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So I did a little test during my 220-mile drive from Indianapolis to Mansfield Ohio last weekend. Let me lay down some stats and observations. You be the judge on which makes more sense.

Route: I-70 and I-71
Traffic: I felt that the traffic on both driving days was the same. There was no stop-and-go and no traffic that slowed me down below 65mph
Road: No significant hills, it's mostly flat with the same amount of incline and decline grades both coming and going. There was also no snow or ice on the road surface.
Wind: There was no wind on the first day of driving, but the return trip home had a slight headwind blowing against me as I drove south on I-71 (for about 60 miles)
Vehicle load: luggage, two kids and a wife both ways
HVAC Controls: Heater on 71 degrees both ways, Seat Warmers off, A/C manually turned off.

Indianapolis to Mansfield:
Fuel: 93 Octane
Tire Pressures (Hot): average 38psi
Oil Life: 50%
Outside Temperatue: 22-23 degrees
Average MPG according to MID: 23.5
Average Speed: 74 MPH

Mansfield to Indianapolis:
Fuel: 87 Octane
Tire Pressures (Hot): average 38psi
Oil Life: 50%
Outside Temperatue: 23-27 degrees
Average MPG according to MID: 21.6
Average Speed: 76 MPH


Both of these driving times were recorded using the following driving technique: Set cruise control at 80mph, shift into manual and select to gear ratio #6 (this kept the RPMs at a pretty regular 2,100 RPMs). Using regular "D" mode was allowed only when a passing situation required or going up a significant grade.

I'm fairly certain that these two days are so close in terms of variables, that we can all assume that they didn't influence the actual test to any sort of degree. So in conclusion, using premium fuel netted me (essentially) a 2 MPG increase over regular fuel. Gas prices at the local stations I use had premium gas at $0.28 more expensive than regular. Some gas stations have their premium at only $0.20 more expensive than regular. I also usually fill up with around 16 gallons each time, so here is my math:

23.5 MPG x 16 gallons = 376 Miles of driving
21.6 MPG x 16 gallons = 345.6 Miles of driving

So you drive 30.4 additional miles if you use premium

That equates to 1.29 gallons of fuel that you'd save using premium over regular.

If you multiply 16 gallons by $0.28 more that premium costs, that comes out to $4.48 more per fill-up. That much money would buy you roughly 1.26 gallons of premium fuel or 1.37 gallons of regular fuel. Now we know, mathmatically, that the difference here is negligible. If you were to use regular gasoline (at let's say $3.37), you could buy that extra 1.3 gallons to drive you that extra distance you didn't have compared to premium for $4.38. Once again, these numbers are negligible.

Conclusion:

If you use premium, you're going to get more MPG's, but the difference in cost is offset by the additional price you're going to pay ALMOST EXACTLY. If you use regular, your goal is to get your highway MPG's within 2 MPG's or better of the MPG's you normally get with premium fuel. Anything less than that and you're losing money and should just use premium.

I will continue to repeat these tests a couple more times at least to ensure that my findings are correct.
 

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Nice evaluation! Your findings are very similar to what we found on our 8500 mile cross country trip. Since we purchased most of our gas at Costco's around the country, we used premium and we were getting about 24.0 mpg fully loaded and with a loaded cargo box. Where we lost milage was when we encountered strong head winds. We noticed a difference when we switched to regular. Our lowest milage was 17 mph and that was going up hill and against very strong winds. Lately I've been filling with mid-grade fuel and getting about 20 mpg.
 

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Nice test.... But aside from headwind, you also need to take into consideration any elevation changes. Since you can't really determine elevation changes, I suggest driving with either 87 or 93 BOTH ways and see if you see a difference in MPG. My bet is that you would see the same exact difference that you currently show...sadly.
 

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Just curious,

Has anyone done a test to see if there's a difference in MPG when using name brand (Chevron, Shell, etc.) gas vs. discount gas (Costco, Sam's, grocery stores, etc.)?
 

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I can't recall anyone comparing the different brands. Myself, I've only used Chevron (all three grades), Sam's and Costco for the 30,000 miles on the car. I really can't tell the difference power wise, but did feel that Costco premium gave me the better mph on the cross country trip. 24.5 mph at 65-70 mph on I-90.
 
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