But my instrument cluster is it the one that's supposed to come with the "L" model?
this will not cause a problem.... but.
if some one removed the speedo needle and re-installed it wrong then that could explain the speedo. maybe.
i have a good bit of experience with speedos. i swapped a legacy L 140 mph speedo into a legacy GT that had a 120 mph speedo. it is not easy and not worth the trouble, but i learned a lot about the speedos in these cars.
question, does the speedo go to '0' when you turn the car off and jump up to 20 or more when you turn the key on??
if your speed sensor is ok, and your speedo is the problem, it is likely that the needle was swapped out and reinstalled incorrectly. there are 2 solutions, the easy one is to replace the whole speedo with one from the same year and model as yours. if it is the speedo this will fix it. and it is easy. you may have to buy the entire instrument cluster to get the speedo, but you can just swap out the speedo. maybe a hour on your first try. look for one at www.car-part.com
the second solution is complicated. basically you remove the clear plastic cover from the speedo and then remove the needle and then reinstall the needle pointing to '0' with the key on.
please note, the speedo needle is very very very fragile. if you bust it you are going to need a new speedo.
if the speedo does not read the correct speed after the fix the next solution is to install the speedo needle point to 60 when the gps says you are going 60. this is very very very dangerous.