the tube will stretch and u'll be fine, but it will be easier to get flats, cause the tube walls are stretched thinner, also idk why but it tends to deflate faster just sitting around, i had an undersized tube on my bike when my shop was out of the right size, and when i checked my tires it was always alot lower than the other tire
2004 Marin bobcat trail, 2006 trek fuel ex7, 2007 iron horse road bike
my bike came with 1.7 tubes and it had 1.95 tires on it ... no extra leakage in fact these were the best tubes Ive had... kenda brand I think.... finally got 3 punctures on both of them.. that's my threshold the 4th flat and it is repaced. They worked better than the maxxis featherweight tubes that were 1.9-2.1. I gave up on being light and now just use the standard cheap tubes.