Why is there such a difference between the milege the Government says we should get and what we do get?
You might have to be more specific. If you are talking about your vehicle not getting the mileage (miles per gallon) that the EPA rating says, it may be due to dirty air filter, old and dirty plugs, partially restricted exhaust, or some other mechanical matter that is not up to par. But it's more likely due to the way you drive, or the terrain you drive on, the traffic you drive in, the amount of time spent idling at signal lights, etc. Go to
this website for some interesting information about the way the EPA estimates MPG. You can read about both the old way and the new 2008 way.
Personally, I don't drive the way I should for maximum fuel economy, but I'm not a hotrodder anymore either. Under the old (or original) EPA ratings for our 2001 Ford Windstar, it was 18 city, 24 highway, 20 combined MPG. Under the new 2008 EPA rating system, it's 16 city, 22 highwy, and 18 combined. And our overall mileage for the last 71,813 miles (nearly 6 years) is 19.587 MPG. In other words we're doing better than what the government says in the new ratings, and close, but not quite as good as the old rating.
The old 2001 Ford Ranger ratings were 18 city, 22 highway, 19 combined, while the new rating system is 16 city, 20 highway, 18 combined. And in the last 22,225 miles in nearly 4 years, our overall average is 18.631 MPG.
So, for what I'd consider "normal" driving; i.e., usually the speed limit, sometimes 5 mph over, a lot of heavy traffic, air-conditioning a good part of the time, very brisk acceleration merging onto freeways, etc., it seems the EPA ratings are about as accurate as they could be.