Thunderbird 900 Tank Range

I have a TB900 tank fitted to my 1200 chop, and although it feels and looks like a big tank that will take me many miles, I'm only getting around 90 miles out of a tank including reserve. I've run completely out twice in the last 3 months, that's 3 times in 25 years of biking. Reserve is good for 5 miles at most.
I regularly rag on Sportster riders' for their bike's short range but am currently being served huge slices of humble pie...

I'm also not happy about it because it costs over 15 quid to fill every 2 or 3 days. Am I being over-optimistic of what range I should be getting, and the miserly reserve capacity, or are these tanks renowned for being too small?

 

It takes about 13-14 litres/£17-18 at each planned fuel stop so it's almost empty. The tank angle is pretty much horizontal, perhaps slightly tilted down at the back. I should point out that the fuel tap is just a universal non-vacuum cheapbay replacement but the feed tubes are about the same length as a standard tap so I don't think that's causing the reduction in reserve capacity.

 

I realised today the rear brake starts binding after about 20 minutes riding. This may well be the cause of my increased fuel consumption. I'll overhaul the caliper and re-evaluate the consumption once that's been done.