I've played around with the speedo settings quite a bit with AE adjusting for larger tires for my truck and my buddies. The ABS system seems to be adaptive as well. We drove my buddies truck with 40"tires on it, except he got a flat the day we needed to adjust the speedo. So one stock tire went on the front right. ABS was so out of whack like that from the different diameters, the small tire kept locking up and ABS pulsing, but the more we drove it the more it cleared up. By the time we got back to the house ABS worked like normal, our at last as normal as possible with that setup. We drove on back dirt roads as well.
As for mileage, I gained a little in mine going to 35s, but it sees mostly highway driving. Lowered rpms enough to make a small difference. My mileage has never been spectacular though. 15 or so.