Amen to what folks have posted here about the M26 Pershing. For both the M4 Sherman & the M26 Pershing THE reference are Hunnicutts two books on these tanks. Zalogas books on the same subject are less expensive & still in print, but contain about 15% of the detail Hunnicutt provides.
There was a remark up thread about the engine making the M4 Sherman hull unsatisfactory. This was not the case. The same engine was used in the entire T20 series (T20, T22, T23, T25, T26) from which the M26 was derived. The height of the M4 Sherman came from having the transmission in front. that required a drive shaft be run from the rear mounted engine to the front. Aside from requiring the turret basket to clear this driveshaft it also added to the maintinance burden on the tank. Removing the drive shaft allowed the T20 ... T26 hulls to be four inches/10cm lower than the M4.
The turret ring of the T20...T26 series were the same size as the M4 (65 inches). That allowed several of the prototype series to mount the same advanced turrets & guns that went onto the late production (1944 +) M4 models.
The second significant difference between the T20...T26 hulls & the M4 was the elimination of the sponsons & their internal compartments. The sponsons were a manufactoring complication, and represented a weakness in the side resistance of the M4 to hits by enemy ammunition. If you look carefully at the M26 Pershing the sides are much better covered by the tracks & suspension, with a smaller portion of the hull sides exposed to direct hits. Unlike the M4 the side armor of the T20 ... T26 tanks has the upper portion sloped toward the top.
The T20...T26 series were the result of the US Army Ordnance Dept sending engineers to Europe & Africa from 1941 to study the existing tank deigns in combat. In early 1942 two officers with a background in tank & heavy equipment design spent several months in Africa inspecting damaged tanks of all makes on the Western Desert battlefields & discussing their observations with the Brits. The report from this team contributed directly to the basic design of the hulls for the T20 ... T26 tanks. Thickness in armor was the principle difference between these tank hulls. The T20, T22, T24 used nearly identical hulls. The T25 & T26 had progressively thicker frontal & side armor.
The primary variation in these tank designs were in suspension: HVSS, VVSS, & torsion bar; transmissions: clutch, hydraulic, & electric; five or six different models of turret, two of which were used on the M4; and four or five different cannon models. Of the five basic prototypes & several sub variants the Ordnance Dept originally proposed production of the T20. It weighed in the same class as the M4, & was to carry the high powered 76.2mm gun of the late models M4. The alternate proposed was to use a electric transmission, that is the engine drove the tracks via a electric generator/motor combination. The Army Ground Forces command rejected production of the T20, but eventually approved limited production of 250 of the electric drive model & some 300 of the T26 with its extra thick hull and 90mm gun as a "heavy tank".