I googled for a couple sources on the breakdown of the price of gasoline, and they seemed to be in agreement that the raw cost of crude is somewhere around half. (And broke refining out separately.)
I'm sure it's not perfect, but it seems fairly reasonable. (And it can be off by quite a lot and still not make a huge difference to the cost-per-km of driving.)
Look at gas prices in your area. Look at the price of crude. Divide.
How could you possibly not be able to estimate the fraction?
And yeah ideally you use an average number over some months and you sample the crude earlier than the gas but those are minor tweaks.
B) They shouldn't correlate by a particularly large amount in a competitive environment. For an approximation as rough as "half" and assuming no other changes it's not a big deal.