Why Uk pilots cruise in low 4000's?

I have a Eurostar on an LAA permit, a 90 mph cruise in mine is done at about 4800 rpm. I know that many of the BMAA permit Eurostars cruise at between 4200-4400 rpm. I was led to belive that this is due to the microlight requirements that specify the MAUM (450 kg) and the minimum endurance with 2 occupants of a certain weight and a specified volume of fuel. If the cruise rpm is too high then the fuel consumption is too high and you can't meet the minimum endurance requirement. I could be wrong. A post on the LAA/BMAA forum would probably illicit the true rationale.

Thanks for repley , I can't see it making a huge difference in fuel economy , maybe a extra 1 to 2 litres a hour . If the engine is me t to be run in the 5000s I be happy to pay the extra fuel consumption , regardles on the extra fuel burn you still need to obey the weight rule if the engine burns 12 or 15 litres per hour it won't make any difference . It's bugging me ??

It's all about the rules, just in the same way that I can fly 480 kg with my LAA permit but if I had a BMAA permit then I can only go to 450 kg. There are also rules about the maximum empty weight, the wind loading and the stall speed. Much of it is not based on the manufacturer's spec, it is all down to the regulations under which the aircraft is operated.

The only thing that changes is the prop pitch , to get a higher rpm to make the engine run how it was meant to run? It's what the engine is intended to be run at ... so it seems.

Agreed, it's all quite mad, mine was a 450 kg microlight when I bought it, then I paid £140 to the LAA and miraculously it can suddenly carry an additional 30 kg! Interestingly mine had a max static rpm of 4800 when it was a microlight and it is no different now. That is on a Woodcomp Klassic prop but I think that the Kiev prop might be different.