Nice Topic!
I have a few pictures taken in Thailand ... but most of them are not motorbike related.
Would you ban me if I posted them here?
Well, actually there is no "right" spelling in English. The only correct spelling is in Thai script. There have never been any set rules for transliterating Thai into English.
So, whoever makes a sign in English can write it however they like. Personally, I like "Terng".