To be honest if you're trying to calculate mpg savings I'd be buying an older van rather than leasing a new one. These companies aren't stupid and they'll be making money out of the leased van (or else they wouldn't be in business). To look at this differently, some of your profits are going to another company to keep your company going.
Another factor is some customers might think you're a little on the expensive side if you're travelling around in a new van so might go to a competitor instead. There's nothing wrong with an older van at all. Many think you'll be good value as you don't seem to spend your money on a posh vehicle (but then some customers might think you're not that good as you can only afford an old tatty one!)
Many swings and roundabouts for choosing a new/old van.
If you own a van you can offset the depreciation, expenses, repairs, etc against your profits (and lower your tax liability). If you lease a van you can only offset the expenses of running the van and payments. The difference between these two figures will easily outweigh your mpg savings!!
If you can't afford to buy a van outright you can always get a loan (and offset these payments (hire purchase)). This means the van is yours and is an asset to your business. A leased van is not an asset - only an expense.
As you can see, I'm not a fan of leased vehicles - it's another form of loan and as the country has found out in the last 2-3 years, loans are expensive and have to be paid at some stage.
If you own a van there is a major disadvantage over a leased vehicle. You are responsible for repairs, whereas a leased company has to pay for these. However, new vans (the ones they lease) tend not to require repairs so some of the money you're paying to the leasor is basically an insurance for the leasing company. You'll only win if your van keeps breaking down.
In my experience (of chatting to others) a 6-8 year vehicle will need to go to the garage twice a year - once for service/MoT and the other time for a repair. The time lost is probably less with your own van as the leasing company will probably want a day or two to transport the van to/from its garage as well as the time to repair. A vehicle under 5 years old tends not to need any repair during the year.
We all want new stuff and reliable vans which is why leasing seems attractive - but, in my opinion, it is a very expensive option for running a vehicle.
