Why MPG Testing Changed After 2020: The Fuel Sulfur Standard Nobody Talks About

You smell that? Rotten eggs, maybe? It's not that Easter egg you left uncollected under your novelty garden gnome. It's sulfur. And, believe it or not, the gasoline in your car has a certain amount of it. Sulfur is a natural part of crude oil, and unless removed, it makes its way into the fuel that you pump into your traditional internal combustion engine or combination gas-electric hybrid setup. Why is that such a bad thing?

Well, in addition to being stinky, gasoline sulfur content negatively impacts the effectiveness of modern cars' emission control systems. In 2000, the Environmental Protection Agency (EPA) – the agency that manages greenhouse gas regulations for cars – instituted tiered gasoline sulfur programs to reduce the amount of sulfur in the gasoline you pump into your car or truck and lower air pollution in the process. However, by the time the Tier 3 transition wrapped up in 2020, measured fuel economy had dropped with the use of the Tier 3 test fuel compared to previous test gasoline. And while there's a difference between efficiency and miles per gallon, no one wants theirs to plummet.

Unlike previous testing and regulatory programs, the Tier 2 Gasoline Sulfur program was the direct result of a Clean Air Act requirement driving the EPA to reevaluate tailpipe emissions standards. That low-sulfur initiative included the implementation of more advanced emission control systems beginning with 2004 models. It also represented the first time the EPA treated vehicles and fuel as one system in testing and subjected SUVs and light-duty trucks to the same testing as passenger cars. And what was the result? These new Tier 2 standards required passenger vehicles to become up to 95% cleaner and reduced the sulfur content in pump gas by up to 90%.

Then came the next stage of the EPA's sulfur-reduction initiative, the Tier 3 program of 2017. As with the Tier 2 program, the Tier 3 strategy considered the vehicle and its fuel as a combined system. Tier 3 required a drop in gasoline sulfur content to no more than 10 parts per million (PPM). Here's the kicker, though. Although the Tier 3 program sought to further lower the sulfur content of pump gasoline, testing revealed a drop in average fuel economy, too — as much as 2.29% in the EPA's Federal Test Procedure (FTP) and 2.98% in the Highway Fuel Economy Test (HFET) compared to the Tier 2 testing.

So, what gives? As part of the 2020 Tier 3 transition, the EPA turned its attention to swapping the zero-ethanol test fuel it had been relying on before — also known as E0 — for a 10% ethanol test gasoline (E10). That tracks, too — the EPA says most American fuel stations nowadays supply consumers with gasoline featuring an ethanol content anyway. That, and the federal agency also acknowledged that the updated test fuel — reduced sulfur content and all – would likely result in different fuel economy results.

"The Tier 3 emission standards include changes to several properties of emission test fuel to make it more representative of in-use fuel, and some of these changes are expected to affect emissions and fuel economy," the EPA reported in its Tier 3 Certification Fuel Impacts Test Program findings. And while average fuel economy did drop by a few percent, the findings also showed a decrease in carbon dioxide (CO2) compared to previous test fuels. In terms of the FTP evaluation, CO2 dropped an average of 1.78% with Tier 3 test gasoline compared to that used in Tier 2 testing.