I can get 57L in my 1980 +8 leaving the hoses to the tank empty. Done 290 miles on a tank full in Poland but the gauge was on E. Sweating!
Yes the original +8 fuel tank was 12.5 imperial gallons (or 56.8 litres). When fuel injection was introduced the high pressure pump was mounted on the tank boards to the right, this meant the tank width was reduced and the 45/50 litre tank became standard.
When the external high pressure pump was changed to the "in-tank" version , there was an opportunity to increase the tank size to the original , but this was not taken, and the smaller tank was the standard item until the end of production.
A friend of mine who experienced a leaking tank which needed replacement , measure up everything very carefully and got a one-off aluminium tank made which used the full width and a bit more height. The resultant capacity was 73 litres. On the one hand this extra capacity is very useful in times of fuel shortages and driving long distances , but on the other hand I rarely drive more than 120 to 150 miles without a stop when touring, so I figured that I would only go down a similar route if my current tank becomes unusable.
