[ghaff]

I'm not sure it's fraud but it's probably not a good practice.

Standard "mileage charges" in the US are actually set by the IRS. I assume it's similar in other places so that's a well-established practice for car-related expenses. I'm not sure how else you would do it as there's no way to directly capture expenses other than fuel.

I assume that in most cases billing some standard travel day rate/per diem (perhaps based on location) would be fine as well with, possibly, air charged separately for the actual amount.

The key is that you're being explicit that you're not charging the actual amount but rather some standard rate that, in principle, approximates average/typical expenses.

[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.