In Texas you have to have cat by law :-(

On my 95 E320 (W124), I would get 27-29 mpg, but that was mostly highway. You're in the 96 W210, which at least looks more aerodynamic - you should be getting good 20's all day long.
Plus, you have OBD II diagnostics, so what could it be that doesn't throw a code?
1. Vacuum line: should give you a mixture code, but easy enough to check all lines (including under the intake manifold, arch your neck and squint really long from the front with a flashlight. Skinny arms can reach under there)
2. Dragging parking brake: jack up wheel, block the others so your car stays put, and rotate the rear wheel in neutral with the brake off. It should spin a turn or two by itself. Adjust with the ratchet device inside the drum. Did someone mess with the adjustment in May, when you had new tires put on?
3. Broken cat: Fortunately I'm not an expert on this one, but I think you will get misfiring with codes first and hear rattling in the exhaust.
4. Fuel leak: won't throw a code, but you would know by now.
5. MAF, ignition problems are code sprouters.

Tires can vary a lot in actual size even if nominally the same size. A taller tire will compute to lower economy but actually be better on consumption. The only way to be sure is to measure the actual diamater of before and after.

Or you could check against the mile markers on the interstate.

Tom W

The tread can also affect fuel economy. When I put more agressive tires on the Suburban -- same size as stock, my mileage went down by 4mpg.

It has taken a lot of tweaking to get it back up to 16mpg combined.

I would also try this to look for fuel leaks. Get some clean cardboard and put the car on top of it. Start and run the car while shining a bright light underneath. You might have a leak that is only happening when the system is under pressure.

I would also run the car and really examine all the fuel lines under the hood.
I had such a leak in the BMW. I got crappy gas mileage until I found and corrected it.

Does this car have a cold engine enrichment booster? If so, it could be stuck open. That was another issue with the BMW that cost me mpg... Check all the temperature senders to make sure they are connected and working.

What about O2 sensors

Both O2 sensors were replaced 4 months ago.

Thanks to all for your contributions.

cat
 

Texholdem ,
An emission test will reveal if you have a faulty cat... But have you noticed any loss of power ?, mostly when you floor it from 60 miles per hour to 80 miles per hour?

I think the next reasonable move is the emission test. Thanks to all.

If that mileage is around town, you have no problem. I can get 25 to 27 on the highway, but would never get 20 around town.

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snake oil?

Benz Merecedes, I don't notice any loss of power, acceleration from 60-80 mpg seems OK.

Holy crap. Does my 94 C280 have one of these?