Difference Between AWD vs 4WD

Many years of driving I have owned several cars with rear-wheel drive, front-wheel drive, four-wheel drive and now I own a Mahindra Thar with four-wheel drive. I bought the Mahindra Thar not because it had four wheel drive but because it was a nice looking car designed for country and off-road traveling, which is what I enjoy doing. But since I bought it I am curious to know the difference between all-wheel drive and four-wheel drive, and here's what I found out?

Four-wheel drive doesn’t work  all the time under normal conditions, only torque is delivered to rear wheels which drive the vehicle forward while the front wheels spin freely. The driver has to push a button or pull a lever to activate a modern four-wheel drive system. But four-wheel drive isn't meant to be ON all the time. So, we have to know when to turn it ON and turn it OFF. But on the other hand all-wheel drive is ON all the time and the computer manages the system by calculating input parameters. Just going down the highway the system will send most of the power to the rear wheels for maximum fuel efficiency. If the road condition changes such as rain or pothole the system adapts and evenly distributes the power evenly to all wheels. So, you can have maximum drive grip with minimum (almost negligible) chance of wheel spin.

In contrast on a normal two-wheel drive car or even a four-wheel drive, in two-wheel drive mode the power transmitted to the wheels will always look for the path with least resistance. This is normally the tyre that is spinning and not able to make vehicle move forward. Most people will be familiar with this, when they accelerate on wet roads at the lights or at an intersection and the drive wheel just spins freely. On the slippery road cars with all wheel drive don’t do this. Since owning my Volkswagen Tiguan AllSpace I have never spun my wheels on a wet slippery road neither taking off the lights nor turning at an intersection. When I drove my four-wheel drive in two-wheel drive mode my car slips all the time and I even spun 180 degrees.

So, what advantage does 4-wheel drive have over all wheel drive? Well in extreme and rough surface conditions such as loose stones, dirt, deep mud, snow or stagnant water a four-wheel drive system will provide more traction & control for the driver. But still vehicle have to set in full drive mode which can be dangerous, once back on dry/ normal roads conditions as axles are not designed to rotate at the same speed in normal cornering. Four-wheel drive is better for off-road in extreme and rough weather conditions and it should only turn ON when required for those conditions. On the other hand all-wheel drive stays ON all the time and is better for more general driving conditions and it gives the car a sporty feel.

Both systems add some weight to the vehicle and we have to end up paying for this at the fuel pump as are not very fuel-efficient. Four wheel drive because they are usually big and heavy with engineering complex drive systems and all-wheel drive because it is always remains active.  I've been driving my Mahindra Thar for one year now and I love it. It is less fuel efficient than my old two-wheel drive car. But I have never had to worry about any rough and extreme road conditions and it feels like a heavier and safer car. But even though you have all this extra safety and peace of mind, 90 to 95 percent of the time we just driving around our local area and it do bug me that I'm not as fuel-efficient as other cars.

To give you an idea of fuel efficiency of my old all-wheel drive car got about 355 kilometers per 26 liter tank in city and my four wheel drive gets about 255 kilometers per 26 liter tank in city. So, with same cylinders I got almost hundred kilometers less per tank. It does make you think is this constant all-wheel drive and four wheel drive really worth the extra money. I guess it all depends on your driving conditions.