The hourly cost of labor for a couple of unionized employees (the minimum would probably be a driver and a loader) is probably greater than the hourly cost of the fuel that's used, especially if they're running overtime. Factoring in employee costs like health insurance, pensions and vacation time, I'd guess at least $100 per hour for a two-man crew. You can get many gallons of diesel for that amount, especially if you purchase in bulk for a fleet of city vehicles.

Also, the number of garbage trucks that the city would need to own would go down, as would the number of employees needed to maintain them.

I think the article emphasized the fuel savings because they didn't want to explicitly call attention to the fairly obvious conclusion that this efficiency improvement, if successful, would cause lots of city employees to lose their jobs.