If your going up wood track roads I would suggest a 6x2 tag axle tractor.unit twin steers tend to loose traction and pusher axle tractor units tend to do that push you when in wet conditions now for the technical bit as for mpg all depends on your driving if you sit with foot to floor no truck is goin to return the mpg you want.also so get one with cruise control and a decent engine brake,and use them as much as possible.if your not weighted up to the max go for a 450 no higher than a 500 but do go for a big engine cc I.e scania 13liter 450 or a Volvo 13liter 480 the last bit is your preference and what truck you like
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