Hi I also have a 2005 VT-1100 and I get about 32 mpg on speeds less than 30mph. But I do get 45mpg at speeds above 55mph to 65mpg. I get about 42 mpg at 70mph.
The mounting of an additional tach would make it look a little weird for it is supose to look retro. But I suppose it could be done. I personally just listen to the motor and shift gears when it sound about the correct time to shift to another gear.