I need to build an electric vehicle powered by 4-1.5 volt batteries that can go as fast as possible and stop at a specific point between 8 and 12 meters. I can use a dc - dc converter to step up my voltage to up to 48V. I also have access to gearing the motor or using a belt or chain if it would help.
What dc motor would you recommend?
"I need to build an electric vehicle"
That made me think of a vehicle that you can ride in. But reading further, I thought "maybe not".
How big of a vehicle do you want?
Please give us some more details about your application.
Roughly 2 1/2ft x 9 in
The deciding factors are the total mass of the vehicle and the maximum acceleration required, which gives you the minimum value of the force that the motor must produce through the wheels.
In turn, that force must be converted into a torque, which depends on the size of the wheels. Figure out the required torque and select a motor to match.
You can learn about force and torque here, or from a basic physics textbook.
What dc motor would you recommend?
You should consider getting one of the RC racing toys.