In the SUV (Honda Pilot), any instrument goes in the rear cargo compartment. In the Chevy Silverado pickup (extended cab) it goes in the back seat. Part of the reason for this is that I like to load/unload everything at one point in the vehicle. And I always have at least the instrument (tuba, trombone, euph) and my gear bag (and sometimes a Manhasset stand). Very occasionally I may put the trombone in the back seat. But never the euph or the tuba.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)