Shoot, I have some hills that I ride 5 mph or less and I painfully watch my speed (and try to keep it above 5)! So heck, 8 mph uphill is smokin'!

That being said, it should read less than 8...good chance it's battery problems. I don't have a cateye, but when my shimano wireless started getting goofy I replaced the battery and !shazaam! all was well.

Even if the computer is relatively "new" who knows how long that battery has been sitting on the shelf. (This was the case with a computer that belinged to a friend of mine, and I think it was a cateye! It quit working after about 3-4 months.) I don't know how new or old yours is, but batteries are pretty cheap!