There isn’t road tax in the UK, only vehicle tax which is based on the about a vehicle pollutes. Additionally all of that tax across the UK goes to the central government, and none of it goes to the London Mayor. There have been attempts to have vehicle taxes collected on London based vehicles be allocated to the Mayor, but it’s always been soundly rejected by central government.

Fuel duty is also collected by central government, and none of it goes to the London mayor either.

The only taxes collected and managed by London are the Congestion charge and the ULEZ charge. Everything else is beyond the reach of the London mayor.

They could also be be referring to London's congestion charge, that drivers pay for driving in inner London.

Neither of those are London car taxes. I think the number refers to the congestion charge. ULEZ raises a similar amount though.