Optimum RPM in top gear

DanVan1990 · Feb 19, 2023

Hi guys. Anyone have an idea of what would be the optimum rpm to drive my Jeep liberty ‘04 in top gear on the highway for best fuel consumption.
I’ve heard stuff about running it at the rpm where you get the full/top torque of the car. So look at the torque curve, and where it peaks, run the car at that rpm to make it most efficient. Does anyone have any clue of this sort of thing?
Thanks.

lfhoward · Feb 19, 2023

On my KK before modding by adding a lift, tires, and roof rack, I experimented with this. I have a Scangauge II so I can see how my driving behavior impacts mpg. On the stock KK the best speed on flat ground for MPG was 62 mph. Any higher and wind resistance (which increases with the square of velocity) started to reduce my mpg. Any slower and I would be a hazard on the interstate, and going slow would drive me crazy. But most semi trucks travel at 62 so I would just tuck in behind one of them (preferably one with side skirts) and average 24 mpg on long highway trips.

mercdudecbr600 · Feb 20, 2023

Around 23-400 rpm seems best but wind resistance is what hurts you at speed

DanVan1990 · Feb 20, 2023

lfhoward said:
On my KK before modding by adding a lift, tires, and roof rack, I experimented with this. I have a Scangauge II so I can see how my driving behavior impacts mpg. On the stock KK the best speed on flat ground for MPG was 62 mph. Any higher and wind resistance (which increases with the square of velocity) started to reduce my mpg. Any slower and I would be a hazard on the interstate, and going slow would drive me crazy. But most semi trucks travel at 62 so I would just tuck in behind one of them (preferably one with side skirts) and average 24 mpg on long highway trips.

I’m glad someone has done a test and experiment with this. Thanks for the information. I was also wondering about wind resistance knowing that would be a factor, just didn’t know at what speed wind resistance would start to effect mpg. I will give this a try at that speed and check it out. Thanks for your help.

DanVan1990 · Feb 20, 2023

mercdudecbr600 said:
Around 23-400 rpm seems best but wind resistance is what hurts you at speed

Yes thanks, also knew wind resistance would hurt at speed. Just wasn’t sure at what speed it starts to affect you. In top gear at 2000rpm I’ll be doing around 62mph.

lfhoward · Feb 20, 2023

I think 80’s cars have a most efficient speed of 55 mph, but in the 2000’s when the speed limits went up, vehicles are geared better for efficiency at higher speeds. Wind resistance still gets you after a certain point. Following semi trucks helps by 10-15% increased mpg in that regard. You don’t have to follow/hypermile at an unsafe distance to reap some benefits in our un-aerodynamic Jeeps!

lfhoward · Feb 20, 2023

Lifting and adding larger tires (especially without regearing) has made a big impact on my mpg. I am now at 17 mpg highway (maybe 19 following trucks) and 12 mpg in the city instead of 14.

Details can be found in my Fuelly data if you want them. (Just passed 800 tanks of gas!)

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Jeepy (Jeep Liberty) | Fuelly

DanVan1990 · Feb 20, 2023

lfhoward said:
Lifting and adding larger tires (especially without regearing) has made a big impact on my mpg. I am now at 17 mpg highway (maybe 19 following trucks) and 12 mpg in the city instead of 14.

Details can be found in my Fuelly data if you want them. (Just passed 800 tanks of gas!)

You must be registered for see images attach

Jeepy (Jeep Liberty) | Fuelly

Thanks for that. Interesting data capture. And ya that’s one thing about the Jeeps, they definitely are not aerodynamic at all. But hey that’s not a major. Thanks for you help. Will definitely be doing a bit of testing myself running with your info.
I’m currently also doing about 12mpg in city and about 21mpg highway. Not sure why the difference is so big.

KJowner · Feb 20, 2023

Start stop driving kills MPG, slowing down and speeding up nearly 2T of metal needs substantial power input, cruising in top gear has minimal load on the engine so lower power input required = less fuel burnt.
If you want economy don't buy a Jeep

mercdudecbr600 · Feb 24, 2023

lfhoward said:
Lifting and adding larger tires (especially without regearing) has made a big impact on my mpg. I am now at 17 mpg highway (maybe 19 following trucks) and 12 mpg in the city instead of 14.

Details can be found in my Fuelly data if you want them. (Just passed 800 tanks of gas!)

You must be registered for see images attach

Jeepy (Jeep Liberty) | Fuelly

Interestingly, the early KJ's 45RLE trans has a lower (higher RPM) OD than the later trans. My early KJ, even with 245/75/16 (30.5") tires, still cruises at 70mph around 2300rpm - stock tire size 235/70/16 (29.0") had cruising around 2400rpm. I guess I'm saying, the older KJ's seem less affected by tire diameter than the later/newer ones. I also think the type of tire (tread design + weight) has A LOT to do with efficiency.

Optimum RPM in top gear

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