I'm trying to settle a debate with my wife. I live in the United States. Given the tag on the gas lid says "Premium Recommended"... How Recommended is it? I have started using medium grade (89), but was having a conversation with friends the other day that made me think I should be using 93. Thoughts?

 

I just recently tried 87 and 93 while towing a utility trailer with two dirt bikes, two mountain bikes, plus gear for two people. From the gulf coast of MS to NC. Averaged 8-9 MPG on 87 and 10-11 on 93. It wasn't a precise comparison as terrain varied and I trusted what the trip computer said. And the vehicle has all terrains, roof rack, etc. But it was all highway driving at 75. I often see a large price gap between 87 and 93. If that difference is only 20-30 cents, I buy 93. But i've seem plenty of stations with nearly a dollar difference. I've run 87 in multiple LRs. V8 LR4, p300 defender, d5. All without issue.

 

One of my recent tanks was on mostly regular grade. It was the best mileage I'd ever seen. I usually use premium but on road trips or in resort towns if I have to fill up I'll use midgrade or regular.
I've never noticed any difference in power, acceleration, or engine noise on any of the fuel grades. I usually drive it pretty easy - not hard on it, only punch it to pass once in a while.

 

Outside of MPG (very helpful data though) is there any known or hypothetical risk to mechanics? One theory I heard is that the bits of the engine (e.g. crank shaft) are built for a specific explosion achieved with the high octane fuel, and using lower octane (although minor) causes added wear on such parts.

 

The US is the worst when it comes to fuel grades. They intentional make it more about marketing than anything, specifically calling the highest octane "premium". While many premiums may be 100% gasoline (no ethanol), there is nothing really premium about them. It's just an octane rating. Always run what is recommended. And they say recommends because if they say "required", well then you are limited on options in some places and can not fill up = complaint to manufacturer. Anyway, if "premium" is recommended, that would generally be the highest octane at the pump which is usually 90+. If you run a lower octane you will get knocking and the knock sensors will adjust engine timing to compensate as best they can, but that does not mean elimination of knocking. And no, you dont always hear knocking yourself. And if you run lower octane fuel, you will have a reduction in power and economy, period. There is no way to get around basic physics. As for damage, you generally won't see anything using lower octane occasionally because the engine is adapting as best it can. With that said, I would never tow or climb mountains or even off-road with less than the recommenced octane. Cruising along the highway in a leisurely way, sure a lower octane would be okay. Long duration damage can occur, but it really depends on the engine and how much wear it the engine already has. But knocking is BAD and damage will eventually occur, especially if you run low octane under heavy loads. Really there is TONS of information online about this topic, it is well researched and well known. But like I said, some engine simply handle lower octane fuel better but it does not mean they handle it without any issue. By the way, the octane recommendation for an engine is pretty much based on compression ratio. The higher the engine compression, the higher the octane is needed to prevent premature detonation (knocking). My 7 series BMW with a 5.0 V12 - regular fuel. It was only an 8:1 compression ratio. People assumed a V12 needed premium when discussing the car. My Rover is 10.5:1, annoyingly. So premium when I can, lower octane when not available.

 

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