The MB is closer to linear than many other cars but it still is not linear. Fuel pressure from a more full tank will not change fuel consumption but lighter vehicle weight from a less full tank will.

By the way, since there is some drag in the sensor, the gauge will read differently going down as it will going up.

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Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY