Not sure how you're getting 8 or 9mpg on an FJ with 35s and a 6spd. It doesn't look like you re-geared. I think your FJ's computer may need to get recalibrated for the larger tires. Your FJ thinks you're driving 200 miles on a full tank, when you're really driving 240 miles due to the speedo being off because of the larger tires.
I've been driving modded Jeeps for 20+ years - whether it was a TJ with 37s, 4.88 gears, 6spd and the 4.0L I-6 or a JK with 35s, 4.88s, 3.8L V6 with a 6spd and I always averaged 13-14mpg in the city and peaked at about 17-18mpg Hwy. Off-road and rock crawling, I would get around ~8mpg. I would go through half a tank at Uwharrie in a weekend and do 70 miles of trails - but I was in 4Lo (4:1) revving at 3-4k crawling over rocks some of the time .
Just doesn't add up.
Miss-communication. I worded it wrong. When I said "That's what I get now" I was referring to the 18 mpg he stated.
When I said "better than 8-9" I was referring to the current gen Raptors. ~My FJ~ gets 5 traffic, 13 city, 16 highway, 17 if I get a nice tailwind