no CEL in Colorado

Just a question. My 02 Jetta 5-spd has had the EGR Delete without the computer being adjusted. I have driven the car about 10K miles in KS. The CEL is frequently on, I get a consistent 42 mpg. I have taken the car twice to Colorado, the CEL doesn't come on and I get 48-50 MPG there. What is the difference or what can I change to get 48-50 MPG in KS. In Colorado I didn't have the AC on much, there are mountains where I go, there isn't as much wind. Any input would be appreciated. Thanks

Was the 48 to 50 mpg based on hour after hour non stop interstate driving with cruise control? Is the 42 mpg in Kansas real world went store, went to other store, visit friend in same or nearby town? All that stopping for lights, getting back up to speed burns more fuel.

The KS driving has been both highway and in town driving. The humidity has been extremely high in KS currently. The Colorado driving was 65-85 mph. But the CEL not coming on in Colorado is what is confusing to me as well.