Mix the 87 and the 91 to get close. 50/50 ought to do it. That's what I do when I gas up at Sam's or Costco.

 

If you have a high performance engine, treat accordingly.91 octane is fine for stock hemi's just like 89, but if u have a lot of mods and consider the engine "high performance" treat accordingly. Now if ur looking at saving money, 89 will b fine in all hemi's. IMO

 

Its not a good idea to mix up 87, 89, 91 in the tank, stick with one octane level per fill up, or use up all the flue from the lower octane before going to a higher octane.

Run a tank empty as possible, than completely fill up, mpg will increase as well.

Altitude plays a big factor in how effective a engine can use higher octane flue.

If your towing, highway, or working a hemi, pony up for 92+, your paying more per gallon or liter for your coffiee, milk or pop than your are for flue.

 

I'm at 7200 feet. Would it bee better to just use 91 then?