I do have a gas golf cart and there are no fumes. If there are fumes, it needs mechanical help. Also, the horses have a tendancy to come around on the side, not stay in the back. The stallion thinks he is supposed to race it or pull it along when he goes out and he loves it.
SAFETY is your first concern. I never take more than two at a time and one is on each side. Also, dont start out running. Go SLOW so they will walk with you and while your helper is holding them, they need to tell you when you need to STOP if there is a problem. If you are walking and start out slow, once they get the idea of going with the cart- it may take several times, they will speed up some. I never go faster than a trot.
Do NOT use a golf cart that has been lifted, or has wheels that stick out past the fenders, or other dangerous issues where your horse could be hurt. I highly recommend a rear view mirror too of some sort so you can also observe what your horses are doing.
Also, leave PLENTY of space between the golf cart and your horse. If your horse is trotting behind, keep in mind they have to have clearance between their forward stride and the back of your cart. You dont want them striding forward to have their leg hit the cart.
Going in a straight line, and out around the neighborhood, is a lot better on their legs than lounging in a circle. It also gets them used to dogs, kids on bikes, etc.... if you live in a fairly quiet neighborhood- but I would not attempt that until they are used to the cart and you are used to ponying them with it and can take your foot off the gas at the first sign of trouble. Know your horses and read their expressions. If mine are startled at something new, we slow to a walk, or even stop, to look at it and then SLOWLY continue on past.