I travel through Arizona 4-6 times a year on I-40. I've found that some places won't give you the "automobile" price unless you ask, and I've found some that won't give it at all. That may be illegal but that was what happened! Other places give it with a smile. The best Ariz. price I usually find is at the Pilot a few hundred yards north of exit 48 near Kingman. But I also like "Little America" truck stop at Flagstaff. I always give them some business just because of their name and attitude!
The best I-40 price in New Mexico I've found is at "Flower Mountain" Travel Plaza at Acoma. "Flower Mountain" is actually an Indian gambling joint, and they use the low diesel price to attract truckers to gamble. I have mixed emotions about doing business with them, so fuel is the only thing I will buy there. I'm not sure if I'm being hypocritical or just frugal!