
I can appreciate Steve's frustration with the speedo. I use a mix of Navman units and a speedo app on my 'smart' phone (to say it is 'smart' is a bit of a stretch - I can't even type words without it making them into something else!),
The trouble I have is the batteries going flat. They never seem to last through the trip. My solution is the installation of a USB charger in my 1924 Morris Cowley to provide the needed power. Having confirmed it works I am installing the same setup in my T Roadster and in my TT.
I looked for an accessory motorcycle USB charging unit to 'hard wire' in the car. My weather proof unit came from a helpful Chinese web site (which used for its price/choice trade off). Check the voltages, but most can cope with 6 or 12v in and automatically adjust to the USB voltage out, so that saves you having to install other voltage changers (my thoughts are the more bits = the more problems)
I remembered to get a two slot version so Mrs W can plug in her phone at the same time – or so I can charge phones for friends (maybe I could actually ‘charge’ for that as well?) The rubber cap keeps the dust out when not in use.
To make sure wires and items being charged didn’t entangle pedals or gear shift, I decided to install the unit on the passenger side at the front of the seat base, low down and discreet but easy to access and ‘plug’ in the phone (and many other USB powered accessories too). This location is discreet (keeps the dash clutter free), is close to the battery (no long wire runs) and makes it easy to plug items in before getting in the car.
It makes a great conversation starter during stops too … “can I charge your phone for you? – Android or Apple? (I had to throw in that ‘tech talk’) - it’s no problem at all!”
Although probably not necessary, I ran the power side through a fuse as well - 'just in case'.
The Result
An easy to reach unit that is out of the driver’s way and is discreet. Now just to figure out how much ‘charging’ I could get away with for those in need of a ‘top up’

Cheers
Adrian