[70times7]

Driving on three wheels... That's a new one. I'm more familiar with other phrases that apply to this guy:
-He's only got one oar in the water
-His rear wheels are in the sand
-His elevator doesn't go to the top floors
-He's a few cards short of a full deck
-He's a few bricks shy of a full load

[KarlInOhio]

He has a mind like a steel trap - full of mice.

[numberonepal]

- An intellect rivaled only by garden tools.
- As smart as bait.
- Chimney's clogged.
- Dumb as a bag of hammers/box of rocks.
- His antenna doesn't pick up all the channels.
- Missing a few buttons on his remote control.
- No grain in the silo.
- Proof that evolution CAN go in reverse.
- Receiver is off the hook.
- He has less going on upstairs than a one story house.
- Several nuts short of a full pouch.
- Slinky's kinked.
- Surfing in Nebraska.
- Too much yardage between the goal posts.
- A few clowns short of a circus.
- A few fries short of a Happy Meal.
- The wheel's spinning, but the hamster's dead.
- One taco/enchilada short of a combo platter.
- All foam, no beer.
- The cheese slid off his cracker.
- Couldn't pour piss out of a boot with instructions on the heel.
- He fell out of the stupid tree and hit every branch on the way down.
- Not the sharpest knife in the drawer/tool in the shed.
- The lights are on, but nobody's home.