The name "Honda" is trademarked, not copyrighted (copyright is for protecting original creations, trademark is for protecting names, brands, etc.).
The specific Honda logos, however, are copyrighted.
Trademark law differs in that you can't use a trademarked term in a manner that is misleading to consumers - Ford can't make a car called the Ford Honda, for example. Someone could start a business called "Honda Widgets" and as long as they don't produce products that are already produced by Honda, or use a logo that looks too much like Honda's, they would be OK. Honda may still get upset, send a C&D letter and threaten legal action, but unless they can prove their trademark is being used to confuse consumers, they don't have much of a case.
If you create a decal that uses the word "Honda" in a manner that doesn't replicate any of the myriad of Honda logos, or attempts to label something as a Honda that isn't really a Honda, then it should be OK. You should use a superscripted TM or R in a circle, depending on the nature of the trademark (registered or not) to indicate that it is a trademarked name/brand.
If you made a decal that says "This is my Honda!" in a font that doesn't look like a Honda logo you should be legally OK.
If you made a decal that says "This is my <Honda logo>!", then you are violating copyright and trademark law.
If you make a decal that says "This is my Honda!" in a font that resembles the Honda logo, then you're only violating trademark law.
Disclaimers - I am not a lawyer, nor do I play one on TV. Your mileage may vary. Ask your doctor if prescription medicine you saw on TV is right for you. Do not feed your mogwai after midnight and never get them wet.