Anyone tell me if a car runs 12.5 and i beat him by 3 cars what does that put me at.is a car length = .1 or something

 

Generally once car length is a tenth. I think that is 10-11 second cars so it might be just less than that. Trap speed will give you a better idea how far ahead of him you are.
If you are looking for a goal at the track I would go with the hope of hitting 12.5-12.6. If you are able to run faster great but plenty of people get there heart broken at the track because they use logic like this and say well I beat my friends 12.5 car. Why am I only running 12.8 at the track.

The only way to know is get your but to the track!

 

Sounds like you raced a guy who runs 12.5s on the street. His car may run 12.5 at the track, but not on the street. There is now way to compare, too many variables.

Goto the track.