My wife and I lived in Oklahoma a couple of months short of two years, so we have a little sensitivity when it comes to wall clouds and tornadoes. The Fujita scale used to indicate tornado wind speed is being revised significantly downward, according to this AP article.
From page 11 of the recommendation (PDF):
| All speeds are 3-second gust speeds in mph. | ||
| Old Range | New Range | |
|---|---|---|
| F0 | 45 – 78 | 65 – 85 |
| F1 | 80 – 118 | 86 – 110 |
| F2 | 119 – 161 | 111 – 135 |
| F3 | 162 – 209 | 136 – 165 |
| F4 | 210 – 261 | 166 – 200 |
| F5 | 262 – 317 | 200+ |
| F6-12 | 317+ | No F6 in new scale |
I cringe when standards get revised, out of principle. We knew that F5 tornadoes were death at your door and really rare, and we had F3s in Oklahoma without blinking. So now some higher F3s will be F5s?
The paper explains on the first page that the National Weather Service estimates F-factor based on the resulting damage from a storm, then converts that F-factor to wind speed. Structural weak points and slow-moving storms can make a storm look more powerful than it is. The new scale also defines 28 measurable damage indicators that help determine the F-factor.
This seems reasonable to me; the only problem is in the comparison with previous storms. When dealing with history, we’ll have to juggle those mph numbers (and yes, the kph for the Canadians are worse). Be careful when goofy environmentalists start crying about the rise in F5 storms. 
If the F-factor changes are based on resulting damage, maybe that’s an indication that newer houses today are easier to destroy than older homes. And today’s cars are a lot lighter (and have paper-thin sheet metal) compared to a few decades ago. In this “throw-away” age, it may not take as much wind to damage things.
Dont go by the F-factor…you will know which is worse by the one that kills the most.