The key formulas are:

P=IV
V=IR

From these you can rearrange to I=V/R, and when you substitute that into P=IV you get the one you want:

P=V*V/R

Sylvania uses 13.2 volts to rate their bulbs, and if you use that in your case you get:

P=13.2*13.2/6 = 29 watts

Even at a more typical 14.5 volts you would only get 35 watts.

I suspect your resistance measured is high. Did you short the test leads together to see what you get? To get the best accuracy at low resistance values I do that first, and then subtract that value from what I measure for the load.

This all assumes the resistance does not change when it gets hot. Normally resistance goes up when it gets hot, so you would expect something lower when cold.

Also it could be that your meter is not accurate enough at very low resistances. If you compared the good bulb to the bad bulb you may get a quick and dirty comparison. Lower resistance gives higher watts.