[PaulNZ]

I've got 30KG of logs on my trailer that helps with traction, in fact I was going up a steep hill the other day on dirt and it lost power before it spun a wheel, had to back off and have a second go at it and it grunted over and only got a slight bit of wheel spin as it went over the crest and the weight shifted.

Mavrick is right though, ali wheels and heavy cut tires will give you what you need, have a look at a real truck, highway tires for highway trucks and off road tires for off road trucks.