Another possibility is to get used to it and leave it alone.
My car is a Euro version originally from Genoa Italy....came to the US completely unmodified including the KM speedo.
Miles to KM: If I see a sign "10 Miles to Paradise" to get the KM's so I can use my KM odometer, I multiply the 10 by 6 then "adjust" the result (60) by adding the leading 6 to the 10 miles to get 16 KM's. Its rough but it works for me.
KM to Miles: If I'm going 85 KM/hour, to get my speed in MPH, I multiply the 8 of 85 by 6 to get 48 then I add a bit since it was 85 (not 80) and also because the actual convert number is not 6 but 6.2...and I find I'm moving at 48 MPH plus the fudge factor so my estimate is about 50 or 52 MPH.
This convert estimate is also handy when on a vacation in a KM country to estimate how many miles to the Church or the Square.
I admit that serving in the US Army in Germany (wandering about lost in the Bavarian forests) for several years kind of fixed this "converter" in my head but I'm sure anyone can do it with a little practice...and it will keep the car original.
And a KM speedo is a great conversation starter when car buffs in the USA ask if my car can really go 220 miles an hour...they come down a bit when I explain it's a KM speedo and the max speed shown is about 140 MPH.
For the math buffs and purists here, I allow that this is not exact but with the usually small numbers used in driving locally the error is small...but even with a sign that says "500 miles to Chicago", my simple calculation would add a 3 to the 5 to get 800 KM's. The more correct figure is about 810...not bad for an estimate.
So if a simple multiplication is OK with you, think about leaving the KM speedo in the car.
Richard M, NYC