I am thinking of changing my belt sprockets on my 2005 883 low sportster. Has anyone done the 31tooth front sprocket and the 66 tooth rear sprocket? Will my bike have enough torque for this combo? Does anyone know what the rpms will be at 75 mph? I ride solo

 

Here you go, have fun.

http://www.nightrider.com/biketech/calc_speed_rpm.htm

 

Your stock gearing is fine for that road speed, as is your engine. My 883 is just great at speeds up to around 90mph, which is as fast as I normally ride it. If you don't do so wear ear plugs when riding at that sort of speed, to improve your comfort.

Your stock gearing is 28/68, giving a drive ratio of 2.429:1. Changing to 31/66 will give you 2.1290:1, which is a change of 14%. In other words your engine revs at any given road speed will drop by that much, say from 3,000rpm to 2,630rpm. That's quite a lot IMHO, also unnecessary as already mentioned. Will your bike have enough torque? Yes, but you will probably find you have to change gear more often, especially when heading into a strong wind, or riding up a long steady climbing road, also to overtake.

Now if you were to fit a Hammer 1275cc kit......

 

I put the 32 tooth front on mine and will put the 66 rear on this winter.

Plenty of torque, pulls any hill I ride on without any downshifting.

 

You say Now if you were to fit a hammer 1275 kit. Please continue your thoughts as that is my plan down the road

 

That was tongue-in-cheek, however to be serious, adding such a kit will give your bike a major boost in HP and TQ, making it much more able to drive a higher gear. While the gearing change above will increase top gear by 14%, a Hammer kit will give a much bigger increase in TQ - HP will increase from around 45 to around 80 - sound attractive?! It's well worth getting your favourite beverage and browsing the Hammer Performance website, where you will find a remarkable amount of Sportster tuning info. Also search for Hammer threads in here, as several members have documented their builds, showing what is involved.

 

This is an rpm/mph chart for Sportsters.
Includes all the available front pulleys (27-32) and the rear 66 tooth for the 2004-Ups:
https://e7a45847-d8f1-42d4-9b57-8adf...0b9a3ff417.pdf

 

keep in mind ur speedo will be off by 14% as well dont know about 05 speedo vss recalibration options im sure there are some used to be speedo healers you could get that wired into the vss your speed will read lower then it is actually at 75mph your speedo will show closer to 65 mph

 

Comparison of 28/68 to 31/66, in numbers, is right. The change results in a 13.6% change. But, this final drive ratio (31/66) is well within an acceptable performance range for the 883.
The "stock" final drive ratio (primary, 34/57 & drive pulleys, 28/68) is 4.07:1. The 31/66 final ratio is 3.68:1. For reference the final drive ratio for the 1200 (primary, 38/57 & drive pulleys, 29/68) is 3.52.
For anyone enjoying a 70 mph cruise rpm of about 3770, or 4850 rpm at 90 mph, the 28/68 final drive is great. But for those that prefer to cruise along at about 3300 the 31/66 pulley combination can get you there without changing the primary. And, if you want to drop another ~125 rpm, there is a 32 tooth front pulley available. Final drive ratio (34/57 primary & 32/66 drive pulleys) 3.46:1.
Increasing the cc doesn't lower the rpm, but it can sure get you to your cruise mph quicker!

 

A possibly better option would be to change the primary gearing to 1200 spec, this would require a 38t (vs. 34t) engine sprocket and a new primary chain...this would bump gearing up significantly (about 12%) but not cause a speedo error, cost would be about $150 or so.
For rpms at various speeds check out https://www.gearingcommander.com/