Typically they're 65 litres. I measured mine from completely empty at the pump. But generally I find that it's difficult to get the last few litres out before the pump starts scavenging. Work on 60 usable litres. In Russia, I ran on the back tank almost all of the time keeping the main 95L tank as the reserve. We just didn't know how much fuel there'd be so when that was empty, I'd stop and fill up. In the UK I get generally around 180 miles on that. Yes it's rubbish. But it's a big truck. As we toured through Europe I logged the range. Getting down to the Chunnel I filled up with super fuel rather than Tesco juice. I squeezed 200 miles dead. In Finland, where we were doing mostly A roads on the way back, I hit 240 miles (twice) on the same litres. Mostly around the 220 using standard forecourt diesel. In Russia it was hard to say given the terrain but I did experience some fuel starvation issues once I reached 55 mph. The truck regulalry hiccuped. This was on either tank which both had Dervski in them. As soon as I hit Finland and filled up, it cleared. The fuel filter was changed and it made no difference. No one else had an issue but I was the only 24valve. Go figure.
I have considered a long range tank, but there's trade offs. It's a lot of weight and they cost a lot. I concluded that it's more a convenience than anything and meaning you can plunder cheap diesel when you find it. But if you are in a convoy, your tank is effectively only as big as the smallest tank in the group unless you don't stop to fill up when they need to!!
The sub tank here is a great addition and doesn't affect the spare wheel carrier. I think it's a really good option unless you really are pushing deep into the Algerian interior. I had 40 L on the roof too and never used it.