How a phased-in carbon tax shift could temporarily reduce gasoline taxes

I've seen a number of blog posts and op-eds saying something like, "How could a carbon tax not increase gas prices?" Well, here is a way to phase in a carbon tax and actually reduce gas taxes temporarily.

Suppose you replaced the federal excise tax on gasoline with a carbon tax and phased it in according to the following schedule:
Year 1 $10/tonne CO2
Year 2 $20/tonne CO2
Year 3 $30/tonne CO2
Year 4 $40/tonne CO2
Year 5 $50/tonne CO2

Then the carbon tax on gasoline would be roughly
Year 1 2.5 cents/litre
Year 2 5.0 cents/litre
Year 3 7.5 cents/litre
Year 4 10 cents/litre
Year 5 12.5 cents/litre

But the federal excise tax is 10 cents per litre! So there you go: replace the excise tax with this phased-in carbon tax and you get three years of some gas tax relief for voters, repeat, voters.

From what I've read it seems that at the $10/tonne CO2 level, what the federal government would lose from motor fuel taxes would be roughly balanced by revenue from taxes on other fossil fuels. That seems like a good place to start.

You may ask, why introduce the carbon tax if you're just going to decrease gas prices and incent gasoline consumption? Well the point is that there is a lot of other fossil fuel consumption that is not taxed: coal in particular. Globally, high oil prices may be reducing demand for oil somewhat, but it's not reducing greenhouse gas emissions so much because people are being driven to burn more and more coal (not in Canada, in other countries). Coal shouldn't be getting that free ride.