We travel with our 2 Weimaraner's 1 at 42 Kilos the other about 25 the last time we weighed them and more often than not have my sisters 2 Guinea Pigs as well (she reckons they are Shi Tzu, Bichon Frise cross dogs but they look more like big Guinea pigs to me) no issues in our 8 metre van and think they would be fine in a much smaller one.
(they are a bit bigger than this now as you may see by my avatar pic but I just found this old photo on my PC and loved it)
(they are a bit bigger than this now as you may see by my avatar pic but I just found this old photo on my PC and loved it)