Good point. Why not have adjustable holders. My car only has two sizes. A standard size and a smaller one which fits those energy drink cans.

Anyway, the 66 mm (2.6") width is not only standard on cans but also on bottles of plastic and glass. It's just not standard for reusable ones, which seem to be designed for fitness use or wherever its popular to have a beefy jug.

I do have a thermal cup that fits a 66mm which is great for a morning coffee (I drink a lot of coffee) but it is too small for water. (By volume I drink a lot more water than coffee.)

I could use a standard PET bottle, but what I'd like is a metal canister without thermal insulation a little taller than 50cl. Like an aluminum pint can, but reusable. (I drink a lot of pints too, but not when driving)

Without going into a rant about sealed copper cups, I just think that water tastes better when drinking from metal, and I'd like to purchase one water canister that fits my cup holder. Just one. Because its reusable.