I replaced our electric range with a gas one and want to use the existing aluminum range wire and splice on a length to go to a sub panel in the garage. I ran 8/3 under house to where I pulled the old range through the floor and found that the range wire is aluminum and only 8/2. Can I splice the new copper wire to the aluminum and tie the ground and common together to the range ground wire?

Absolutely not.

So yo u are saying that I should buy another piece of 8/3 to replace the 8/2 aluminum?

Completely agree with Harold. Even if you could splice it, the 8/2 is only good for straight 240 and you wouldn't have a neutral. Were you thinking of connecting the ground and neutral of the 8/3 to the ground of the 8/2? Bad ideas, do it right! You could pull a copper 8/3 to the range and use the old range box as a junction box and go from there. However, you may need a screw clamp for connecting the wire since wire nutting an 8 AWG is not easy.

Yes, I have screw clamps for the splice and rubber tape. The splice will be in a new junction box mounted on a joist under the house. I just though that since the neutral and ground attach together in the panel box that they could at other end. I guess I'll have to get another 50 ft of 8/3. Thanks

Well if you want that sub panel to be 120/240 I hope that you mean 8/3 with a ground. I, of course could be wrong, but something tells me that you are calling 8/3 is really 8/2 with a ground.

Yes, 8/3 with ground.