You can use a light-meter (professional photographers use them) to compare the actual brightness. Take measurements at different/measured distances from the bulbs. You may want to do it in a darkened garage or at night.
You cannot use LED bulbs in most signal/hazard lamp applications because the bulbs do not draw enough current to make the signal/hazard flashing unit work properly or at all (there are minimum current values for the units to start flashing). You could, though, use the bulbs in parallel to existing incandescent bulbs by tapping into their POS leads but you would need to install new sockets/housings...
LEDs that burn out do so mainly because of mismatched resistors (yes, cheap as in manufactured using incorrect or substandard resistors). Properly manufactured, they should outlast 2 to 4 vehicle battery changes.
to the responders, and especially to Brandon b, I was able to<BR>find a wealth
of information on LED tail lights via Google. The<BR>decision on what to
do seems to rest on a few things:<BR><BR>What's the size of your light
receptacle vs. the size of the LED (you<BR>can't go with the smallest); the
site shows diameter and length<BR>measurements of the two larger, more
applicable, LEDs. Will they fit?<BR><BR>What sense do you make of
'"actual" vs. "apparent" brightness" as stated<BR>in the text of the
site.<BR><BR>How do you rate the cost ($25 -- $37 USD per lamp) vs. the
hopeful gain<BR>in safety achieved?<BR><BR>I'm measuring and thinking about
it, hoping that someone has some input<BR>on what is the actual achieved
brightness on the conversion to LEDs.<BR>Thanx again.<BR><BR>