Octane Rating vs. MPG, Test Results

Mr Nick · **Posted:** Wed Aug 26, 2009 12:47 am

Many people believe that higher octane gas gives them more power and mileage. Some argue the gain in mileage isn't worth the additional $0.10 - $0.20 or more per gallon compared to regular 87 octane.

I decided to put this to the test, MPG only. I had zero desire to pay and have this thing dyno tuned with different gas, not even an option.

The results are pretty much what I expected. Over a test period that was 20 fill-ups long, 87 and 89 octane were pretty much an even match, and premium (92 or 93 depending on station) the MPG went down slightly. I felt ZERO difference in power, smoothness, or any other drive-ability changes between the octane ratings.

I feel there was an even mix of all driving types for each octane rating.

The Liberty has a K&N intake on it, which was the same for all tests. Stock computer (no chip), stock exhaust.

If you removed the 2 tanks with comments next to them, the results would be even more clear: Higher octane on my application hurt mileage.

Discuss!

87 octane
Miles Gallons MPG
322 17.5 18.4
236 12.7 18.6
296 15.8 18.7
277 16.7 16.6 idled with A/C for approx 3 hours this tank
266 14.4 18.5
152 8 19.0

87 Octane totals
1549 85.1 18.2

89 octane
Miles Gallons MPG
270 14.8 18.2
294 16.4 17.9
298 16.5 18.1
307 16.4 18.7 ALL highway miles
220 12 18.3

89 octane totals
1389 76.1 18.3

92/93 octane
Miles Octane MPG
241 12.9 18.7
244 13.5 18.1
278 15.8 17.6
217 12.2 17.8
246 14.1 17.4
127 7.6 16.7
262 14.34 18.3

92/93 octane totals
1615 90.44 17.9

Grand total
Miles Gallons MPG
4553 251.64 18.1

Mr Nick · **Posted:** Wed Aug 26, 2009 12:50 am

I forgot to adjust for my slightly larger tires, 255/65/17 (30") compared to stock of 225/75/16 (29.3")

That would INCREASE my mileage data by 2.5%, but for comparison purposes, it's not important. Tire size was consistent during all testing.

hrogi · **Posted:** Wed Aug 26, 2009 5:33 pm

Well this sounds right...

The higher Octane burns to slow and hurts the BANG part of the 4 stroke.

Good test...
I like it...

Diggerfreek · **Posted:** Wed Aug 26, 2009 8:18 pm

Thanks for the numbers

E2 · **Posted:** Thu Aug 27, 2009 1:39 am

Don't have my numbers and too lazy to go to the jeep. But my numbers will show no difference.

Mr Nick · **Posted:** Thu Aug 27, 2009 7:59 am

I got some platinum plugs at work ($4.95 each

), I'll toss them in when I get a chance and see what happens.

33,000 miles on it so far...

nightdrv · **Posted:** Mon Sep 07, 2009 10:04 am

any updates?

louder2kj · **Posted:** Mon Sep 07, 2009 2:19 pm

Some state require ethanol to be added I believe in Illinois the 87 doesn't have any and 89+ has 10%. Since ethanol lowers MPG the results make sense for Illinois. This is also why before e85 got huge you could find stations where the 89 cost less then the 87. Check what your state requires before you compare these results.

Mr Nick · **Posted:** Mon Sep 07, 2009 5:20 pm

nightdrv wrote:

any updates?

Didn't have time to put the plugs in before my last 2 road trips. 700 miles last weekend, 300 this weekend. :shock:

On Saturday I did get 18.2 from home to our cottage, 120 miles 95% highway at 75-80 mph.

I took Tuesday off, if I take a break from the Cutlass I'll toss the plugs in and hope for the best.

1quick1 · **Posted:** Thu Sep 10, 2009 11:04 pm

Putting higher octane is a waste of money and how even his test results showed the MPG went down.

The fact is with most vehicles running higher then needed octane does more harm then good.

People think that it makes their car have more power which is not true at all and probably decreases power slightly if anything.

tjkj2002 · **Posted:** Thu Sep 10, 2009 11:15 pm

1quick1 wrote:

Putting higher octane is a waste of money and how even his test results showed the MPG went down.

The fact is with most vehicles running higher then needed octane does more harm then good.

People think that it makes their car have more power which is not true at all and probably decreases power slightly if anything.

In certain conditions higher octane is better,like towing and the more extreme offroading.My KJ was tipping the scales at 5500lbs without me sitting in it,so when I was in it I was over the max GVWR and I always got better mpg's with premium gas(about the same as towing all the time).I now live in CO and there low grade gas is 85 octane,2 octane less then the lowest recommended for the KJ.My KJ pings like crazy running 85 octane,not much better with 87 but runs like a champ with 91 octane in the tank and can get just shy of 22mpg's on the highway at 75-80mph.

hrogi · **Posted:** Fri Sep 11, 2009 11:18 am

Well Tjkj2002 brings up a good point.....

Octane has to be adjusted from time to time based on the what is happening.

85 Burns super Fastest
87 burns fast
89 burns slower
91 burns really slow
93 burns really slow
100+ burns way to slow for most cars unless heavly modified or running supper hot like 20 minutes on a 2 mile track at max RMP the hole time.

Most normal cars use 87 at sea level
High out put cars use 91 at sea level

past sea level things change, so adjust for what your car needs.

Ping is the bang going off to soon, higher octane slow down the burn so fixess the ping as seen here.

Diggerfreek · **Posted:** Fri Sep 11, 2009 11:22 am

tjkj2002 wrote:

1quick1 wrote:

Putting higher octane is a waste of money and how even his test results showed the MPG went down.

The fact is with most vehicles running higher then needed octane does more harm then good.

People think that it makes their car have more power which is not true at all and probably decreases power slightly if anything.

In certain conditions higher octane is better,like towing and the more extreme offroading.My KJ was tipping the scales at 5500lbs without me sitting in it,so when I was in it I was over the max GVWR and I always got better mpg's with premium gas(about the same as towing all the time).I now live in CO and there low grade gas is 85 octane,2 octane less then the lowest recommended for the KJ.My KJ pings like crazy running 85 octane,not much better with 87 but runs like a champ with 91 octane in the tank and can get just shy of 22mpg's on the highway at 75-80mph.

75 to 80?? Come on troy, I have followed you down I 25 and I don't remember dropping down to 75 once :twisted:

Mr Nick · **Posted:** Sat Sep 26, 2009 8:14 pm

Mr Nick wrote:

I got some platinum plugs at work ($4.95 each

), I'll toss them in when I get a chance and see what happens.

33,000 miles on it so far...

After 4 tanks the new platinum plugs have been either the same or 1mpg better then the stock plugs with 33,000 miles. So new plugs pretty much gave me .5 mpg improvement. I'm sure new non-platinum would have shown an mpg improvement as well, but I like less maintenance with the platinum. :wink:

Octane Rating vs. MPG, Test Results

Who is online