I think for these minivans you can get 10-12 avg only.
You can make ur van drive more efficient by using correct viscosity engine oils (like 5w30, as mentioned under the bonnet).....replace/change trans oil every 50k km....check sparkplugs condition.
I m not sure about Suzuky Every/Mazda Scrum or Honda Acty, but pretty sure about Nissan Clipper and daihatsu hijet that they have 4spd auto trans....i hope urs doenst hav a 3spd auto because it has bad fuel economy
