I'll chime in. It's been a while since I've had my 40 but I still think I've got some experience with them. I think Kurt (cruiseroutfit) has given an excellent breakdown of pros/cons of a 40 series for this type of travel. The biggest issue is going to be driver fatigue resulting from the short wheelbase and stiff leaf sprung suspension along with the noise. I personally think I had a 40 that was extremely comfortable (for a 40 series) but comfort is relative. I daily drove mine 60+ miles/day for more than 8 years and it was the primary offroad vehicle as well and it only got trailered twice in its life. Otherwise I was driving it to and from everywhere I took it which were some pretty long distances.
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The things that *helped* in terms of comfort with my FJ40 was first having a drivetrain that ran well and ran smoothly (as smoothly as a 2F is capable of running). As has been mentioned they are a glorified tractor and the motor feels that way but having one in good mechanical condition will be key not only for comfort but for reliability on long trips. Every single part in the drivetrain had been rebuilt or replaced on my FJ40. Also, for suspension there's only so much you can do. A spring over axle conversion can do a lot for ride quality if done right but will result in a minimum of 5-6" of lift which might be more than you want. I had suspension that was sprung under in the stock configuration but I did all I could to dial it in for ride quality and offroad performance. I had custom Alcan springs made for it that were built to my specs and for the weight of the vehicle. Matching those with some quality shocks made for a good ride as far as a FJ40 is concerned.
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Adding Power Steering is also a nice comfort mod and also makes life off-road much nicer. The manual steering that is stock on FJ40s will give you arms like Popeye in no time if you have larger tires and spend time in the dirt/rocks.
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Seats. Everyone is totally different when it comes to seats and which ones are comfortable. I had a set of Procar Rallye seats in my FJ40 and I thought they were VERY comfortable. But I'm 5'8" and 160lbs so I'm not that big. Some of my friends that were bigger than me found them to be too tight and not as comfortable. But for me they made a big difference and made the long trips much more comfortable. I could drive for hours and not feel fatigued.
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Tires. If your trips involve a lot of pavement a smooth AT type tire will make the drive much nicer. I always had aggressive MT style tires on my FJ40 because of the terrain I liked to drive. The aggressive tires were more noisy and not as smooth going down the road but I compensated by running ~25psi in the tires consistently on the road. This improved the ride a lot.
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Noise. This is a hard up-hill climb with FJ40s that are old. There is no sound proofing and most of the FJ40s you'll find have dry, brittle weather stripping and window tracks/felts (if they have them at all). If the weather stripping and window tracks/felts are in poor condition it will make the truck a massive rattle-trap that will make you want to shoot yourself in the face. As part of the rebuilding process with my FJ40 new weather stripping was possibly the single best thing I did. It made the doors fit tight again, it made the windows tight and smooth and it sure quieted things down a lot not only with the rattles, but keeping road noise out........sort of. I also had the inside and underside of the tub lined with bedliner that helped reduce some of the road noise also. But don't be fooled, even with all of that FJ40s are still not quiet.
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Room for gear will be the next hurdle. Room for gear is hard to come by in an FJ40. When my wife and I would take multi-day trips the back of the tub was packed from floor to the top of the rollbar, and we didn't have any fancy stuff like fridges etc taking up room. An FJ40 definitely makes you streamline your packing and causes you to be very selective about what you take and what you leave home. Even before my wife and I had our kids, we were considering an offroad trailer to pull behind our FJ40 because we felt like we needed more room for gear on our longer trips. If you are solo most of the time, then I think it would be possible to fit all the necessary gear for an extended trip.
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I haven't had my FJ40 for 10 years now and I gotta say I miss it badly. There is just a feeling you get driving an FJ40 that I haven't experienced in any other vehicle. I genuinely miss the noise, the smell, the tingle in my hands and legs from the constant growl from the motor after I finished a long drive. It was awesome and I hope I can be back in another 40 someday. But I'll be honest it won't be a real "overland" vehicle. There are just much better options out there.
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