Effect of Spare Parts/Maintenance Effort to the Second World War

Dear historians...or history buffs.

It's easy to look up production figures of tank production during WWII (49K Shermans, 84K or so T-34, 8.5K or so Panzer IV, 6K or so Panthers, under 2K Tigers). However, are there figures available for the production of spare parts, and how much of an impact this had on readiness rates for these vehicles?

I have read that the German Army was very short on spare parts compared to the allies. So, did the allies not only out-produce the Germans when it came to armored fighting vehicles, did the allies also out-produce the Germans when it came to the maintenance effort? Is there any way to determine what effect a lack of spare parts (assuming there was an effect) had on the German war effort?