Dear Tom and Ray:

My husband's latest great idea is to convert our 2001 Ford F-150 truck from gas to diesel. I would very much like to know if this is worth the time and money to do. The truck currently has more than 100,000 miles and is driven maybe a few times a month, but he is convinced that this is a good idea.

I can only imagine what it would cost to pay a mechanic to do the work. Can you help by providing me with some straightforward and unbiased advice? I'm counting on you to help burst the bubble.

-- Laura

RAY: It's actually very easy to drop a diesel engine into a Ford F-150, Laura -- if you happen to be a Ford factory!

TOM: For the rest of us, it's much more of a project. So perhaps doing the math will make your decision easier.

RAY: If he's going to buy a new diesel engine for this truck, he's looking at a $10,000 expense to get started. Then he'll need a matching transmission to go with it.

TOM: So let's say he buys a used diesel engine/transmission combination. That'll run him about $6 grand. Plus shipping. Add another $4,000 in labor and other parts, and let's say it's a $10,000 project.

RAY: But then you have to look at all the money you'll save once you have the diesel truck. The cost of fuel obviously varies, but these days, diesel costs about 50 cents more per gallon than gasoline. And let's say -- for this calculation -- you get 20 percent better mileage with a diesel.

TOM: So, instead of the 15 miles per gallon you get now, you'll be getting 18. You say you drive the truck only a couple of times a month. So I'm going to guess you drive this thing 300 miles a month, or 3,600 miles a year.

RAY: If gasoline costs $3.50 a gallon and diesel costs $4 a gallon, your annual fuel cost for the gasoline truck is $840.

TOM: And for the diesel, it's $800. So your annual savings will be $40.

RAY: That means you'll recoup your original $10,000 investment in only ... 250 years.

TOM: So it's a close call, Laura. I wish you guys luck as you struggle over this very difficult decision!