Our newer 190E does not have an economy gauge, but I am guessing it works like most - measures manifold vacuum. High vacuum would indicate good mileage, abnormally low readings while idling or under decel could be some serious problems, or just a vacuum leak. Check all vacuum hoses and idle air supply hoses/gaskets.

The O2 sensor really should have little effect on mileage. It is only supposed to help lean out the engine from the stochoimetric ideal air/fuel ratio for emissions control. The mechanical adjustment alone should give nearly as good fuel mileage if done correctly. The latter is also is unlikely to change much over many years. More likely in that department would be a leaky fuel distributor. Many report raw fuel pooling below the airflow sensor vane when that happens. Normally, there should be little or none there.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7