Correct, for modern trucks.
yep, current trucks do carry 10, but older trucks carried nine. The phrase has been around for a long time. Trust me, I'm Italian. And we know all about mixing up the mud. Here is one, the phrase "hell on wheels." In the olden days (again) first class was up near the engine of a train. The heavy locomotive added weight, so the first car behind it rode pretty smooth. The heavy exhaust was pushed to the far back of the train. Conversely, the last car before the caboose was the worst ride. Bumpy, noisy, and the train exhaust would smother the car. The last car was also then the cheapest fare. So, the gunslingers, prostitutes, low lifes, scalliwags, rapscallions, tatterdemallions, and ne'er-do-wells would always ride in the last car on the trains. What did the train conductors call the last car? "hell on wheels." Out.