Jens,
I live on an acreage 38km from Edmonton. When I go for a load of coal,that trip is 105km round trip. When I go somewhere,it is almost always to Edmonton or Dodd's coal mine in Riley.
I would say that about 1/2 litre of diesel is needed for a 10 minute warmup at 1000rpm. If I am going on errands in town,I dont need to purge. Thats if I am in and out of the store. In summer,I am good for an hour. So best case scenario would be 1/2 litre for warmup. I purge about 4km from home@100km/hr,so that would consume about another1/2 litre.
If one doubled this diesel consumption for winter,then about 2 litres of diesel would be used. A trip to the city with some driving around woul use around 12 litres. Calculate 'Miles Per Purchased Gallon' or MPPG. Isuzu Elf gets 25 mpg on diesel,average.
On best case scenario,1litre of diesel to 11 litres of veg oil would yield 250 MPPG.
On worst case scenario,2 litres of diesel to 11 litres of veg oil would yield 125 MPPG
It would even be better than 250 MPPG,if say, I went to calgary,a distance of 400 miles round trip. Normally that would burn 16 gallons or 72 litres. Under good circumstances this would use 2 or 3 litres of diesel. This would be over 500 MPPG!
Short trips in town,like you do,well your still saving money on diesel. But not as much.