+1 on what Ryland wrote.
With LEDs, to judge brightness, you compare the LED's lumens vs the stock bulb's lumens (see more detail below on that), you don't compare watts.
You don't compare watts for brightness because watts is only a rating of power consumed, not of light output. Of course if comparing incandescent vs. incandescent of the same voltage, watts consumed gives a good indication of light output. But now when comparing bulbs of a completely different design principle.
You can compare watts of the LEDs vs. of the incandescents to get an idea of how much power you'll save. However, the watts draw of brake and tail light LEDs is so low, you can pretty much just assume you're saving nearly the whole wattage of the incandescent bulbs you're replacing. Save yourself a whole lot of detail math.
HOWEVER when comparing lumens for a
colored LED bulb, the LED's lumens count for more than the incandescent lumens. Here's why. The incandescent needs filtering (usually in the housing lens) to convert the white light to (say) red. That cuts the actual output to about 1/3 of what the bulb puts out. Because you're losing all the green and blue and other colors the bulb is putting out. The LED on the other hand, puts out only red light. So its output is not cut to achieve "red". Yes the housing lens is there but everything coming out of the LED bulb is red so it all gets through the red filter.
Here's my LED thread:
http://ecomodder.com/forum/showthrea...3-a-23045.html
And here's a pic of my setup. Done in the usual way where the bulbs show up as a discolored blob. Bigger brighter blob = brighter bulb. The LED setup is on the left, the stock incandescent is on the right.
See my LED thread for exactly which bulb I used, from superbrightLEDs.com. They were essentially $25 each, in summer of 2012.