I calculated a rate of 40 cents a mile for operating costs for my car. Insurance, depreciation, wiper blades, car washes are part of the 56 cents a mile allowance but are averages based on factors other than miles (wiper blades dry and crack whether you use them or not; cars get dirty even sitting in a garage; insurance and license must be paid regardless of miles; even tires will age and crack and need to be replaced even if not worn out). Your actual cost per mile is based on your miles per gallon and whether you do some of your own services yourself or pay a dealer for those oil changes. The IRS rate I think is based on 20-23 mpg or so. Most economy cars get 30 or more. My hybrid gets 40. So the 56 cent rate is going to be very generous for most passenger cars, and inadequate for most of the rest.
Each person needs to calculate their own costs based on their own mpg and their own tires and their own service costs. If you buy 30,000 mile tires you will go through them faster than if you get 75,000 mile tires. Take the price of the set of four tires, mounted and balanced, divide it by the guaranteed mileage, and you'll get the cost per mile for the tires. $500 for 50,000 mile tires = 1 cent per mile. My next tires will be 75,000 mile tires for about $600 total so it will be even less than a penny per mile for tires. Gas is down to $2.50 a gallon or less right now. For 40 mpg, that's less than 8 cents a mile for gas. I paid $20,000 for my car and expect to drive it 80,000 miles before I get tired of it. That's only 25 cents a mile depreciation assuming I would throw it on the scrap heap at that point (and it would probably still be worth 1/3 to 1/2 of what I paid for it at that point). 25 plus 8 plus 1 cent for tires plus 2 cents for oil -- that's 36 cents a mile. So I can afford to go do a job for 40 cents a mile and know that I am not going to be out of pocket for the trip.
The point I have repeatedly tried to make is that the incremental cost of operating a typical economy car for a mile is *not* 56 cents a mile. If someone passes up work because they can't get 56 cents a mile, they may be missing an opportunity that would actually put money in their pocket even after feeding the car. On the other hand, if your only vehicle gets 15 mpg, you should plan only to do local shops because the travel cost will almost certainly eat up most of your fees.
And if the trip is for several shops, I just figure how much went to feed the car and the rest is my pay for the day. If I don't have any other options for making money that day, it's my decision if I want to go for a drive and end up with $50 net for the day when I'd rather have $100 or if I want to sit on my butt and make nothing, secure in the knowledge that I taught some cheapskate MSC a lesson -- while they simply give the job to someone else anyway and put me on their "difficult and demanding" list of shoppers to call last.
I've put $8000 in the bank *after* paying for the car expenses by taking jobs under conditions that some people here would sneer at. But I shop for my reasons, not theirs.
Each person should do their own calculations and make their own decision and figure up at the end of the year if they have more money than they would have had if they had not shopped. That's really the only thing that matters, after all. Not whether they "won" by refusing to work for less than 56 cents a mile for travel.
If you had a wage job and a 15 mile commute, that would be 150 miles a week nobody was reimbursing you for that you can't deduct from your taxes. So don't lose perspective by focusing on the notion that you should be reimbursed for driving to a mystery shop. You're self-employed. How you get to the jobsite is your problem, not the MSC's.
Time to build a bigger bridge.