Centexguy

I've run gas from Walmart, Costco, shell, Exxon, and everyone between. In my g37s Infiniti, my 2011 f250 diesel, and my 2012 Eco. My fiancé only runs Walmart in her Xterra. Neither of us have ever had any issue with pinging or mileage differences. That's between premium and regular and diesel.

ThreeOhTwo

The 5.0 is a pretty high strung engine with a 10.5:1 compression ratio and indirect injection. Asking it to perform its best on 87 octane is a pretty tall order particularly when put in a three-ton truck that can tow another four tons. We can get 15+ hp gains out of it by using premium or 96-octane E85 so I am strongly betting that it's retarding timing quite a bit on 87 and 87 is likely the absolute minimum the engine can run on.

It sounds like you got something less than 87 octane from Costco. There is actually a lot of sub-87 octane gasoline floating around out there today despite nothing using sub-87 octane fuel. This is because a lot of fuel is blended with ethanol. Ethanol is really good at increasing octane so distributors can use some really crappy base stock gasoline and add a little ethanol and voila- 87 or 89 octane fuel at lower costs than using 87 or 89 octane straight gasoline. However actually getting the full 87 or 89 octane requires the distributor to blend in the proper amount of ethanol, not let it get contaminated with water (ethanol sucks up water and the water reduces octane by a few points), and not accidentally confuse straight 87 octane gasoline with sub-87 octane base stock that WILL be 87 octane after the ethanol is added and give you a tanker of low-80s-octane garbage gasoline.

We had a lot of problems with knocking on early ethanol blends in the St. Louis area in the '90s as we are an EPA non-attainment area and mandated to use ethanol-blended gasoline for emissions purposes. Various groups investigated the issues and the problems nearly all turned out to be adding too little ethanol to the terrible base stock gasoline and letting water get into the fuel. That and people running old beaters which were grossly out of tune...

I personally take care of the problem by just about never buying 87 octane gasoline. I use E85 or if that's not available, 89-90 octane mid-grade. Those are much more forgiving for a distributor having a bad day than 87 octane and cheaper too since I now live in the buckle of the corn belt.

dwysywd

So a 10% ethenol would equal what in octane if 87?

CDC5.0

90% of the time my truck gets Shell 87 octane and it runs great on it. I have put a couple tanks of Shell 91 octane in it and it does run a bit smoother but there wasn't enough of an improvement in performance / economy to warrant the change. I have never tried costco gas but in a pinch I put a tank of 7/11 gas in it about a month ago and it just didn't seem to run the same. Maybe its just in my head though. I try my best to use Shell mainly because I can collect air miles .

ThreeOhTwo

If you add 10% ethanol to regular 87 octane, it would give you 90-91 octane.

If your 87 octane has 10% ethanol in it, the actual gasoline component is somewhere around 83 octane.

95stepside

I never put that much thought into why 89 ethanol is cheaper than 87 straight gas, other than the fact that is has ethanol in it. Learn something everyday.

dwysywd

I've never thought to run E85, but it sounds interesting. Does E85 = better performance and better MPG's?

ThreeOhTwo

It gives better performance and never pings. Ford's numbers are that the 5.0 makes 375 hp/390 ft-lb torque on E85 vs. 360/380 on 87 octane. Seat of pants dyno says that there is probably a little more than a 15 hp gain.

Fuel mileage is about 15% worse than non-ethanol gasoline on average. There is a little less of a drop when working your truck hard (or driving in stop-and-go traffic) and a little more of a drop in the winter as the ECU runs the engine pretty rich on E85 in the winter until it warms up. I'd not recommend running E85 in your truck in the winter if it gets below 0 F regularly and you park outside as E85 has about the same ability to cold-start in the winter as spring or summer gasoline would.

NLterminator

Same here, in my 03 Cobra Costco 91 gas seems to work much better especially when the temps are cooler.

HOT150

Is there any HP gain by using 91 vs 87?