That's what I wanted to know. I'm going to install an H55F with a header on my FJ43. I don't cruise on the highway much, but I'd like to keep up with the traffic. Not concerned about the fuel mileage etc, just a more relaxed ride on road trips. ThanksMy experience is in a 2F 60 with 33" tires and 3.73 gears but I'll share for reference. I did see an increase with my H55.
At 70mph with the 4-spd I was getting 11-12mpg. With the H55 at 70-75mph, I get 14-16mpg depending on how many mountains on my trip. My 2F is stock with a header. 75mph in my truck is ~2500rpm with the H55 compared to ~3000 with the H42.