That looks like a filter differential pressure gage.

Essentially it's measuring the pressure drop across the filter.
New filter should have very little DP, old (plugged) filter will have high DP.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles