carpool

Forgive me I don't know where else to post the question on Wen xue city. I figure people here would probably have some insights.

I carpool with a few guys, but chose not to drive. I am the only one not driving.
One guy suggested that my I pay the rest of them based on IRS mileage for business, which is $.55 per mile for 2008 and 2009, plus tolls.

I think this is a little unreasonable, because:

1. $.55 is for business which may have included compensations for hardship, and our carpool is for commuting.
2. My riding is incidental, meaning they drive to work with or without me, and they don’t incur any additional cost because of me.
3. $.55 was set in 2008 when the gas price was the highest.

Also, how much I pay depends on how many people we have. The fewer the people there are, the more I pay. But I think I should pay a fixed amount calculated based on 5 passages (the full capacity of a car). And when there are fewer people carpooling, each of them will have to drive more but will get paid more too since now there are fewer people sharing the payments received from me.

Does my reasoning make any sense?

Thank you.