This has probably covered before but the search has not helped. The fuel consumption on my '89 K100RS seems a bit high. I am seeing about 46 MPG, admittedly on a lot of short trips. New air filter, new fuel filter. Any suggestions would be recievede with gratitude as they say.

My '87 K100RS regularly returns 5 litres per 100 kilometres (20 kms per litre), and always has in a combo of urban, rural and touring riding over a full tank of premium 98 octane fuel. There are engine mods done, however. Others I've ridden with get 22 or 23 kms per litre.

Doing the maths to work out a five quart gallon produces a headache this early in the morning. But approximately 4.55 litres for an imperial gallon and travelling the distance you state suggests that your bike is chewing the juice a bit more than is 'usual'.

Tyre pressure, right hand twist percentage (giving it the stick?), uphill with a headwind, idling in city traffic, low compression, ignition timing, valve clearances, & etc, are things I'd consider having a squiz at.

I wouldn't be too concerned with that considering the short trips round town.
I used 98 octane as anything less was a waste as consumption increased markedly I usually got 50 - 52 MPG using 98 and I live in a country area where the shortest ride is 12k into town.
46 isn't bad enough that I would go looking for a problem but a general tune up and check it on a long ride over a full tank and just keep an eye on it for getting worse.