It sounds like you are paid on a "piece rate" basis. In these cases, so long as you are making (1) at least the minimum wage (currently in CA it is $8 per hour) for all hours worked and (2) getting paid overtime if you work any overtime hours, then the employer is complying with CA law.
For example, let's say you work 50 hours this week. You spent 40 of those hours doing repairs and got paid $30 for each car you repaired. You repaired 10 cars so you earned $300 this week for all the cars you repaired. You also spent 10 hours filling up the vehicle with gas for which you did not get paid.
To determine if you got paid at least the minimum wage, you divide the amount you made this week ($300) by the total number of hours 50. This equals $6 per hour. Your employer has not complied with minimum wage laws and owes you an additional $2 per hour. Your regular rate in this case is $6 + $2 = $8. In this case, you are owed an additional $100.
You also worked 10 hours of overtime this week. To determine the overtime your employer should have paid, take one-half of your regular rate ($8/2 = $4) and multiply it by your overtime hours (in this case 10 hrs) which equals $40.
So this means that you should have been paid $300 + $100 +40 = $440 this week.