Older cars have more trouble than newer ones. There are exemptions for cars old enough to be classics, but it is effective at weeding out those in the 15-20 year old range.
My 1993 Ford Escort Wagon (yes, I still own it) has absolutely no trouble passing State of Washington emissions laws. It's not even close to the limits.
My vehicle currently gets roughly 35 mpg highway and 30 mpg city. I know it's simplistic to use gas mileage as a proxy for gasoline conversion efficiency, but - it is not obvious to me that there's been significant improvement in gas engines over the past 20 or 30 years. I don't see a lot of similarly-sized new cars that do better than - or even as good as - my old beater.