The national average price for regular gasoline decreased at a more or less continuous rate in September, from $3.647 on September 1st down to $3.433 on September 30. This is a decrease of $0.214 for the month.
In August the price decrease was $0.074, about 1/3 of what it was in September. Despite this decrease in the price of gasoline, the cost of food and most other items increased, so the CPI-W increased 0.3%, despite the gasoline price decrease.
The same trends regarding food and other items probably have continued in September, but they will have a much bigger decrease in the cost of gasoline to overcome, so at this point I suspect that the September CPI-W will not exceed August’s by as much. However, we could be surprised. The cost of all fuels is about 6% of the total CPI-W, so this may tend to mitigate the sharp drop in the price of gasoline.