First, apologies for short comment lacking in specifics.
YouTube has a channel called Free Documentary which has some videos from some small, developing countries in Asia where you will see some small engines powering large vehicles(usually leftover Soviet stuff) through the gearbox. Now these are not highway worthy vehicles, but they get the job done apparently. At first I couldn't understand what I was looking at. What I thought I was seeing was a large portable gasoline powered generator(minus the generator)mounted on a small platform on the front of some fifty year old Soviet military truck. How is this possible? I assume the driver can only use the low range gears. You have to see it to believe it. I'm still thinking someone will correct me on this.