The guage for each tank of every car I've owned does not drop in a linear, or even, manner. In my MB, I can get 350 miles at the 1/2, but that doesn't mean I'm going to get 700 miles to the tank.

I run my tanks down to the reserve on the theory that the less fuel is in the tank, the lighter the car will be for better ultimate economy.

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'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate