Farming Simulator

Absolutely use the Lizard truck! (Or the Golf Cart?)
Cruse control 3, with a rubber band. (as used in Darth Chimps post)

I did testing on over a dozen of the vehicles.
Here are a few stats with some (fairly rough but indicative) number crunching

Case IH Steiger 600
Fuel capacity: 1722 liters,
Constant speed: 26mph,
Liters per mile: 28L,
Total Range: 61.5 Miles,
Total driving time: 2.37 hours,

Case IH Magnum
Fuel capacity: 635 liters,
Constant speed: 27mph,
Liters per mile: 10L
Total Range: 63.5 Miles
Total driving time: 2.35 hours

As you can see the magnum gets almost identical range for the same amount of driving time, but uses only 635L of fuel, instead of 1722L. This means you have a much smaller digital carbon footprint! (and also it takes much less time to refuel).

Lizard Truck
Fuel capacity: 990 liters,
Constant speed: 34mph,
Liters per mile: 13L
Total Range: 76.15 Miles
Total driving time: 2.24 hours

The Lizard truck will get an extra 13 miles per tank over the others, and does it around 6 minutes faster. Nothing else came close to the Lizard truck, anything that had the same run time had much less miles driven.

I also would not bother refueling the vehicle, if you buy the truck just drive it where it spawns at the dealership, there is plenty of space. When it runs out of fuel just press "Y" to open the shop and go to your "owned vehicles" page, sell the truck, and if you don't change to another tab it will still be in your owned vehicles section but says you own 0. Just by it again. This is much faster than refueling any of the vehicles.

Terrin has pointed out in the comments that the golf cart didn't run out of fuel after trying on multiple saves. It drives at only around 16mph, but can be left going overnight or as long as it takes to get the achievement.

I'm not able to check this myself, but if it works is a great tip.
Thanks to Terrin.

Cheers,
Matt

I'm finding the best way of grinding out the miles is to use one of the bigger tractors,

currently using the Case IH Steiger 600 - at cruise control level 3 and with the stick fully banded, it's still doing 27mph in a tight turning circle, so don't have to worry about space and has a large fuel capacity so can go for longer, it has a fuel tank of 1722 liters so will be good for a longer time, but if you're only planning on an hour or two, then as Netrodude says in the comments, the Lizard truck is faster, though has a smaller fuel tank so won't last as long as the Case, so you can play around a little with the options you have

one problem you may encounter while idling this one is that your tractor will run out of fuel - easy fix for that, reset it,

Start menu - Vehicle selection and change it so it selects the tractor out of fuel and it will put the tractor literally feet away from a refuel tank so you can fill it up again without needed to get a mobile tanker to it

621.371192 miles you can track it in game as miles (second stats page on the pda) so you can use cruise control and rubber band your stick so it just goes in a circle or grind it out

This is quite the daunting achievement, when you're just using the golfcart method. It requires roughly (depending if you actually play a little in between boosting sessions) 45-55 hours to complete. It takes about 5 mins to do 1 mile. So 621 miles=1000 kilometers. 621x5=3150. 3150/60=51.75 Hours to complete. Again depending on how often you do it. And this is only using the golfcart. The golfcart of course has unlimited fuel which is why we can use it overnight, at work, at school etc. Good Luck!

for this achievement you need to drive in total 621.40 miles any driving that you do will count towards this cheevo hiring workers to drive for you wont count