I recently put on two new tires for my 83 urban because I bought it with the original ones still on. The new ones are much heavier and bulkier and the rear tire is slightly larger than standard but fits without issue. The problem is that I lost around 4 mph with the new ones. I'm curious if the heavier tires really could have this considerable effect or did I make a mistake reassembling although seems to run fine. Please let me know if anyone else has encountered this before. Also if they know where or if new tires similar to the originals can be purchased.