BUT I still have my OEM drive gear because when I bought the gauges, the supplier did not have any of the separate rear wheel sensors so I used what came with the E-Speedo, which plugs into the stock speedo drive gear. So basically, I spent the money on E-gauges to replace perfectly functional stock gauges and STILL have the Speedo drive gear. 
I calibrated mine using a GPS app on my iPhone. I taped the phone between the gauges so I could easily see it and zeroed the gps mileage and activated the speedo calibration while stopped. I then rode down the straight road at around 40 MPH and slowed down at 9/10ths/stopped as the GPS hit 1 mile and pushed the speedo cal button.
Seems quite accurate - measured using the same GPS app and also based on the OEM gearing/sprocket/RPM/MPH info in the service manual.
FWIW - I almost removed the E-speedo after installing it, becoming quite frustrated with the calibration, especially since my OEM speedo worked fine! I found it rather difficult, frankly, the button pushing - or my failure to do it properly - caused me to have to re do the measured mile several times before it actually "took." I tried the static method first and got so PO'd doing that that I decided to quit for the day before taking a 3 pound sledge to the e-speedo!
TBH, as far as a recommendation, If the stock gauges work properly, as mine did, I don't see any worthwhile benefit. I bought them primarily for the speedo, with the intent of eliminating the speedo drive gear. I have never have a problem with it but while on the IOM last year, there was a guy in a pub parking lot dissembling the rear wheel of his Commando because the drive gear had become red-hot and seized. I thought...hmmm, might be good to get rid of that!
I calibrated mine using a GPS app on my iPhone. I taped the phone between the gauges so I could easily see it and zeroed the gps mileage and activated the speedo calibration while stopped. I then rode down the straight road at around 40 MPH and slowed down at 9/10ths/stopped as the GPS hit 1 mile and pushed the speedo cal button.
Seems quite accurate - measured using the same GPS app and also based on the OEM gearing/sprocket/RPM/MPH info in the service manual.
FWIW - I almost removed the E-speedo after installing it, becoming quite frustrated with the calibration, especially since my OEM speedo worked fine! I found it rather difficult, frankly, the button pushing - or my failure to do it properly - caused me to have to re do the measured mile several times before it actually "took." I tried the static method first and got so PO'd doing that that I decided to quit for the day before taking a 3 pound sledge to the e-speedo!
TBH, as far as a recommendation, If the stock gauges work properly, as mine did, I don't see any worthwhile benefit. I bought them primarily for the speedo, with the intent of eliminating the speedo drive gear. I have never have a problem with it but while on the IOM last year, there was a guy in a pub parking lot dissembling the rear wheel of his Commando because the drive gear had become red-hot and seized. I thought...hmmm, might be good to get rid of that!
Used my Zumo to check against the electronic Speedo. GPS says 60 and Speedo reads 69
I have recently fitted one because, although speedo head was working fine, the drive gearbox on the rear wheel was failing and I was fed up with the poor quality replacements.
…...or it may make the odometer accurate.
Is the 2mph difference speedo over- or under-reading compared with the GPS?
The main difference - and it's a big one - is that speed cameras are not used for enforcement in most of the US EXCEPT in a few states where they are used in school zones or other "special," low speed areas. The only common use of cameras is in some states for traffic light violations (running a red light, etc). And even for that, they can only be used to fine for the offense, points cannot be assessed unless an enforcement officer personally issues a ticket.