R.O.I or B.S. (Case 1)

As I promised in my previous ROI blog, I am posting proposals I receive from vendors that seem too good to be true. My first case is from a manufacturer that compares the operating costs of gasoline vs. propane powered lawn mowers. First, let me state that I am all for the use of alternative and clean-burning fuels. My argument is not against the use of propane fuel, rather their implied cost savings assumptions. So here is the proposal I received. Is it a valid ROI or BS? 

The ROI evaluates the operating cost comparison for a fleet of 6 riding lawnmowers using propane vs. gasoline fuel over a five year period.

Current Gasoline Price:  $3.85

Hours Mowing Per Day:  7

Days Per Week:  5

Weeks Per Year:  40

Gallons per hour (average): 1.0

Number of Mowers:  6

Total Hours per Year:  8,400

Total Gallons per Year:  8,400

Propane Price:  $3.00

ESTIMATED SAVINGS

Fuel Cost                             Gasoline                              Propane                               Savings

Fuel Cost                             $32,340                               $25,200                              $7,140

Fuel Cost                             $3,234                                  $0                                       $3,234

(theft & spillage)

Maintenance Costs             $10,000                                $3,500                                $7,000

TOTAL ESTIMATED SAVINGS

Estimated Annual Savings: $17,374.00

Estimated 5 year savings: $86,870.00

Now…unlike my previous posts, I will not provide immediate comment on the proposal. I am looking for the fleet community to scrutinize the proposal and give me your comments. Is the ROI realistic? Do you feel that the cost savings are justifiable? What are the variables that would affect any anticipated cost savings? After I receive some responses to the ROI, I will post my analysis.