The Panzer IIF
With the completion of Rheinmettall's work on the MK-101 for the Luftwaffe, a 30mm autocannon was now available for use on a version of the Panzer II. The Heer was not interested, currently, in putting more powerful anti-tank weapons on the Panzer II chassis, seeing the Panzer II (in it's tank form) as becoming a recon vehicle for the panzer divisions, supplementing the armored cars that would perform long distance recon missions. The 30mm autocannon would increase the vehicles punch while not encouraging commanders to engage targets that they should leave alone. The MK-101 was a heavy weapon for an aircraft gun, but on an armored vehicle, the weight was acceptable, and the 10-round magazines, while requiring frequent reloading, did allow the crew to choose between HE and AP projectiles.
The new Panzer IIF also benefited from the previous year's work on the Panzer II Arg and Panzer II Bel variants. Renk had developed a new transmission, too late for those vehicles, but it's design allowed the vehicle designers to install it and the 240 hp diesel engine from those vehicles into the 4.5m chassis used by the Heer. Combined with the wider tracks and road wheels from the variant vehicles, the more heavily armored turret, and additional belly armor requested by the Heer, the little tank was still able to maintain a weight of 10 metric tons.
In it's trials, one concern was noted: if the vehicle attempted a tight turn at high speeds (60+ km/h), there was a high likelihood of it shedding the slowed track.
The Panzer IVF
The Panzer IVF was the first German tank designed from the ground up to mount the new 75mm/48 KwK 38 cannon. Visually, it looked much like the earlier Panzer IVDs with the exception of the long-barreled cannon, re-arranged turret hatches, and the lack of bolted-on armor, but it incorporated a number of improvements over the preceeding Panzer IVD. The bolted-on applique armor had been replaced with thicker plates of armor (not always as thick as the combined thickness on the Panzer IVD, but thicker than the Panzer IVC/B) and thicker deck, belly, and rear plates were used, the tracks were 20% wider to compensate for the increasing weight of the vehicle and the engine was a 330 hp model instead of the older 300 hp model. All in all, the new vehicle weighed slightly over 24.5 metric tons, and could attain a road speed of just over 24 mph.
The Panzer IIIH
While the Panzer IVF was the current and latest design of the panzer III/IV series, there was zero chance that the Heer would receive the funds needed to replace all of the already built Panzer IIIs and IVs with the new model. Even if the money was available, which it wasn't, the production of that many tanks would take several years.
Happily, another option was available. The turret developed for the Panzer IVF used the same size turret ring as the existing Panzer IIIs and IVs, so a refit program to replace the old turrets could be put in place at a fraction of the cost of an all-new tank. Upon return to the factory, the older tank (already upgraded to the Panzer IIIE or Panzer IVD standard) would have it's turret replaced, along with the installation of new ammunition boxes for the hull machinegun and new radios. Then the tank would be returned to the Heer.
This upgrade was referred to (somewhat confusingly) as the Panzer IIIH. In theory, it should have been called the Panzer IVE, but the Panzer IIIEs had priority in conversion over the Panzer IVDs, so (for reasons buried in the Heer bureaucracy), it was given the Panzer IIIH title.
The Panzer IIIH weighed just over 23.5 tons and had a top speed of 23 mph on road.