The fuel is factored into the estimate for my regular jobs however, If I am running a discounted followup special, working out of my local area or giving a cut rate deal for referrals my customer's are aware of the $.50 per mile fuel surcharge before I confirm the deal.
Most of my details are not to far from my house and I have been eating the fuel if it's not more then 15 miles round trip. I have a few customers out side of that 15 miles. My business is growing and I am trying to keep cost down on my side and still offer a good price and quality products to my customers
if your mobile.. it should already be factored. UNLESS, someone wants you to go out of city (not that i have done this) then you could make a deal where the original customer can get 2-4 other full corrections to make it worth your wild to drive out of city and you cut out traveling cost
The way I see it is that if I was stationary, I would have to pay for rent and upkeep. I would not charge extra to the Customer for rent and utilities As a mobile detailer gas is my overhead... So no overcharge. But as others have said, you have to calculate it in your cost of doing business and adjust your prices according to this. But it's transparent to the Customer.