 30s is still just-about acceptable, but given the cost of fuel and the climate crisis, ideally you’d be looking for something that can reliably return close to 50mpg.

It's can be a little frustrating for a response to essentially say "it depends", but that's the case here. There are three main types of automatic gearbox used by car makers today, with dual clutch (DCT) and continuously variable transmissions (CVT) and more conventional torque converter 'boxes going toe-to-toe with manuals. Each has its own advantages and disadvantages, and while a manual gearbox might produce a fractionally better official fuel economy figure than an automatic, autos have become more and more sophisticated over the years so the difference has become increasingly negligible. How you drive has a far greater impact on economy than what gearbox your car has, and real-world driving is somewhat different than the processes involved in official testing, so while the on-paper figures might slightly favour manual cars, you may find it easier to get closer to those official numbers in a car with an automatic gearbox, simply because there are fewer variables involved (EG at what revs you change gear, which gear you select on hills, and so on). Maybe we should have just left it at "it depends"...

Yes. We can say that with certainty; like for like, a diesel engine will give greater economy than a petrol one. Mind you, with diesel much more expensive than petrol at present and diesel cars costing more to buy, whether you'll save money in the long run really does depend...

If in a manual car, change up a gear as early as you can without the engine bogging down in low revs. Remove roof boxes and roof rails, use cruise control when you can, keep an eye on the road ahead so you don't accelerate unnecessarily, don't carry excess baggage and surplus items in your boot, make sure your tyres are at the correct pressure, and drive at 60 or 65mph on the motorway rather than 70mph.