Works with both V-belts and multiple V-belts IF: 1. Belt width 7/8 inch or less; and 2. The maximum measurable tension does not exceed 150 pounds. This information is from the Gates website and my testing. For example, Mercruiser recommends the following tension for 8.1 engines: New belt: 110-130 lbs / Used belt: 70-90 lbs (target tension). Here's an easy way to check belt tension without a gauge: Take the longest section of belt and rotate it 90 degrees. If you really can't turn it anymore, the suspense is good. Search "belt tension trick" on YouTube.