Deep learning: Training from COCO/imagenet weights or from a class-specific trained weights when classify 2 similar objects
If you have a previously trained model to classify only cars, you can use it considering the fact that you have less data. The trained model for cars classification that you have already, would have been fine-tuned on the Imagenet or COCO in the first place. This way I think you would be able to train the model in less amount of time compared to fine tuning on Imagenet weights. Try both the methods once and try to find out the amount of performance increase in terms of time and accuracy over each other.