Let's take, as an example, the Pennsylvania Turnpike and a trip I made upon it earlier this year, from Pittsburgh to Philadelphia. Using their calculator, using exits 57 and 362 results in a total distance of 269.7 miles and a toll of $19.70. That's a per mile rate of 7.3 cents, three times what we might expect if California's gas tax actually sufficed. Orange County's own CA 241 toll roads are even worse. Santa Margarita Parkway to the I-5 comes to a total of $2.50 in tolls over an 11.3 mile distance. At 22 cents per mile, that's ten times the cost that we would expect if the gas tax were sufficient. What's more, the 241 and other toll roads receive additional funds from the gas tax.
The truth is, the gas tax comes nowhere near close enough to actually paying for transit. It retains some appearance of doing so, although falling short even still, because a large portion of driving is on roads that are paid for by other taxes, such as property taxes, thus subsidizing the highways. Take away that subsidy, however, and it's nothing more than a hollow shell.