Almost all cars and trucks currently made with EFI and a Knock
Sensor can run just fine on 87 Octane (R+M/2) Regular, they pull the
timing back a few degrees and adjust the fuel map to stop the
knocking.  You do lose a few horsepower, but the savings per gallon on
the fuel easily makes up the difference.  (IMHO 5 to 10 HP loss is
over-estimation, I'm betting it's closer to 3 to 5.)

 The higher performance modern cars will have language in the owners
manual encouraging 91 or 92 Octane Premium fuel, but very few cars
made flat out require it.  Heck, for some of the people who own
restored 1970's Full-On Detroit Muscle Cars like GTO's and Judge's,
you need 104 octane racing gas or better - which used to be readily
available at the pump as Premium when the cars were built.

 Where the "Use Premium" pressure really comes from is the oil
companies - they want to get people to spend another 20 to 40 cents a
gallon for the intangible cachet of Premium, just so you can "Put a
tiger in your tank!"

     --<< Bruce >>--