I shift into 6th gear at 65 mph and over. Seems that at thant point, I still have enough torque to accelarate decently if I have to while in 5th gear, but can mope along in 6th whenI settle at plus 65 mph. Anything less than 65 mph in 6th gear and the whole powerplant seems to "struggle" if you get on the throttle. I gotta believe that the same would hold true for an '08 Fat Boy.
Please keep in mind that since there is no tachometer on my '07 Fat Boy, I have no clue what RPM's the bike is running at. I suppose I can figure it out. This is the first bike, ever, that came with no tachometer. I do miss have a tach on a bike that I ride.

 

I don't change into 5th below 45?

 

umm, I agree with whitebread, not changing into 5th gear until 40-45 mph, otherwise sounds horrible if I am in 5th before 40th..Def. not changing into 6th to at least 65-70 mph.

Also, I changed out to Redline and helps with the shifting clunk. You can get the powerpack which will save you money and give you enough for all three holes.

http://www.redlineoil.com/Products.aspx?pcid=19

 

Got my Fatboy in 2007, my salesperson said save 6th gear for speeds of 70+ so, I normally do not hit 6th until i am going 70 mph or better. I try not to lug the engine, and that appears to work well.
Transmission- using synthetic and am used to the HD clunk - it's a Harley, not a rice product.

Enjoy your ride!

 

I was told by the dealership not to shift to 6th until 70mph. They said it isn't good for the tranny.

Speaking of odd tranny noises though. My tranny on my 08FXSTC has a faint whining/winding sound when accelerating in the lower gears. Dealership told me this is normal, but it's really annoying when I hear it.

 

I put the redline oil in my tranny and I really dont think it made much of a difference. I was a little disapointed because of all talk about how good it is and how much it lessons the clunking and noise from the tranny. I guess that is only the real complaint I have about my bike. To me the tranny leaves a little to be desired over the other bikes I have owned in the past.

 

45mph in 6th is shifting way to early, I don't even like to go that slow in 5th..

I have a chart on MPH vs RPM's and at 45 in 6th Ur turning less than 1700 rpm's which is way to low to be pulling 6th. I have read it will really stress the motor and lead to early motor failure.

I personaly used to shift into 6th around 65 and even then the motor felt like it was not running free. I also got my worse fuel mileage in 6th. (38-39mpg)

This spring I installed the cams and can now run in 6th around 60 (2300 rpm's) and it pulls it easy and get my best mileage (43-45) the cams made a big difference.

To make it real easy on getting Ur rpm's in the area where most feel U need to be I just remember to add a "0" to what ever gear I am in..
2nd gear, no slower than 20---usually ride at 25
3rd gear, no slower than 30----usually ride at 35
4th gear, no slower than 40---usually ride at 45
5th gear, no slower than 50---usually ride at 55
6th gear, no slower than 60

By doing this Ur RPM's will still only be above 2000 and under 2500, but if U take care or Ur motor it should last longer.

Some of the motors that have been modified a lot say they never run under 2500rpm's

 

My '07 FXSTC loaded with 2 up and 45 lbs of luggage: 55 mph + in 6 th gear. No problems, pulls hard. Completely stock setup. Below 55 I'm in 5th.
Mobil 1 75W-140 in tranny, Mobil1 20-50 in crankcase and primary. Have oil cooler.
9900+ plus miles, no issues no problems, no noise. Just finished a 3300 mile trip 2 up.
1st and 5th gears are straight cut for strength, all others are helical cut. Harley gear boxes have no synchrrnizers, they use shft dogs to engage, ergo the "clunK'. Not to worry.

 

Helical cut gears are stronger than straight cut (spur gears) Helical gears are used in large industrial applications. Ask any Industrial Gear Box Designer or Mechanical Engineer.

http://en.wikipedia.org/wiki/Gear#Helical_gears

 

i love to hear the clank of the tranny when i shift.