Before you convert their vehicle to propane, you will want to know much it will cost to drive.  We know that propane has a lower specific energy than gasoline.  Taking information from the US Department of Energy:

• Energy Density of Gasoline           115,000 BTU/gallon
• Energy Density of Propane            84,500 BTU/gallon
  • Ratio of Energy Densities    1.36

We need to know the price of each fuel.  This could vary quite a bit from one location to another so check prices in your area.  The ratio of the price of gasoline to propane must be greater than 1.36 or else the conversion will not make sense.  For example, let's use the following prices:

• Gasoline Price (regular - 87 octane)               $0.85/litre ($3.21/gallon)
• Gasoline Price (premium - 91 octane)            $0.95/litre ($3.59/gallon)
• Propane Price                                                    $0.55/litre ($2.08/gallon)
  • Ratio of regular gasoline to propane       1.55
  • Ratio of premium gasoline to propane    1.73

Let's assume your car gets 10 litres per 100 km.  Your equivalent gas consumption would be:

• On Regular:    10 x 1.36 / 1.55 = 8.8 l/100km
• On Premium:  10 x 1.36 / 1.73 = 7.9 l/100km

Driving savings would be 

• On Regular:     (0.85 -1.36 x 0.55) x 10 = $1.02/100km
• On Premium:   (0.95 -1.36 x 0.55) x 10 = $2.02/100km

Let's assume that you drive 50,000 km per year, your annual savings would be:

• On Regular:    50,000 x 1.02 / 100 =   $510/year
• On Premium:  50,000 x 2.02 / 100 = $1010/year

You can see that even with long commuting distances, it is difficult to recover the cost of conversion with a 30¢/litre differential and a fuel economy of 10 l/100km.  However, by having a suitable conversion vehicle and doing much of the work yourself, you can do a conversion that pays for itself in less than 2 years, not including the savings in maintenance.

For our American friends, let's assume your car gets 20 miles per gallon.  Your equivalent gas mileage would be:

• On Regular:    20 / 1.36 * 1.55 = 22.8 mpg
• On Premium:  20 / 1.36 * 1.73 = 25.4 mpg

Driving savings would be 

• On Regular:     (3.21 -1.36 x 2.08) / 20 = $1.91/100miles
• On Premium:   (3.59 -1.36 x 2.08) / 20 = $3.81/100miles

Let's assume that you drive 30,000 miles per year, your annual savings would be:

• On Regular:    30,000 x 1.91 / 100 =   $573/year
• On Premium:  30,000 x 3.81 / 100 = $1143/year

You can see that even with long commuting distances, it is difficult to recover the cost of conversion with a $1.13/gallon differential and a fuel economy of 20 mpg.  However, by having a suitable conversion vehicle and doing much of the work yourself, you can do a conversion that pays for itself in less than 2 years, not including the savings in maintenance.

Sources of Information:

http://www.propanegas.ca/FileArea/PGAC/Propane%20properties.pdf

http://en.wikipedia.org/wiki/Propane_(data_page)