Detective Kim was called to a crime scene, where a 16 wheeler truck had just been hijacked. There he met a very upset Lieutenant Lee.
"The truck is heading for the Golden Gate bridge, and we can't stop it," said Lieutenant Lee. "My plan was to let the truck go over the bridge, because we have an ambush set up on the far side. But now I hear the bridge is being worked on, and has a very low weight limit. If the truck is over that limit, I'll have to dynamite the ramp leading to the bridge. I have to decide this in the next ten minutes, and I have nothing to go on."
Detective Kim nodded and then walked over to a discarded truck tire. "That's from the truck," said Lieutenant Lee. "The driver had pulled over to replace a flat tire. He had just brought out this spare when they shot him. They left the replacement, and drove off with the flat tire." "Having only 15 wheels won't change the weight, though," said Detective Kim.
Detective Kim looked at the dried blood sprayed across the road. "They shot the driver right by the truck, before they took it," said Lieutenant Lee. "You can tell because of these bare spots in the road where the tires were. Everything else has little drops of blood."
Detective Kim went back to the patrol car, and got something out of the glove compartment, then went over to the discarded tire for a moment. Then he looked carefully at the blood spots on the road. Finally, he jotted some numbers down in his notebook. Suddenly, he turned to Lieutenant Lee and said "The truck is too heavy! Blow up the ramp! Do it immediately!"
How did Detective Kim know the truck was over the bridge weight limit?
I give up, show me the solution.