[rahimnathwani]

The mileage charges set by the IRS are for 'computing the deductible costs of operating an automobile for business, charitable, medical, or moving expense purposes', i.e. they're used to work out your tax bill.

There are similar mileage rates in the UK, published by HMRC. They have the same purpose.

The mileage rate you agree with your client needn't be related to those rates at all.

[ghaff]

Fair enough. It's pretty common to use them for billing purposes as well but if, for example, you mostly drive to client sites and want to use mileage as a proxy for billable travel time it would be perfectly reasonable to add an uplift to mileage rates over the IRS rate.