Hyundai is under investigation by the EPA for over-estimating the fuel economy, on the Monroney (consumer info) sticker, by about 3%, affecting about 900,000 vehicles.
This is going to be a very expensive mistake for Hyundai, in cash and market perception. And with a big election so close, I can only assume EPA is going to hammer them with fines.
From Detroit News:
Sung Hwan Cho, president of Hyundai America Technical Center Inc., said the company in 2010 changed testing procedures to calculate road resistance that accidentally overestimated the fuel economy.
So what happened? It sounds like Hyundai screwed up their dynamometer calibration for road loads. In order to get a close match between road performance and lab results, Carmakers calculate numerical coefficients which are used to estimate the effect of air resistance, rolling resistance, vehicle inertia, etc. on a vehicle on the road. These coefficients are used to correct the load that a chassis dyno puts on a vehicle, so that the fuel economy and emissions results are a reasonable match to real world performance.
With this change, Hyundai can't brag about having so many 40mpg vehicles.