Off-Roading
Four Wheel Drive Concepts
   Four Wheel Drive Types
   Petrol vs. Diesel
   High & Low Range Gears
   High & Low Range Gears
   Differentials
   Suspension Types
   Tyres
   Vehicle Recovery Points
Offroad Driving Techniques
Vehicle Recovery Techniques
Reference
Calculators
Concepts
Petrol vs. Diesel
Much off-roading is done at low speeds (as slow as possible, as fast as neccessary), using a lot of power. This requires an engine that produces high amounts of torque at low revs (aka grunt). Diesel engines, by their very nature, are low-revving, high-torque engines, although modern, purpose-built petrol engines have similar characteristics. Entire rain forests have been destroyed documenting the virtues of one or the other, and I present some arguments in favour of each.
Diesel Engines
As mentioned above, diesel engines are well-suited to off-roading because of their low-revving, high-torque nature. However, diesel engines are much less responsive than petrol engines. Other considerations include availability of diesel, and their suitability for wading.
Availability of fuel in your country must be the first consideration. Diesel is available almost everywhere, although, because of its high sulfur content in widespread areas (such as Africa) require a very high degree of maintenance and therefore cost. Diesel injection systems are complex and therefore difficult to understand, and expensive to fix or replace, especially in remote areas.
Lastly, because diesel engines induce combustion through compression, they are well-suited for wading through water.
Petrol Engines
Petrol engines, on the other hand, produce similar torque, but at much higher revs. This becomes especially aparent when descending steep slopes, as it will be seen that diesel-powered vehicles descend much slower than their petrol counterparts. Bear in mind, however, that a large engine in a small car (such as the 4.0l straight six found in the Jeep Wrangler) provides an excellent power-to-weight ratio. Even though maximum torque is produced at higher revs, it will still descend a slope slower than a Defender TDi or Pajero 2800 Turbodiesel. Petrol engines are also much more responsive than diesel engines. "Feathering" the throttle in a petrol engine, coupled with effective clutch control, allows for greater driving control and better driving flexibility than diesel-engined vehicles.
Of course, most mechanics world-wide understand petrol engines, and frequency as well as cost of maintenance and repairs are substantially lower than on diesel engines. On the down-side, however, many modern petrol engines use electronic injection systems which can be as complex as diesel counterparts. Further, most modern petrol-powered four wheel drives run on unleaded fuel, which is not always readily available in remote countries such as Morocco, Mozambique and Zambia.
The last point worth mentioning is that while practically feasable and still cost-effective, it is much more difficult and expensive to water-proof a petrol engine for wading than it is a diesel engine.