The factory used to produce a "Sebring" tank which was reputed to hold 19 gallons or 86 litres, but I think this could have been US gallons in which case it would have been closer to 15.8 Imp Gallons or 72 litres.
I had one of these on my Plus 4 racer , and when full it transformed the car from mild understeer to dominant oversteer, when cornering at any decent pace.
It was useful for long distance touring of course as Simon points out.
My 1999 +8 has a 45 litre tank but I have never been able to get more than 41 litres in it, even after travelling for 60 to 70 miles after hitting empty on the gauge.
BTW Jon , 90 litres of petrol will not weigh 90 kilos, quite a bit less due to the specific gravity.
