Hello,
As far as I am aware the Series 3 FFR - that is making the assumption from part of your username - only had a single tank that is fitted under the driver's seat.
If you have a rear tank it could be an aftermarket one or an optional extra that someone fitted or swapped out from something like a station wagon. It could also be a custom job too.
If you post up some photographs someone may be able recognise the tank and give you an estimate. Otherwise it is a "how long is a piece of string" exercise. The old formula of length x height x width will give you some idea of volume
Kind Regards
Lionel


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