My '99 TJ Sahara, 4.0L, is getting p-poor mileage. I just drove about 45mi (80km) on the first 1/4 tank. This is consistent. A couple of months ago I put on 31 BFG's (no lift) from 30's; recent tune up/oil change, K&N filter. It idles a little rough for a minute on a cold start, but other...