The M113 is a tracked Aluminum hall Vehicle designed in 1959 for use by Mechanized Infantry. IT was replaced by the M2 Bradly.
The Humvee was a transport/communication/cargo vehicle design to replace the M151 Jeep, the M35 series of 2 1/2 ton trucks and the Various trucks between those two vehicles (The US Army in WWII had used Jeeps and 2 1/2 tons trucks since WWII. US Jeeps and 2 1/2 ton trucks were the best in their class. On the other hand America never did come up with a good 3/4 to 1 ton Vehicle until the Humvee).
Anyway the Jeep had to go for it was Gasoline (Mo-gas for you people still in the military, Petrol for the English readers of DU). The US Army since the 1970s adopted a policy of using only one fuel, Diesel. While Diesel has better fuel economy than Gasoline engines, Gasoline engines have better low range acceleration (You can peel out with a gasoline engine, you can NOT with a Diesel). In off road use this was important for the small Gasoline engine in the Jeeps could keep it out of mud that a diesel of the same size would not have enough power to pull the Jeep out of. Thus decision of the Army to go to Diesel forced the Army to up size its smallest Vehicle to an engine size (a V-8) that could pull it out of mud even if it was a diesel. Thus the Humvee was born.
The Humvee was intended to be a cargo/communication/troops transport. It was NOT to be used in direct combat missions. When you are in combat in such a Vehicle you are to abandon the Vehicle to fight on foot. Hopefully only the lead Vehicles would be attacked, the rest would be dismounted before the attack hit that part of the Convey. The early WWII 2 1/2 trucks had solid steel tops over the cabs, these were removed as the war progressed not only to save steel for other uses but to permit easier exit if the truck was attacked. Cargo vehicles were NOT intended nor design to be used in actual combat.
Now if that is the case why do we have Humvee and 2 1/2 ton trucks in the Army? The reasons are as follows:
1. Rubber tires last 50,000 miles Tracks last 2000 miles
2. Humvee get 10 miles to the Gallon Gasoline (350 miles on a 35 gallon gas tank), M113 gets 3 miles per gallon (300 mile range on a 95 gallon gas tank).
3. Maintenance of the track must be done daily (Including greasing the track). Other than bleeding the air brakes and bleeding the fuel filters, Humvee do not need "maintenance" more often than your car.
4. The Humvee can be towed by another Humvee or the Standard US 5 ton Wrecker. The M113 has to be repaired on site or hauled by another M113 or a tracked 15 ton capable wrecker (which means you have to transform the entire maintenance section to be able to handle the switch to M113s, the increased weight of the Humvee do to the add on armor did not put them beyond the 5 ton limit of the standard Army 5 ton wrecker).
5. Humvee cost the US Army $50,000 a piece. The latest M113 purchased cost $1 Million a piece (The M113 already in stock are the one's the Military plans to use so actual cost were much less but still much more than the Army spent on Humvee).
6. The main enemy of the US forces today is the RPG and roadside bombs, the Aluminum armor of the M113 was design to stop 50 caliber bullets NOT RPGs. Worse since RPG use "Heat" antitank rounds, such rounds have a tendency to catch the Aluminum Armor on Fire destroying the M113 and killing its Crew. If the Roadside Bomb is a HEAT Round the same effect (Through most Road side bombs are NOT HEAT rounds).
Thus the Cost to convert from Humvee to M113 will be on the order of 10 (i.e. if the cost today is 1M, when the M113 are introduced it will be 10M for the same level of patrols). Thus just to stay EVEN the US army will have to increase our cost of occupying Iraq by 10. This war is getting costlier and costlier.
More on the M-113 (and where I obtain 300 miles range on 95 gallon gas tank):
http://fas.org/man/dod-101/sys/land/m113.htmMore on the Humvee:
http://fas.org/man/dod-101/sys/land/m998.htmPrices for Military Vehicles:
http://www.strategypage.com/fyeo/howtomakewar/databases/price_database/ground.asp