In mileage terms, I reckon I save, on average, 8-9p per mile - sometime more, occasionally less

. Thats a saving of £8-9 every 100 miles (my commute is around 57m round trip). Or £80-90 per month for my usual mileage. So, even om my modest miles, 7month payback isn't far off

I should also add, this is an unfair comparison, as (once the car is warmed fully, and always let it cool properly) due to the low fuel costs, you do tend to drive, errrr, 'less economically'. You think you won't, but you do

And you realise how less economically when you do the cardinal sin, and run out of LPG and have to run on petrol, and see how the gauge goes down

Petrol wise, I brimmed the MV6 in early Decemeber to get weight over rear axle due to snow, its been complaining about Range for about a week now. It will probably run another month or 2 before needing filling (I'll only pop £30 in, as its summer), but I'll do it sooner rather than later as:
a) The lights annoy me

b) I have a tendency of running out of gas

, so I need some range on petrol
Petrol use in Winter is higher, as the car takes longer to reach a temp where LPG works (esp if you just leave it on the drive, idling, defrosting

), and I tend to increase the switchover temp and times - though the Stag 300 Plus ECUs do have a temperature correction facility that we need to play with.