I had MTR's on my truck for several months and my mileage did not drop drastically. 10.4 to 10.6 average, which is about where I was give or take .2 miles with the stock 35's.
Sound like you have another issue. The tuner might be running really rich. How's the engine running?
2001 H1 Opentop
Former 03 H2 Adventure Series
http://homepage.mac.com/babiak1/