Happy Holidays.
On my 1989 300E I have new Goodyear P195/65 R15's.
The fuel door says 26/29 F/R.
The decal on the driver door says 26/33 F/r.
The Tire Sidewall says 44psi Cold Max!
It's a 3.0L six.
I'm guessing 36/40 ?
Could you make me a recommendation ?

I'd recommend 26-28psi front and 29-30 30 psi rear (if you carry loads in the trunk often, or passengers in the rear, increase a few lbs). Are your tires wearing well, or are they new? Just curious.

I try to stay as close to spec as possible (meaning the numbers specified in your owners manual or on the car, rather than the max inflation listed on the tire). Under normal use, at spec pressures the tires will wear as evenly as you can reasonably hope. I'd vary the pressure based upon a couple of things:

Wear - Watch and measure the tire's wear pattern. Increased pressure will cause your tire to stand a bit higher and will wear the center more (but also ride harsher). Less pressure conversely will ride better, but wear the edges more and respond less well to aggressive driving.

Driving Style - If you're aggressive, then put a few more pounds in the fronts...just a couple of lbs will cut down on tire squeal and improve handling (but ride firmer)

On the W124, I recommend rotating the tires every 5k miles. Your car beats up the front tires pretty hard, so it's best to distribute the beating. Good luck, and I hope this helps!

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Best regards, Michael
'92 500E
'88 300TE

rlumsd,
I'd pretty much agree with everything Michael said. The 44 cold psi number is for the tire alone. The recommended pressures on the fuel door (or other official MB sources) are what is a good compromise for that car based on normal driving style/use. These sources should list normal recommended pressures, traveling with a load, and for traveling over 100mph. Start with normal spec then adjust to your taste.

If you are an aggressive or even occasionally aggressive driver you may want to consider having the car re-aligned to Sportline specs. The M103 300E's (3.0 motor) all handle great if given good tires. You shouldn't need more than a couple extra lbs unless you are going at it so hard you'd be better served by a sport suspension and/or wheel+tire upgrade.

Hope this helps...Lee