Guess I needed to look at it on a per 100 mile basis vs my per tank basis.
However, the 1,111 gallons you saved with an 18mpg avg would only get you an extra 20K miles but the 833 gallons you saved with a 40mpg avg would get you an extra 33,320 miles. A diff of +13,320 miles of driving pleasure...and a lot less CO2/NOx per mile... You can look at it both ways, one is per 100 miles driven and the other is per 100 gal used, which is more efficient??? or cleaner??? Guzzling gas is definitely less green/less efficient for the environ cuz you're burning more hydrocarbon per mile.... IMO, GM still doesn't get it.....

My old Hummer gets 7mpg and my new one gets 8mpg.....so I use less fuel per 100miles driven but at what expense???

Can't compare an 18mpg vehicle with a 40mpg vehicle, apples and oranges again.....