Since you are talking about a digital odometer Bonnie, I should add that changing the sprockets will not affect the relationship between the speedo and the tach. Because your speedometer reads at the transmission, it also doesn't know you changed the sprocket. On Bonnies with a front wheel reading speedo, you will notice a difference with a sprocket change.
Where you will notice a difference on a digital odometer Bonneville is when you compare your bike speed to a GPS or one of those speed check signs that tells you how fast you are going.
As I said, I re-calibrated my speedo for the 17 tooth sprocket with TuneECU. Now, when I am traveling at 65 mph in fourth gear, my tachometer says the engine is turning 5000 rpm. If I went back to an 18 tooth sprocket, the tach would still read 5000 rpm when the speedo reads 65 mph. My GPS, however, would tell me that I'm actually going faster than that.
Here is something I am not sure about but I think it is right. When Triumph builds the Bonnevilles, it sets the speedometers to read a little high, but I think they set the odometers to read correctly. They read from the same sensors but might have different programs in the ECU.
When I changed to a 19 tooth sprocket for a while, I found that my speedometer agreed with the GPS, but the odometer said I was going further than the GPS said. Now, with the 17 tooth sprocket and re-calibrated speedometer, the GPS says I'm going further than the odometer says. This leads me to believe the tach and speedo are running on different programs. That makes sense to me because Triumph has a good reason for wanting the speedo to read optimistically and it also has good reason for wanting the odometer to read correctly. I'm not sure of this, but it is my observation.
So in order to re-calibrate your speedo with TuneECU, you will need some way to accurately check you actual -- not indicated -- speed. As far as the odometer goes, I think it will only read correctly when you have the stock tires, sprockets, and the optimistically reading speedometer.