Hello,
this is quite the conundrum
I currently have a BMW 328 with LPG setup (basically the same kit as CNG) and a Diesel VW Golf that I run on a mix of veg/diesel (a poor man's bio, if you will)
For work my commute is circa 17000 miles p/a, and then I do 10-15k on top running about, so similar overall mileage to you- here's what I think:
LPG/CNG
1. My 328 gets circa 26 imp mpg using an old style single point set up- its not the best for economy, or power, but it gets the job done, and car is fast and smooth
2. In terms of servicing costs, mine needed a new vapourisor in Nov last year, was £75 (less than a new petrol pump for in the tank). Filters are peanuts (£5 ish max). The gas means less bore wash of the oil by petrol, which means the oil lasts longer, and comes out golden.
3. Range- with a 60L tank holding 50L (can only fill to 80%) I get circa 270-300 miles depending on what I'm doing. this makes the PEMPG (Petrol Equiv MPG) circa 45-50 imp mpg. On straight unleaded the best I've managed over a tank is 37mpg, so it does offer a saving. My car had the kit already fitted, but if I installed it today (£900) the payback is approx 12 months. Between the Petrol and LPG, I'm close on 800 miles range
4. Availability- Having moved house, I no longer have an LPG station near my home, but I still have 3 within 5 miles of work. If I need to fill up at home, I combine it with a trip to see my parents (earn some "good son" points!). I also have an App on my phone that has maps of all the LPG stations in the UK, and has prices updated by users daily.
5. Cost- Our Chancellor (head of the treasury) announced on Wednesday that the LPG taxation was being held at current levels for the next couple of years- so its approx 1/2 the cost of Petrol here
Diesel/Bio Diesel
1. No noticable difference in MPG between running pump diesel vs 30% veg oil. It has to be noted though, the car is nowhere near as engaging or entertaining to drive as the 328i- but runs ok on veg (unless its too cold, then its stuttery and wont hold an idle!)
2. I reckon I need a new fuel filter, due to the gunk cleaned out my lines (£12) and the glowplugs are having a hard shift in the cold mornings- but blending the fuel seems to help. Regular bio diesel wont have this problem as it wont be as thick as Veg
3. Being Diesel, range is not an issue 650 miles is not uncommon- veg/no veg appears to have no real impact on range. this is a big thing for me, means less than a tank a week- rather than 2 fills of LPG
4.Availability- Pump diesel available everywhere, Veg oil available from all supermarkets ( I use a couple of sites to compare prices- aiming for £1 a litre of veg). Regarding Bio- Diesel, I reckon discussions with local take aways etc could be fruitful- but I dont know how happy the girlfriend would be with me refining bio at the house
5. Still using pump diesel (£1.459/L) and paying £1/L for veg. Assuming I can get to using more veg- say 50% in summer, my costs per mile should go down.
I use the golf for commuting, my Mrs uses the 328 for her work (she gets 15 days commuting to a tank of LPG)
Both cars have their place, but at the moment, diesel wins for my purposes (low cost commuting on boring roads!)
In short- in your position, I'd be asking myself " do I need my truck as it is?"
If you need load hauling capacity- then probably yes,
but if you just need 4wd, then something like a Subaru with CNG would be a better bet.
or do you have any smaller 4wd pickups with diesel- like the Ford Ranger etc?