The simple version

The used cars and trucks index rose 0.4% in August, after rising 0.4% in July. Over the last 12 months, the same index is down 2.3%.

That looks like a contradiction and is not one. A monthly change asks what happened since last month. A 12-month change asks where prices sit compared with one specific day a year ago, and the answer depends as much on where they were then as on where they are now.

The numbers

• The index for used cars and trucks rose 0.4% in August 2026, seasonally adjusted (Bureau of Labor Statistics, Consumer Price Index news release, August 2026)
• The same index rose 0.4% in July 2026 (BLS, Consumer Price Index news release, August 2026)
• Over the 12 months ending August 2026, used cars and trucks are down 2.3% (BLS, Consumer Price Index news release, August 2026)
• The monthly path from February through August 2026 ran negative 0.4%, negative 0.4%, 0.0%, positive 0.1%, negative 0.2%, positive 0.4%, positive 0.4%, all seasonally adjusted (BLS, Consumer Price Index news release, August 2026, Table A)
• Compounding those seven months gives roughly negative 0.1%, which is our calculation rather than a published figure (BLS; calculated)
• The earlier part of the window is where the decline sits. From the published seasonally adjusted index levels for the series, the monthly changes from September 2025 through January 2026 work out to roughly negative 0.2%, positive 0.7%, positive 0.1%, negative 0.9%, and negative 1.8%, which compound to about negative 2.1%. Those five figures are our calculation from BLS index levels, not BLS's published rounded changes (BLS series CUSR0000SETA02; calculated)
• New vehicles rose 0.3% in August and are up 0.6% over 12 months, moving in the opposite annual direction from used vehicles (BLS, Consumer Price Index news release, August 2026)
• All items rose 3.4% over the same 12 months, while used vehicles fell 2.3%, so the two moved in opposite directions over the same span (BLS, Consumer Price Index news release, August 2026, Table A)

A 12-month change has two ends, and only one of them is now

Any year-over-year figure compares two points: this month, and the same month last year. When the number falls, it can mean prices dropped recently. It can also mean prices a year ago were high and the comparison has not caught up yet.

The used car figures show the second case. Prices have been rising since July, yet the annual comparison is still negative, which puts the decline somewhere earlier in the 12-month window rather than in the recent months.

The visible part of that window supports it. From February through August the monthly changes were negative 0.4, negative 0.4, 0.0, positive 0.1, negative 0.2, positive 0.4, and positive 0.4. Compounded, those seven months come to roughly negative 0.1%, close to flat, which is our arithmetic on the published monthly series.

So seven of the twelve months were roughly a wash and the last two were increases. The 2.3% annual decline is therefore mostly a description of the five months this release does not show, from September 2025 through January 2026, and the published index levels for the series bear that out: by our calculation the index fell about 0.9% in December and about 1.8% in January, and those five months compound to roughly negative 2.1%.

This is what people mean by a base effect. The starting point of the comparison is doing the work, and as those older months roll out of the window the annual figure can turn positive without prices accelerating at all.

The Real Cost lens on a $25,000 used car

Index percentages are easier to weigh in dollars. Take a $25,000 used vehicle, a figure chosen as a round illustration rather than measured from any listing or sales data. The index tracks price changes for a category, not the value of any particular car.

• A 2.3% decline over 12 months on $25,000 is about $575 (calculated)
• The 0.8% increase across July and August on $25,000 is about $200 (calculated)
• Those two figures point in opposite directions and both come from the same release, covering different spans (BLS)
• Over the same 12 months, all items rose 3.4%, so a category down 2.3% moved about 5.7 percentage points differently from prices generally (BLS; calculated)
• New vehicles rose 0.6% over 12 months, so the used and new markets did not move together (BLS)
• The $25,000 figure is a stated assumption for the arithmetic, and no individual vehicle tracks a category index (assumption stated)

A shopper comparing a listing today against a memory of last year is using the annual figure whether they know it or not. A shopper comparing against a listing they saw in July is using the monthly one. Both comparisons are valid and they give different answers because they start in different places.

What this means

Annual and monthly changes get quoted interchangeably in coverage of prices, and they answer different questions. When the two point opposite ways, the useful next step is asking which part of the window is producing the annual number, rather than deciding one of the figures must be wrong.

It also explains something that shows up every year in inflation coverage. An annual figure can shift noticeably without any change in what prices are doing right now, purely because an unusual month from a year ago drops out of the comparison. Knowing that is most of what separates reading a price statistic from being steered by one.

What this is NOT

This is not a prediction of where used or new vehicle prices go next, and nothing here forecasts the direction of the annual figure as older months leave the comparison window. This is not advice on whether to buy, sell, trade in, finance, or wait on a vehicle, and it recommends no dealer, brand, model, marketplace, or loan. This is not a valuation of any vehicle or of the used car market, and it takes no position on whether current prices are high or low. The compounded seven-month figure, the September 2025 through January 2026 monthly changes and their compounded total, and the dollar conversions are our calculations from the published monthly series and index levels, not statistics BLS publishes in that form. The $25,000 vehicle is a stated illustration for the arithmetic, not a measured average price, and a category index describes the weighted basket BLS tracks rather than any individual car, whose price turns on model, mileage, condition, and location. The statement about where the annual decline occurred is an inference from the published monthly series, the published index levels, and the published annual figure, not a separately published measurement.

Sources

• Bureau of Labor Statistics, Consumer Price Index news release, August 2026, USDL-26-1496 (Table A carries the February through August monthly series): https://www.bls.gov/news.release/cpi.nr0.htm
• Bureau of Labor Statistics, Consumer Price Index by expenditure category, August 2026: https://www.bls.gov/news.release/cpi.t01.htm
• Bureau of Labor Statistics, series CUSR0000SETA02, used cars and trucks, U.S. city average, seasonally adjusted (index levels used for the September 2025 through January 2026 path): https://data.bls.gov/timeseries/CUSR0000SETA02