tyre size and economy

Unless the tyre size is matched to the torque and speed that torque is deliverd when compared to your average travel speed, then bigger tyres may make it quite a bit worse, it is possible to get better if the matching already isn't optimum.

One circumstance for distance travel where 5th/Odrive may be used is a larger tyre WIll make it worse, but dropping a gear will/may find the optimum and it could be between the normal highest and next lowest gear.
Therefore the bigger tyres used in the lower gear may provide that effficiency for you.

Manufacturers gear and tyre their vehicles for intended use, but can't match all possible use patterns or loads carried or predict the terrain.
Unfortunately you need a few sets if different tyres to check it all out.

Overgearing, ie, bigger tyres generally will use more fuel as the load on the engine is more and greater fuel input is required to match it. Overrevving for the conditions also uses more. so too low is no good either.

A wider tyre is known to require more fuel to drive it, no matter what the diameter is.

As was posted above..any well designed vehicle will have the gearing optomised for the factory tyre size.

How close that is would be fiddly to establish.

One thing that is a certain fuel saver....and its free.....is a few extra PSI in the tyres.

It reduces the rolling resistance.

but if you go overboard, it will ride like a dray, they tyres will wear unevenly and it wont handle or stop properly.

cheers

Every vehicle has it's sweet spot some are good from the factory and some improve with slightly larger tyres, tread pattern makes a difference as well my diesel GQ gets better economy with 33" A/T tyres than it does with smaller ones and my petrol GQ was at it's best with 32" tyres. It's a matter of experimenting on your own vehicle to find out which is a timely process you learn by your mistakes unless you own a tyre outlet and you can keep changing them to you find the right one. If you have the clearance under the wheel arches I can only suggest if you think a higher profile will help only go up one size for now but you need to keep a record of your economy for both sets to compare. Also check your speedo with a GPS and use it to calculate your economy because your speedo may be out and give you the wrong reading so many people use their speedo or the cars electronic lph display for economy which are quite often wrong by up to a litre sometimes. Then gain or loss in economy can only be figured out by the individual vehicle because of what accessories and gear you have.