I have done it both ways if you strap it to a sissy bar ... make sure you use some solid straps as bongees can move around and let go ... that hard shell guitar case catches a lot of air force ...

a friend let me try his back pack style soft case and that I felt was much safer and more sure of and I bought one and that is what I use now

good luck either way ...

play on

 

As above there are some ideas....yet

If the guitar is valuable I'd take the car.
Wouldn't want to fall over with a "priceless" guitar.

 

i ride all the time with my golf clubs strapped across my back .. sometimes doing 70 but i would say my golf bag might be a tad more aerodynamic seeing as it doesnt have what im gonna call the "base" of a guitar .. but **** it man you can do anything on a sporty!