The EPA prints two 1-to-10 scores on a new car's window sticker. One is greenhouse gas, driven almost entirely by fuel consumption. The other is the smog rating, which covers the pollutants that affect air quality where you're standing — nitrogen oxides, particulates, carbon monoxide.
People read them as a single "how green is it" number. Here's what happens when you sort by smog rating and check the economy.
| Smog rating | Average mpg | Configurations |
|---|---|---|
| 10 (best) | 93.2 | 1,082 |
| 9 | 27.3 | 1,250 |
| 8 | 28.5 | 286 |
| 7 | 28.5 | 2,327 |
| 6 | 22.0 | 8,586 |
| 1 (worst) | 18.5 | 1,346 |
A 7 is more economical than a 9
Cars rated 7 for smog average 28.5 mpg. Cars rated 9 — two grades better — average 27.3. The scale runs slightly backwards in the middle, which it shouldn't if the two measures tracked each other.
The pattern only holds at the extremes. A 10 averages 93.2 mpg because that band is almost entirely electric, and a 1 averages 18.5 because that band is large petrol engines. Between those, the relationship falls apart.
Why they diverge
They measure different chemistry.
Greenhouse gas is arithmetic. Burn a gallon of petrol, emit a fixed amount of CO2. The score is fuel consumption expressed differently — nothing about the engine changes it.
Smog is emissions control. Nitrogen oxides and particulates depend on combustion temperature, catalyst design, particulate filters and how the engine is calibrated. A manufacturer can build a thirsty engine with excellent aftertreatment, or an economical one with modest aftertreatment.
Diesel is the clearest illustration. Diesels return better mpg than equivalent petrol engines — 27.9 highway against 24.6 — so they score well on greenhouse gas. They historically scored poorly on smog, because NOx and particulates are exactly what diesel combustion produces.
Which one should you care about?
Depends what you're optimising for.
Fuel cost: neither, directly. Use the mpg figure and the annual fuel cost estimate, which is what you actually pay.
Local air quality: the smog rating. This is the one that affects the street you drive on and the children walking down it. It's also the one that matters for emissions testing in states that run them.
Climate impact: the greenhouse gas score, which is close to the same information as fuel economy.
On a used car, verify it belongs to that car
Both scores are calculated per configuration, so the same model year can carry several — a third of recent model years have more than one rating, as we found in two 2025 Civics can differ by 18 mpg.
The original window sticker holds the figures for the exact vehicle as built, and it can be retrieved from the VIN — we covered how here. A VIN check decodes the engine and drivetrain so you know which set of numbers applies. We also looked at which brands emit most overall in a Lamborghini emits more than two Volvos.
Frequently asked questions
What is the EPA smog rating?
A 1-to-10 score covering pollutants that affect local air quality — nitrogen oxides, particulates and carbon monoxide. It's separate from the greenhouse gas score, which is essentially a restatement of fuel consumption.
Does a better smog rating mean better fuel economy?
Not reliably. Cars rated 7 for smog average 28.5 mpg while cars rated 9 average 27.3. The two only line up at the extremes, where 10s are mostly electric and 1s are large petrol engines.
Why do the two environmental scores disagree?
They measure different chemistry. Greenhouse gas is fixed arithmetic from fuel burned, while smog depends on combustion temperature, catalysts and particulate filters — so a thirsty engine with good aftertreatment can beat an economical one with modest aftertreatment.
Which rating should I pay attention to?
For fuel costs, use the mpg figure and annual cost estimate directly. For local air quality and emissions testing, the smog rating. For climate impact, the greenhouse gas score.
Denis Kataev is a serial solopreneur and the founder of ForCar. With 15+ years in software engineering and 10 years in SEO, he builds data-driven products end to end — backed by a sharp eye for design. At ForCar he mines proprietary vehicle datasets, turning raw numbers into buying advice you can actually trust.
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