Horse vs Car

Data:
(a) For pulling carts, four Mongolian horses can pull 2000 kg between 50-60 km a day. (See source).

(b) Average mass of horse is 500 kg. (see source).

(c) For each kg of body-mass a body needs on average 40 calories. (see source).

(d) Grass, the primary food of horses contains 2.0 calories per gram. (see source. See also here and here).

(e) Each 1k acre on average produce 1600 kg grass-hay per acre per year. (see source).

(f) One liter of petrol contains 33.2 million joules. (see source).

(g) One calorie = 4200 joules. (see source).

(h) A 1.25 tons car has 28 mpg mileage. (see source. See also here).

Working:

From the data:
"For pulling carts, four Mongolian horses can pull 2000 kg between 50-60 km a day"
means 4 horses, 2 tons, 50 km
= 4 horses, 100 km-ton
= 1 horse, 25 km-ton.

1 horse = 25 km-ton/day x 300 days = 7500 km-tons/year. Mass of horse is 500 kg, energy intake is 40 calories/kg. Total energy intake is 20,000 calories/day or 8 million-calories/year or 923 liters petrol.

A horse fully grows up at age 3 and work till age 15 therefore 20% of its life time is spent in childhood when it not work. A mare also has average life 15 years but in addition of 3 years childhood it also has to raise (feed & take care of) two ponies each of whom need full attention in first 1.5 years of life so 3 years needed for that per mare. Therefore the mare works for 9 years.
Out of total 30 years average lifetime available to the couple the horse works 12 years and mare 9 years total 21 years. Divide 30 by 21 and we gets 1.43 herd-mass for each working-mass. Total food energy consumed by the 500 kg working horse is equivalent to energy in 923 liters petrol, multiply that by 1.43 and we get 1318 liters. Total work done by the working horse is 7500 km-tons/year so 5.69 km-tons per liter petrol.

Lets compare that with engine:
1.25 tons, 28 miles, 1 gallon.
= 1.25 tons, 28 miles, 3.75 liters
= 1.25 tons, 44.8 km, 3.75 liters
= 1.25 tons, 11.95 km, 1 liter
= 14.93 km-tons per liter of petrol.

A car takes almost as much energy in completion as it consume in its life time. Lets assume that the making of engine and related body parts takes half the energy and the remaining luxury stuff like seats, air conditioner, tape, speakers etc takes the other half energy. Since in making of a loading truck we can avoid or drastically decrease the amount of energy consumed in making of luxury stuff we have to reduce the work done per liter of petrol in our calculation only by 75% instead of 50%. 14.93 x 0.75 = 11.2 km-tons/liter-petrol. Comparing this with horse's work done per equivalent liter petrol of 5.69 km-ton we find that horse has 50% efficiency of a car engine.

1 1k acre of pasture produce 1600 kg hay which is equal to 404.8 liters of petrol.

World oil production is 30 billion barrels. Oil provides 40% of energy. So total energy usage is equal to 75 billion barrels. In a population of 6 billion it means 12.5 barrels which means 2000 liters petrol. So that means 5 1k acres or 1 modern acre. Modern acre is one on which agriculture is done in modern way that is by using artifical fertilizers, pesticides and tube wells. Since although 1 1k acre can produce equivalent of 400 liters of petrol the using of horses reduces the efficiency to half so 10 acres are needed per person to feed horses to maintain the current world average life style.

We can use donkeys in place of horses. Infact donkeys are atleast 33% more efficient in work and 25% efficient in food. So double efficient. The higher efficiency in work is due to working larger number of hours per day and larger delivery-load to body-mass ratio. The higher efficiency in food is due to eating many side things in crops that horses don't eat.

In terms of power 2000 liters of petrol means 2104 watts of continuous power in form of primary energy. Lets round that to 2000 watts. In developed countries the average is 10,000 watts and in developing countries the worst is 200 watts. Note that we are not counting food energy consumed by people which for a 2000 calories/day average diet is 100 watts. In countries like Pakistan the non-food energy consumption per capita is just 200 watts which means only 1 pasture 1k acre would be enough to maintain the current life style post peak oil.