if you have a friend, or a GPS that gives you a speed reading you can get your speedo pretty close. Fill up on gas and put the needle at full... if you know what your car idles at you can also do the tach. that's about it though
If youve got a real big parking lot, or a long straight road with very little traffic, you can try this. Get together with a buddy and pre determine a speed. Have your buddy drive in front of you on the road, and when he/she gets to the pre set speed, set their cruise control and turn on the hazard flashers. Youll be following behind. When you see the hazards, match speeds as best as you can and then set your cruise control. Then, CAREFULLY line up the speedo needle with the pre determined speed and lightly press it on. 50 mph is usually a good speed to aim for. But please be cautious when you do this cause a lot of things can go wrong at 50 mph if your looking down fiddling with the speedo.
P.S. This involves bolting the guage cluster back in place without the lens on.