Along the I-5 corridor from Kettleman City to Santa Nella, Calif. the Diesel is running + or - $1.50. The regular gas is right at $2.00. Forty miles North/East in Modesto the regular could be as much as twenty cents cheaper. The only Diesel I could ever find there was at the high price, big names and it's always out of sight.
It wasn't too long ago that the Diesel was a dime to fifteen cents more.