This really is a heated subject, and gets much worse when people start bashing any of the big 3. (most) of those who are into firewood will do anything they can to stretch their gas, especially in todays economy. I own an 85 HD f250 6.9 4 speed with 1 ton running gear and E rated tires and have no issues filling the bed with mixed species poles, the tailgate down, and cab height. I have no issues slowing down either--yet I don't speed when hauling, and gear down in a pinch. Even so my brakes are more than up to the task as I usually have a trailer in tow (usually a pickup bed with sides and no brakes)
I often dream of finding a dana 70 front axle, double stacking all leaf springs, flat-bedding, and adding dual wheels out back and adding air bags for overkill, yet for now it does the job.
I know others that boast putting 1-1/2 cords in the back of their 1/2 tons, and I've seen an 89 toyota 2wd pickup repeatedly loaded with 3k lbs of gravel, and I have recycled scrap that has squatted my truck more than a cord of dry fir (still not bottoming the suspension at 3700 lbs)
Yet a cord of dry oak weighs inbetween 3000+ for dry black oak and 5000 lbs for dry live oak, and wet live oak can weigh in at 8k lbs. I personally wouldn't load my truck up with a cord of dry live oak, yet others might be willing to make the trip to gain some $$$. There are also other factors to consider, I live in the mountains, on a 2 mile dirt road with 17 or so 2' water-bars, the roads here are windy with steep ascents and descents. I likely wouldn't think twice if I lived in town and only had to drive half a mile to deliver a cord of oak. I likely would do it here if I had overloads on my truck.
Here is a cord weight calculator for different species.
Firewood
Keep a tape measure and a calculator handy when he does show up. Remember that a cord is 128 cubic feet of wood stacked, and 180 cf loose. No pickup truck bed except a stepside is going to be a perfect 4' wide, so don't expect the stack to be 4' tall unless the bed is less than 8' long. Use your judgement, and don't be afraid to refuse the load. While he might not be happy about it, he may think twice about shorting people the next time. (A wasted trip in gas really does hurt the wallet)
On a side note, I personally will deliver 1-1/5 to 1-1/4 cords for the 1 cord price. Most people appreciate this and call me back the next year, or when they have enough cash for another load. There are many swindlers out there, and there are a lot of people who are just looking for a way to survive these hard times. Looking for the cheapest deal will usually put you face to face with the former. Find someone that does you right, then stay with them.
Just my .02