Maintenance costs for gas-powered trucks?

This data-file tabulates the maintenance costs incurred by a fleet of 42x CNG-powered trucks, over 16M miles in the United States. Maintenance costs averaged 8c/mile, of which 1.6c/mile (i.e., 20%) was specifically attributed to running on CNG. Specifically, gas spark plugs must be replaced every 60,000 miles, niche maintenance operations are more expensive and in one instance, the truck engines were damaged by ‘wet fuel’.