I don't know much about the car you have specifically, but it's not uncommon for reverse to be slower than forward. The ESC usually has 4-5 forward-drive FETs but only 1-2 reverse ones. So the resistance is quite a bit higher and they don't go as fast.
Additionally, a lot of motors are timed to run better forwards than reverse.
If it's brushless, then it's likely just a setting in the ESC or transmitter. You'll have to consult the manuals for either a reverse/break endpoint/ATV or a setting in the ESC for reverse drive power.
You may also need to recalibrate the ESC in either case. Sometimes they lose calibration or your transmitter's trim setting will change and require a recalibration. Also should be instructions for this in the manual.
Hope this helps, and good luck!