I have just purchased an '01 E320 wagon - really a sweet ride. I have filled the tank a couple of times now and the dash display tells me that there are 17.2 gal in the tank when full. Manual says tank is 21.1 gal capacity. Wondering where the missing 3.9 gal have gone to. Thinking either a faulty sensor or the car just does not tell me about the reserve.

Appreciate any helpful feed back.

Thanks

 

21.1 gall is from dry.
Unless stated otherwise this figure will also likely be to 100% full.
It is not the usuable capacity and in addition the tank doesn't normally fill to "brim" since space is allowed for fuel to expand if the car is filled "full" (auto cut off on pump) of fuel (say 50 deg F from underground storage) and then left parked in the sun (ambient say 80degF). The capacity for the expanded fuel has to be allowed for since the system has to be sealed for emmissions /legal purposes.
Also there may be considerably more gas in the tank when you think it is empty (as signalled by the guage) than you realise.
Manufacturers ALWAYS err on the side of safety when graduating tank level since for the car to run out before the guage states empty would have liability issues associated.
e.g. Stalled part way across a junction due to lack of fuel and guage stated there was plenty!
In addition modern engines (or more accurately emmission control systems i.e. Catalysts) can be damaged when a car misfires which occurs when a vehicle is running out of fuel.
Since cars are mass produced an allowance has to be taken into account for manufacturing differences between the components in one vehicle and another and especially the interface between the instrument pack and the fuel sensor.
I suspect that when you have allowed for some of these variables the gap between actual and stated will decrease.