Aha! Been looking at a Harley parts catalog? hehe
The speedometers you have been looking at are likely the mini-gauge type used on Harleys and customs. I think if you look at the JP Cycles catalog, they have the best variety. There is one that has BUILT IN LEDs for idiot lights which would work really well as you could retain idiot light function going to a speedo only set up.
The 2246:1 is definitely as Sportster only ratio. I used a 2:1 on a 1996 Vulcan 1500A I owned so if I were going to try one, that would be the one I would try but I could be wrong. If you called the JP Cycles folks, they have great tech support and might actually know the answer. See if they will let you return the guage if it doesn't work as it should. Also keep in mind that the handlebar bracket used to mount the gauge will be for a 1" bar and you have a 7/8" so will have to use additional wrapping; probably some rubber of something to build up the bar diameter to use the provided mount bracket.
