Other than an adv gimmiick..What is the reason for the .9 at the gas pump?

http://www.users.uswest.net/~taaaz/AZgas.html

a. It is not possible to purchase one gallon of motor fuel at the advertised price - a circumstance totally unique in commerce.

b. Changes in motor fuel prices are always made in whole cents, not fractional cents.

c. The banning of 9/10 cent motor fuel pricing by Iowa state law was well received by Iowa motorists from 1985 to 1989 before special interests reinstated the deceptive pricing practice. The Iowa experience demonstrated that motorists preferred whole-cent pricing of motor fuel and the Arizona State Legislature could easily provide Arizona motorists these advantages by State statute.

d. The 9/10 cent pricing practice was initiated at a time when a 1/10 cent saving per gallon of motor fuel represented a much larger portion of the total purchase price than is the case today.

e. Fractional cent pricing other that 9/10 cent can easily be programmed into a dispenser computer but there is no recorded instance where this has ever been done since the technology became available.

f. Motor fuel price changes are among the most volatile of retail transactions and are always made in whole cents, yet the 9/10 cent portion of the per gallon price is, and always has been, a constant, whereas the remainder of the price continually changes. This pricing concept is totally unique in commerce.

g. The industry-wide and unchanging 9/10 cent pricing practice for a gallon of motor fuels should be considered price collusion and a deceptive marketing practice.

h. Active price competition exists at 1/10 cent per liter for motor fuel sold in Canada. One-tenth cent per liter approximates 4/10 cent per gallon. One-tenth cent per gallon pricing implies a transactional accuracy that is unachievable, given failure to volume correct for temperature, the effect of residual volume from the previous purchase and the permissible error in dispenser accuracy.

i. Motor fuel purchases involving 9/10 pricing must always be rounded either up or down to the nearest whole cent except for those multiples of 10.000 gallons (i.e. 10.000, 20.000, ...gallons.) For exact gallon purchases, the need for rounding would disappear once 9/10 cent pricing was discontinued and would only be required when divisibility of the purchase is not possible. When rounding exact half cent purchases, motor fuel dispenser meters should be programmed to apply commonly accepted mathematical rules for rounding, thus eliminating the current 15 to 10 bias toward rounding up.

j. Arguments against prohibiting 9/10 cent motor fuel pricing by State statute must provide evidence that consumers and dealers would, in some demonstrable manner, be harmed. The benefits have been presented and it is not a reasonable or sufficient argument that motor fuel has traditionally been priced in this manner.

A joke:
Thank you for your purchase of 13.401 gallons of regular unleaded gasoline priced at $1.399/10/gallon.
Rather than the 13.4 gallons + 3/4 teaspoon of gasoline shown on the dispenser meter display, it is more likely that your purchase ranged somewhere between 13.339 and 13.463 gallons.

Because this transaction was for less than 5000 gallons, the gasoline you purchased was not temperature compensated, therefore the energy content is unknown.

Regardless of the fact that it is not possible to accurately measure the amount of your purchase to the nearest 9/10 cent, gasoline has traditionally been priced in this manner.

HAVE A NICE DAY