tyre puzzle

The 2007 Hi Rider ranger was originally fitted with 235/75/15 tyres which have a circumference of 90.7ins.
The previous owner replaced these with some 18in CSA rims (probably off a Rodeo) fitted with 255/60/18 tyres having a circumference of 94.4ins.
The result is that the engine feels as if it isn't turning over fast enough when towing a a lightly loaded dual axle trailer at 90kph (2000 rpm in 5th)
I use 4th if carting a decent load.
Because I'm towing a lot at the moment I put the original 15in steel wheels back on but, surprise - the rpm's at 90kph are the same - 2000.
I expected them to increase. Why not, if the circumference has decreased ?
I've just spent a lot of dosh and am about to fit some 16in Ford allloys with 235/70/16 tyres (circumference 91.0 ins, which is very close to the original 15in rims 0 but it looks like I'm going to be disappointed.

The 2.5 l Ranger runs 215/70/15's (84.4 ins) and the gearing feels just right.
Putting bigger wheels on Rangers to up circumference and therefore the gearing is not a good idea and just makes them feel intractable. I suspect it does nothing for the fuel economy either.
Any answers?
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