One of the problems with attributes is the whole idea of scaling... a character with STR 12 (a really, really strong normal dude) can't lift a car up over his head. It's not going to happen. A character with STR 16 (a superhero with 'Incredible' strength), is going to be able to lift a car over his head with some effort and luck. That's only a 4-point difference in ratings, but a HUGE difference in what they can do. If we set lifting the car at target 30 (for instance), this means that the guy with STR 12 can lift it on a roll of 18 or better (so 15% of the time), but the guy with STR 16 can lift it on a roll of 14 or better (so 35% of the time). Neither of these is right.
So, we introduce the idea of 'super-human checks'. We keep all of the granularity on the player end (with STR ranging from 2 to 25, Professor X to the Hulk), but we put checks on two very simple scales... anytime you want to try something 'super-human', you need a rating of 14 or better to try it. STR 13 and want to lift the car? No chance. STR 14? Grab those dice and let's see! There's a gulf between 13 and 14 for game purposes. The default check for a super-human task is 30, with a modifier of either +4 (pretty easy by super-human standards) or -4 (tough for even a super-human). On this scale, we have three simple designations for STR checks:
+4 to the roll. Up to 10 tons.
no modifier (vs. target 30). Up to 100 tons.
-4 to the roll. Over 100 tons.
That car is under 10 tons, so it's target 30 with +4. That super with STR 16 rolls 1d20 +16 +4, target 30... he now needs a roll of 10 or better to lift the car over his head (so he has a 55% chance of success). DING. We have a winner. This is reasonable.
And the big HULKy guy with STR 25? He can lift that car on a roll of 2 or better (he just can't botch), can lift up to 100 tons with a roll of 5 or better, and needs a roll of 9 or better to lift in excess of 100 tons. That sounds pretty hulky.
And, the GM always has the option of giving +2/-2 instead if we're in the murky area... if something is 15 or 20 tons, I'm giving +2 to the roll... if it's 120 tons, I'm giving -2.
This goes against my love of granularity on the check side of things, but makes for a fast game, and still allows a 1-point difference between STR scores to matter on every single roll. I know that there's a big difference between 25 and 75 tons, but at the end of the day, you don't want to sit there with your laptop trying to figure out how much a bulldozer actually weighs. This gives you a ballpark where you can make informed decisions and keep the action moving.