This is a tough one. I have lost riders for this, but fair is fare. If you are driving all the time and putting your car on the road, then you will have expenses to cover sooner or later such as tires, oil change, and others. To help you with this expenses, and have everybody share gas and car maintenance, you should charge $1 per every ten miles. One-way ride for 55 miles should be $5.50 and $11 for a two-way ride per person. Do you have another system?
I have been carpooling for almost three years, and I found that if you are the one driving and using your car, then it would be a good idea to charge per ride as sometimes riders might not ride one day and will the next or have variable working days. This way there are no confusions or misunderstandings. This has helped me a lot. What is working for you?
If you are putting a lot of miles on your car, then:
I tried all of them such as Chevron, Mobil, AM/PM or so, 76 and I forgot if there any more for at least a month to see how my car performed. I settled for Shell. I know it is a little more expensive (5 to 10 cents more for a dollar or two more when filling a gas tank of 17 gallons), but it is worth it. I have used it for more than six years. My car has that power when starting and passing and lasts longer. What do you think?