I've used in the past an OBDII Bluetooth dongle linked to OBD Check app on my Android phone.

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Impressive to see all the dials light up when you start engine. You also get max and min dial markers for each session.

The Speedo on my car, what you can see of it and allowing for parallax, seems reasonably accurate.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone