Got a new iPhone 4S or recently upgraded your iPhone 4 or 3GS to IOS5 and noticing unusually high cellular data usage? Are you close to exceeding your data usage limit when you never have before? Have you already exceeded it? You are not alone.

My wife recently traded in her old iPhone 3G for a shiny new iPhone 4S, and within 10 days had exceeded her 200MB AT&T data plan limit when she has never exceeded it before. So what gives? It’s a new phone, and Siri does transmit voice data back to Apple’s servers, but could that really have caused the usage?

Fixing The Data Usage Problem

Turns out the problem is that Apple ships iOS5 with iCloud data syncing over cellular networks ON by default. This means all the documents and data stored on your phone for all the applications you have installed on your phone will be automatically uploaded to iCloud, decimating your puny cellular data plan you thought you’d never burn through.

If you are experiencing super high data usage on iOS5, just follow these steps:

1. Navigate to: Settings -> iCloud -> Documents & Data -> Use Cellular
2. Turn it OFF
3. Fight with AT&T (or other wireless carrier) about your excessive data usage charges

Why Apple would ever think its a good default to sync data and documents for all your phone’s applications over a cellular network in the world of severely limited data plans and usage based billing is beyond me. This, and the widespread  battery  issues with the iPhone 4S and iOS5 seem to indicate Apple didn’t do nearly enough field testing with the new phone or OS. Let’s home they learn from this in the future.

Usage Before iPhone 4S

Some months have clearly higher data usage than others depending on how long we were away from home or what we did while we were out with out phones, but we were never in any real danger of going over the low 200MB limit before.

Data Usage after 10 days with the iPhone 4S

On the left (green) is my data usage with an Android device, Right (red) is my wife’s data usage this month with the new iPhone 4S.

My wife got a text alert for 65% data plan usage on Monday, and then one on Friday for 80%, and one the day after on Saturday saying she had gone over her limit (exceeding 100% of the plan) and that we would be billed an additional $15 by AT&T for the overage.