One cannot take historical movement rates and just divide by the number of turns to determine movement rates for the simple fact you don't know how much time the actual units were *not* moving. If you have never been in the Army it's difficult to understand how much time is spent waiting around, not to mention how much time is spent not moving getting ready to move (maintenance, supply, briefings, etc). There is also this little thing called the "command cycle" which we as players skip altogether. It takes time in real life to make plans, disseminate orders, and then prepare and implement them. Then thrown in time spent discovering and correcting all the mistakes and misunderstood orders.
So in real life those units you push to the maximum each turn should move 1 turn and then sit for 2-3 turns before moving again.
Not saying there isn't room for some tweaking the movement rates, but be careful before you make it impossible to achieve historical results at all. There is a cascading effect when you make a change here are there.
A few more points. The mud affected the Soviets as much as the Germans in terms of operational mobility. They knew and planned for it (and halted operations for the most part if you noticed...) . The Germans of course were not ready, but if you follow their movements during the Vyzama and Briansk operations, they managed to move a heck of a lot more than you would just reading about their difficulties. What really killed them was the breakdown in supply which occurred before the mud period and only got worse with the mud.
Also, you should take note that mud is only mentioned as a factor when one side or another needs an excuse why they lost. Take note on how mud is *not* mentioned in the fall of 1943 (Zhitomir offensive) but is mentioned as a problem for the Germans during the relief of Cherkassy. And while the Soviets launched a major offensive in the spring of 1944, the mud somehow only became a problem when they started to lose control of the situation. Oddly enough, that same mud did not seem to bother "Hube's Pocket" that much. And there are plenty of examples of mud being a non-issue for both sides despite being in it.
Before I get off my soapbox, please remember one thing when reading military history - every writer has an agenda. And remember Dunnigan's oft-quoted remark (paraphrased here) that for a wargame to be successful it has to at least maintain the illusion of movement. (this is supposed to be fun, no?)
My 2 cents - from one who has moved real units in knee-deep boot-sucking German mud.