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Green Vehicle Guide

Frequent Questions on the Smog Rating

Q: Why does the Model Year 2018 smog ratings chart look so different from previous years?

New federal (EPA Tier 3) emission standards are in effect for MY 2018+, with many auto manufacturers taking advantage of the early phase-in option starting in MY 2017. The California (CARB LEVIII) emission standards are also in effect. As a result, the older (Tier 2 and LEVII) emission standards will no longer appear on the smog rating chart as of MY 2018. 

The good news is that with the new standards in place, the light duty fleet is getting even cleaner. Implementation of the new Tier 3 standards, however, can make smog ratings comparisons across model years challenging. A vehicle model may see its smog rating drop between MY 2017 and MY 2018. This does not mean the vehicle is getting dirtier. In fact, a MY 2018 smog rating of 3 is about the same as a MY 2017 smog rating of 6. 

The updated scale will also impact MY 2018+ SmartWay ratings required for certification. Due to the shift in ratings, it may appear that the SmartWay thresholds have loosened for smog when it’s actually the opposite – the thresholds for smog are tighter than ever.

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Q: Why is there a 1-10 scale if only 7 ratings are being used?

The new rating scale has seven levels that reflect the seven bins represented in the Tier 3 standards. When the smog rating was launched in MY 2004, there were 11 bins under Tier 2, so we had a rating scale of 0-10.  In MY 2009, we dropped the “0” rating, as it was no longer needed. Over time, and factoring in California’s LEV standards, the number of ratings actually in-use has varied. Moving forward, we decided to maintain a 1-10 scale, as it provides consistency and flexibility, should the federal or California emission standards change in the future.

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Q: Will the smog ratings update affect vehicles that were SmartWay certified in MY 2017 and earlier?

No. Once a vehicle has been SmartWay certified, it will always keep that designation. SmartWay highlights the top 20% cleanest vehicles in a given model year, so once you get the designation, you keep it. 

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Q: How can one vehicle have different federal and California smog ratings?

All light-duty cars and trucks must meet either federal (Tier) or California (LEV) emission requirements. Individual states can choose whether vehicles sold in that state must comply with the federal or California emission standards. Within both the federal and California systems, automakers may choose from a range of emission standards for their vehicles, which is why you see a range of smog ratings.

There are cases in which two vehicles may look identical (same make and model), but have different smog ratings. This is generally due to the vehicle being certified to, and/or sold in, different states (one with federal standards, and another with California standards). Two situations can occur:

  1. The vehicles look identical, but have a different emissions configuration. You can distinguish between these vehicles by locating the 12-digit “Engine Family” or “Test Group” ID on the certification label under the hood.
  2. Two vehicles look identical and have identical “Engine Family” or “Test Group” IDs, but the automaker has decided to certify the vehicles to different emission standards. Ultimately, the fuel economy labels that are affixed to these vehicles on dealership lots will always reflect the federal standard. allows you to input your state of purchase, which will use the applicable state standard to determine the smog rating.

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