Different people have different theories. I have always been advocating people to buy cars whose price is equal to their salary. But given higher interest rates and spiralling fuel prices,
OTR = (0.75 * CTC) is roughly what I believe it should be.
Deviations from this might be due to other commitments (house, children's education, investments etc). Some people think this is going overboard, but this is how I see it. If a person (single), earns 6L per year, 45 K per month roughly, he buys a car for 4.5 L and take a car loan for 4 L, his monthly EMI is going to be 10,000. Add fuel consumption of 4-5K per month and maintenance of 2K per month, he will be shelling out 16-18K per month for his car, i.e., 35% - 40% of his income.
Given that house and car are the two biggest expenses one incurs in his lifetime, I think this is fine. Some people say married men should take a more conservative approach given additional commitments, but if you consider an additional income (varies from case to case), it shouldn't matter much