Welcome to the Club, John.
Thomas has made a good suggestion, I had a faulty wing mirror on my s type. A replacement would cost £286, eBay had one for £27.90, All is now well.
Let us know how it goes.
Regards,
Peter.
You may find it's the guage it's self that's at fault experienced same problem the solution was to replace entire binical with a secondhand unit around £40 from ebay and another 40 to have correct milage reset a lot heaper fix