Hoping theres a quick and simple answer to this.

I've got a '95 TS V6Si, searches on the web bring up that it's a 65L tank. But I've never had anything more than 45L when I fill up. Am I missing something?

Yep. The tank is never empty when you fill it, plus there is a space for fuel expansion built in and they probably count that too. Try it when you run out of fuel. lol

If it's anything like mine, my orange light glows and the needle shows empty when I still have about 18 litres in it, I ran it another 150km after this happened and I managed to squeeze 62 litres into the tank.

I wouldn't be counting on how accurate your fuel guage is.

The upside is you'll never run out of fuel if it shows empty early.

Good idea really, to really show a fuel level lower than it actually is. If you run the fuel tank almost dry on a regular basis, the fuel pump sucks air too often and can burn out, or the filter can clog up from any rubbish in the bottom of the tank. Just ask people who run dual fuel and regularly run the fuel tank dry. lol I just put $50 in it when it gets to a quarter on the fuel gauge.

FYI, the 2nd gen sedans had 64L tank according to the manual for my old KS.