Ok, I guess trial and error are the best methods, but I am curious was everyone uses as their baseline between stops for fuel?

300 km = 187.5 mi.
320 km = 200 mi

I'm running a 2009 FLHX.  

I use 250 kms. This gives me a bit of a buffer. One of the guys in our crew can only do 150 kms though.  

What do you ride ironhead?

Do you pack light or heavy?

I'm 265 lbs. + Gear  

Depending on the area I am in ie Desert/City I will run 160-175 miles with a five gallon cell pulling a Bushtec trailer.
However I now have an 09 with a six gallon cell and hope to change that up to 175-200, although most of the time my wife and I are quite ready for a streach and butt break after 175 miles.
I will add here as well that it is nice when time and distance over the journey is not a factor, the best rides/tours I have done have been those of the path less traveled and only directionally conceived, aim and plan the route and road selection as you head that direction, this will almost always yield the most rewarding journey.

Slick  

I also have a '09 FLHX. I just purchased it in February and haven't made any long trips yet. I'm only getting about 37 MPG after I added Khrome Werks HP-Plus mufflers, SE Air kit and Power Commander 5. I weigh about 255. I like to ride as far as I can before stopping so the more mileage the better. I probably need a dynotune to see if I can get better gas mileage. Half the time I ride with a group so of course I have to stop when the group stops. Sometimes there are guys with Sportsters so we have to stop pretty often. My last bike was a '06 FLHX. I had a SERT on that one and got better mileage. The '09 is a smoother riding bike though.  

Thanks guys for all the info.

I went for a rip twice over the weekend. I used Shell 91 gas and I have the Heavy Breather, SE Touring mufflers, and the factory [and warranty friendly] tune. I had empty bags, and myself [265 lbs]. I also had the soft lowers on [wind resistance??]

ANYHOW, I got 320 km[200 mi.]  on a tank. The dummy fuel light was on for about 15 km [10 mi]. Topped 'er up and off again. I just did the first service, so things should be breaking in nicely.

2 up I'm planning on 250 km, so long as she can go that long... [ha ha]

My long trip this summer will be solo.

Was that ever a long winter.  

Ok here i go...riding solo most of the time at freeway speeds of 65-70 mph i'm getting about 230mi per tank full. So I'm pretty happy but ridin a stock flhx wonder what the result would be after doin some mods (exhaust, heavy breather & fuel tuner)? I'm sure that will go down a lil.  

Here's something to keep in mind for those trips into the awesome unknown.  When riding into a big headwind, riding hills, and/or riding two up, the gas mileage falls precipitiously (I probably should have looked up how to spell that).  As I noted in a different forum, a solo, 250mile ride on my stock flhtc into a headwind took 2 1/3 tanks of fuel.  Had to ride with the throttle wide open in 5th at times to keep up with the toyotas.  Bike's in for pipes, air, and EFI.  Next desert wind, I'll repeat the trip and see what happens.  

For you guys who don't live near the desert, that wasn't just a breeze, it was a full-on 60+ mph, sustained wind blowing directly at me.  sitting up felt like you would expect--sand, wind, and who-knows-what hitting the face shield (and my neck) at 130-140mph.  I stayed down behind the shorty windshield.  I expect the added hp from the current upgrades will help.  I just don't know how much more HP is needed and what effect it will have on mileage.  I'll post that one of these days.  

i think somebody already said it, but i'll burn about 150 before i stop for a break--not just for gas, but to  stretch the legs and eyes.  thats right, stretch the eyes.  I find after burnin for a couple hours on the road, its a good break to focus on something else for a second.  although you all probably know this just as well as I ***  

I have been getting 200 mi [300 km] per tank lately. [by myself.]

Have not hit a sandstorm, but hail hurts too...

[Shoulda wore my full face, shoulda wore my full face.]  

last 3 tanks been 238, 234, and 236 on stock 09 flhx heavy throttle on launch and speeds of 70 to 75 mph avg temp from am at 70 to pm at 94. waiting for my long haul over to midwest to really let me know what i can push out of it. oh yeah i usually wait to the "dummy" lites and it tells me "lo".  

I run a FLHRCI 06 with a 95ci and 203 cams, Power CMDR, and Reinhart true duals.  I am about 225 lbs, average about 160 miles to the tank with about 40 lbs of gear (tent, clothes, tools, leathers.....).  I think the gas miliage sucks, but hey,  it is still fun.  

I ride an 08 FLHRC with a Fairing. Before upgrading to a 103" and other goodies, I did Wash. D.C. to New Orleans and back. I tracked my mileage, gallons and cost over this 2400 mi. trip.
I averaged 47.5 MPG while cruising 70 - 83 mph most of the time. I only rode down the whole tank a couple of times but got somewhere around 260 miles before the dummy made me stop. I wiegh 203 and had about 35 lbs. of gear.