How much do I need to worry about lower than 91 octane being used in a Mazdaspeed3?

Hello, while I’ve been at university my Mazdaspeed3 has been at home and used by my dad somewhat regularly since the lease on his car ended and he hasn’t bought a new one yet. Long story short, I recently found out he’s been filling it with 85 instead of 91 like I told him to do. He says he’s gone through about five tanks of 85 and I’m worried this may have caused unnecessary wear to the engine, or damage. I’m only in my hometown for today so I feel it’s unlikely I’ll get to see my car. Do I need to worry and do I need to do anything like siphoning the tank and using fuel cleaner?
 

sm1ke

Work In Progress..
Moderator
Contributor
:
Canada
:
'18 CX-9 Signature
Hello, while I’ve been at university my Mazdaspeed3 has been at home and used by my dad somewhat regularly since the lease on his car ended and he hasn’t bought a new one yet. Long story short, I recently found out he’s been filling it with 85 instead of 91 like I told him to do. He says he’s gone through about five tanks of 85 and I’m worried this may have caused unnecessary wear to the engine, or damage. I’m only in my hometown for today so I feel it’s unlikely I’ll get to see my car. Do I need to worry and do I need to do anything like siphoning the tank and using fuel cleaner?

If the car has already been through five tanks, I don't think siphoning the remaining 85 octane gas in the tank will make much of a difference. I'd skip the fuel cleaner too. From what I've read, using low octane gasoline in a vehicle designed for higher octane can damage engine components and cause severe engine knock or pinging. I think using higher octane becomes even more important on turbocharged cars that are specifically designed for that octane rating.

I would fill it with a couple of tanks of 93 (if available) and then go back to 91 (and never let your dad fill your gas anymore, lol). The 91 octane gas will mix with the existing 85, which will change the octane rating of the gas depending on how much 85 is in the tank right now. For example, if you have a half tank of 85 and fill it with 93, you'll have an approximate average of 89 in that tank. Then, if you siphon the gas or drive the car enough to get the 89 tank to the half tank mark, then fill it with 93 again, you'll have an approximate average of 91 octane for that tank.

If you notice engine knock after going back to 91, chances are the damage has been done.
 
Last edited:
:
Pueblo county CO
:
CX-5 Sport 16.5 6M
84 or 85 seems to be common around where I live (Colorado). They say it's ok because of the altitude. I try to use 87 (recommend in my manual) and 91 or 93 occasionally. Using “Top Tier" gas might be more important.
 
Top