Yes this can be included as part of your earnings.
I received my Tier 1 extension successfully by showing basic salary + car benefit. And I also know other people who have done so for their ILR application too.
Supporting documents I sent for previous earnings -
- Payslips & bank statements
- Employer letter - confirming total earnings = PAYE income + cash equivalent of taxable benefits (Car & Fuel)
- P11D form provided by employer
Additionally you can add your HMRC tax coding notice or a tax calculation you received from HMRC.
Read more here -
http://www.immigrationboards.com/uk-tie ... 33904.html