I didn't realize that every 80 and 100 here in the US is a "high revving runaway train" on the trails.
...OK, one last try... I promise... If SHOTTSUZJ100 won't believe me, maybe he'll believe someone else...
This is from a site in Oz called "4WD Encounter Australia" but I thought it offered a good, simple explanation of this concept:
http://www.pps.net.au/4wdencounter/articles/dieselvspetrol.html
"Another distinct advantage of the diesel engine is its engine braking. The design of the diesel engine means that it sucks in air unrestricted and the amount of diesel fuel injected determines how much power the engine makes. A petrol engine requires a constant fuel/air ratio so the amount of air drawn in by the engine is regulated by an inlet buterfly in ratio to the fuel being used. What this means is that at idle, a diesel engine still draws in a large amount of air (RPM x engine capacity) while a petrol has the butterfly almost closed and there is very little air being drawn in.
When using engine compression for downhill braking and factoring in the compression ratio of 20:1 for a diesel and only 9:1 for a petrol, the diesel engine offers significantly greater resistance to an increase in engine RPM. The diesel engine offers significant resistance as large quantities of air are still being drawn in through the unrestricted air intake and being compressed to 20:1. The petrol engine has the air intake butterfly closed during downhill braking and so only minimal quantities of air is being drawn in and then, its only being compressed to 9:1. This is why a petrol engine tends to "run away" compared to a diesel."