I always figure about 50 miles per hour, it usually comes out pretty durn close. Most service intervals I've seen that list both miles and hours have also been 50 mph. 58k would be 1160 hrs. Most heavy duty diesel engines need to rebuilt at about 10,000 hours or 500,000 miles. Idling hours are harder on a diesel, leads me to think that it doesn't have as many hours as you think. Many diesel engine hour meters continue to run if the key is on even if the engine isn't running and can rack up erroneous hours. We have a Kubota tractor at work whose key was accidently left on when it was parked for the winter, gained over 2000 hours(100k miles) before the battery went dead, all while it was parked. If the oil hadn't been changed and hours recorded before it was parked I would have thought it was old beyond it's years.