Fuel mileage
07-19-2014, 10:11 AM
#21
JK Junkie
That is pretty consistent with what I see. Slowing down to 65 MPH gives me about 2 MPG over 75 MPH. From somewhere around 12-14 to 14-16. My speedometer is calibrated, numbers are hand-calculated.
Just curious, who else has noticed that the Jeeps get a couple more MPG when burning mid-grade fuel at higher altitude (bear in mind that mid-grade at higher altitudes is 87 or higher)? I get a little improvement at regular (85-86 octane), but definitely see an improvement in performance and gas mileage at high altitude if I stick with the 87 and higher octane. I run with a lot of different folks, and many of them have remarked the same thing to me.
Just curious, who else has noticed that the Jeeps get a couple more MPG when burning mid-grade fuel at higher altitude (bear in mind that mid-grade at higher altitudes is 87 or higher)? I get a little improvement at regular (85-86 octane), but definitely see an improvement in performance and gas mileage at high altitude if I stick with the 87 and higher octane. I run with a lot of different folks, and many of them have remarked the same thing to me.
Regarding the octane, using an OBD gauge on the dash that can read the KR (knock retard), my experience is that 87 octane is a fairly good number to use -- Which is what the owners manual recommends. Sometimes there's more KR after a fill-up with the same octane, and suspect we don't always get the advertised rating. With 85, KR seems to happen even at mild throttle opening on mine, and would probably be worse going uphill. Haven't used 85 octane much because of this, but it's a reasonable assumption to me that KR is going to reduce MPG a little (when driving in a KR condition). The logic that lower octane is required at high altitudes fails when KR is happening at partial throttle. Also, the PCM is pretty quick to respond, can't usually hear pinging when small amounts of KR is applied.
One thing about higher altitudes is that tire pressure increases (roughly 1 PSI per 2000 ft), so rolling resistance probably tends to decrease a little as altitude increases -- If tire pressure wasn't adjusted, there might be some increase in MPG regardless of octane because of that. Also, air friction goes down with increasing altitude, so that's good for MPG as well.
Last edited by Mr.T; 07-19-2014 at 08:39 PM. Reason: More about affects of altitude on MPG.
07-20-2014, 08:41 PM
#22
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Fuel mileage
Last year we went on another trip to Death Valley, 1775 miles of it was on-road highway type driving. For this trip we mostly followed the slower trucks in the slow lane in Oregon & California. Speeds were ~60 on the flat, and usually less than 55 going up the steep grades. There was a little city driving visiting relatives, and the average over 1775 miles was 19.8 MPG. Earlier this year we did a similar trip running the the speed limit +5, and spent more time in Nevada where the trucks have the same limit as cars, and that's the basis for the 65 and 75 MPH highway estimates. What I noticed really sucks the MPG down is maintaining the limit +5 going up steep grades -- That little 3.8L feels good around 4500 RPM, but life in the fast lane gets expensive.
Regarding the octane, using an OBD gauge on the dash that can read the KR (knock retard), my experience is that 87 octane is a fairly good number to use -- Which is what the owners manual recommends. Sometimes there's more KR after a fill-up with the same octane, and suspect we don't always get the advertised rating. With 85, KR seems to happen even at mild throttle opening on mine, and would probably be worse going uphill. Haven't used 85 octane much because of this, but it's a reasonable assumption to me that KR is going to reduce MPG a little (when driving in a KR condition). The logic that lower octane is required at high altitudes fails when KR is happening at partial throttle. Also, the PCM is pretty quick to respond, can't usually hear pinging when small amounts of KR is applied.
One thing about higher altitudes is that tire pressure increases (roughly 1 PSI per 2000 ft), so rolling resistance probably tends to decrease a little as altitude increases -- If tire pressure wasn't adjusted, there might be some increase in MPG regardless of octane because of that. Also, air friction goes down with increasing altitude, so that's good for MPG as well.
Regarding the octane, using an OBD gauge on the dash that can read the KR (knock retard), my experience is that 87 octane is a fairly good number to use -- Which is what the owners manual recommends. Sometimes there's more KR after a fill-up with the same octane, and suspect we don't always get the advertised rating. With 85, KR seems to happen even at mild throttle opening on mine, and would probably be worse going uphill. Haven't used 85 octane much because of this, but it's a reasonable assumption to me that KR is going to reduce MPG a little (when driving in a KR condition). The logic that lower octane is required at high altitudes fails when KR is happening at partial throttle. Also, the PCM is pretty quick to respond, can't usually hear pinging when small amounts of KR is applied.
One thing about higher altitudes is that tire pressure increases (roughly 1 PSI per 2000 ft), so rolling resistance probably tends to decrease a little as altitude increases -- If tire pressure wasn't adjusted, there might be some increase in MPG regardless of octane because of that. Also, air friction goes down with increasing altitude, so that's good for MPG as well.
Today I traveled about 400 miles keeping about ~70mph 5th gear and my peak mpg was 21.5, wow... And I averaged 21.2 for the whole trip. I had the front 2 windows down the whole time, and when I also had the rear windows down too I got better mpg's than when I just had the front windows down
My specs are still the same as the first post btwSent from my iPhone using Tapatalk
