Looks like the computer on mine is a bit optimistic, but still, 28.7 is pretty good for mixed driving, and rush hour at that. Next tank should give me some more solid data.
After nearly 3 years of tracking my MPG very closely, I've come to the inescapable conclusion that the trip computer is more consistently accurate than the errors introduced with the manual calculation method.
The trip computer can meter fuel more accurately than a gas pump gushing out gallons/minute. The biggest variable with manual MPG calculations is that pumps shut off at different levels of fill. The Trip Computer meters the fuel as it is used so it isn't affected by this variable. The other potentially large variable the Trip Computer doesn't encounter is pump accuracy. In Washington State, where I live, the Department of Weights and Measures does unannounced random pump accuracy and fuel quality testing. It's legal for the pump to read under but even a slight over reading is a violation.
If your state doesn't have such an effective program for keeping station owners/operators honest, your MPG numbers may be lower simply because you are not getting quite as much fuel as you paid for. Also, your fuel could have more than the Federally allowed maximum of 10% alcohol. Washington State tests alcohol content and levies enforcement fines when it finds alcohol over the limit. If your state doesn't measure and enforce this very effectively, I absolutely guarantee there will be considerable fuel over 10% alcohol in the marketplace. That's what Washington State found when they started measuring alcohol content but enforcement actions have brought the cheaters (mostly) back into line. There is a double incentive to cheat on alcohol content;
1) Alcohol is cheaper than gasoline
2) It's cheaper to make gasoline of marginal octane and a little extra alcohol can ensure it measures out to the desired minimum octane.
The bottom line is that you can trust the CX-5 Trip Computer at least as much, if not more, compared to the manual calculation method using fuel pump figures.