If you look closely at the prop receipt for Blast from the Past that Doc pulls out of the DeLorean in 1985, you can see that it cost $29 and Marty paid $3.19 in tax for a total of $32.19.  This is an 11% sales tax rate.

On January 1, 2013, California adopted a standard statewide tax rate of 7.50%.  Local taxes on top of this tax rate make the sales tax higher in most communities.  As of April 1, 2015, 496 cities and towns in California use the standard 7.50% sales tax rate while 1,287 cities and towns have sales tax rates ranging from 7.625% to a whopping 10%.  Petaluma, one of the locations originally considered as the setting for Back to the Future, has a sales tax rate of 8.25%.

If Hill Valley was a real California town in 2015, it would have the highest sales tax rate in California.

Prediction Accuracy: Failed