What Most Hurts MPGs: Weight, Diameter or Width

[ohthatguy]

I’m wondering what everybody has seen impact MPGs the most. I’ve tried 3 set ups on my 392XR (stock suspension)-

OEM XR392 Wheels 41 lbs
OEM 315/70/17 Tire 62 lbs 34.4” (less under weight)
Total 103 Lbs MPGs 12.6 average

20x12 Moto Metal Wheels 37 lbs
Mickey Thompson Baja Legend 36x15.5 36.4” (35.5” under weight) 91 lbs
Total 128 lbs MPGs 12.3 avg dropping to 11.8 once the OEM rock sliders were removed and I started driving more aggressively (post rubbing)


20x10 Moto Metal Wheels 34 lbs
Toyo Proxes III 305/50/20 32” (shorter under weight) 44 lbs
Total 78 lbs MPGs 13.0 (updated because it bounced up once I took OR+ off and ran a 30 minute errand in a 45MPH zone).

I honestly thought the much lighter, much shorter, narrower, way less aggressive tread pattern Toyo street tires would more dramatically impact the MPGs. The acceleration, braking and lateral grip are all massively improved but not so MPGs.

I’ve got a trade worked out to swap the 94 lb Mickey Thompson 36.4” mud tires for 34.6”, 83 lb Mickey Thompson 35x15.5 AT tires to see where that puts MPGs. I’ll update with the results.

What do you guys thinks causes the MPG hit, height, weight, width, level of tread aggression?

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[JEEP4U]

vehicle height.

 

[rickinAZ]

Weight. Specifically rotating mass.

 

[Zandcwhite]

You admitted you're driving more aggressively with the street tires, that has the biggest impact on mpg in my experience. Beyond that I'd say height is number 1 as you're making bad aerodynamics worse. Then width. Then lastly weight. That is assuming most of your driving is at speed. If most of your driving is stop and go, weight would likely jump to number 1. Once the weight gets up to speed I'd say it's pretty insignificant in mpg. Aero is the opposite, mostly insignificant in stop and go/ getting up to speed but a steady drag that's ever increasing with higher speeds.

 

[ohthatguy]

You admitted you're driving more aggressively with the street tires, that has the biggest impact on mpg in my experience. Beyond that I'd say height is number 1 as you're making bad aerodynamics worse. Then width. Then lastly weight. That is assuming most of your driving is at speed. If most of your driving is stop and go, weight would likely jump to number 1. Once the weight gets up to speed I'd say it's pretty insignificant in mpg. Aero is the opposite, mostly insignificant in stop and go/ getting up to speed but a steady drag that's ever increasing with higher speeds.

Makes sense. It had been at 12.7 with OR+ on and my getting into the throttle more to enjoy the sound. Then I ran a 30 minute errand down a 45MPH road with my wife (OR+ turned off as she hates the exhaust burbles) and in that short of a trip it rose to 13.0 and was still climbing steadily when we got back home.

 

[ohthatguy]

Weight. Specifically rotating mass.

I dropped my rotating / unsprung weight by 40% and MPGs only went from 11.8 up to 13.0. I was hoping to see closer to 15 MPGs.

 

[6.2Blazer]

Did you re-calibrate the speedometer each time you changed tire size?

Did you drive the exact same routes, under the same traffic conditions, in the same weather every time?

 

[ohthatguy]

Did you re-calibrate the speedometer each time you changed tire size?

Did you drive the exact same routes, under the same traffic conditions, in the same weather every time?

No this is just informally observing the changes in MPG with different set ups. It probably thinks I’m getting (and displays) better MPG than I actually am with the smaller tires since they’re rotating more often as it burns the volume of fuel but I’m not covering the same distance per rotation.

 

[Ratbert]

You're driving a 392 and you're concerned about tweaks to mileage???

 

[multicam]

If you really want to draw meaningful conclusions you need to compare MPG over the course of tens of thousands of miles, dozens of tanks of fuel, and not use the computer to determine your mpg (hand calculate it).

For example in my thread comparing MPG with and without ESS enabled I analyzed data of 122 tanks over the course of two years and 30k miles:

https://www.jlwranglerforums.com/fo...n-analysis-2-years-122-tanks-30k-miles.73702/

…and even in that analysis I fully admit it is only meaningful for me in that particular situation (my commute now is drastically different so I expect my results would be different now than when I lived in Texas).

The larger the sample size, the more you can trust the data!

 

[ohthatguy]

You're driving a 392 and you're concerned about tweaks to mileage???

Yes, my prior car was a 575 hp twin turbo V8 X6M that averaged around 16.5 MPG and I never gave the mileage a second thought. But with this 392 I’m hitting the pump every 4 days. It’s ridiculous.

In years past I’ve also had this same 6.4 engine in 4 chargers and challengers and once in a Durango and it was never this bad.

 

[lowmpg]

From my own personal experience, weight was definitely the big hit on me. I have a 2.5" lift and 37s, I noticed a MPG increase when I dropped weight on my tires even though I moved from a 35 to a 37 inch tire. Going from MTs to ATs dropped my weight considerably from close to 80lbs a tire to 62lbs per tire. I immediately saw my MPGs go from 15.3 to 17.5 and they have stayed that way consistently.

 

[Pinky Tuscadero]

Not nearly enough information... yet
And the difference in MPG you've shown here is negligible and would be even on a non changed set up. It appears to be less than 1 mpg on all of them
You didn't even change the speedometer setup for the various tires ???
All that work and you got lazy at the very last moment ?
Besides, you bought a hemi for the power to drive like a dick so lets not hear any complaining about the mileage ?

 

[multicam]

Yes, my prior car was a 575 hp twin turbo V8 X6M that averaged around 16.5 MPG and I never gave the mileage a second thought. But with this 392 I’m hitting the pump every 4 days. It’s ridiculous.

In years past I’ve also had this same 6.4 engine in 4 chargers and challengers and once in a Durango and it was never this bad.

This is one area the 392 guys and the 2-door guys can really relate to each other: terrible range. I almost always get <300 miles per tank. We don’t even get that much better mileage than you and we have that tiny fuel tank.

I don’t really care that my jeep gets bad gas mileage but I care insofar as it’s annoying to have such limited range.

 

[Ratbert]

Yes, my prior car was a 575 hp twin turbo V8 X6M that averaged around 16.5 MPG and I never gave the mileage a second thought. But with this 392 I’m hitting the pump every 4 days. It’s ridiculous.

In years past I’ve also had this same 6.4 engine in 4 chargers and challengers and once in a Durango and it was never this bad.

I'm guessing that those other vehicles weren't shaped like a brick. Our horrid aerodynamics play a massive role in our mileage, especially as our speed increases.

From what I understand we start with a .45 coefficient of drag. That increases as we throw more crap on our bumper, lift it, put on fender extensions (XR package), bigger tires (XR package again), etc.

Extremely light aluminum skid plates theoretically could help, but I haven't done that analysis.

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