Gas milage vs gear ratio vs tire size
 

Alright the world of gears confuses me, and i need to get some things straight in my mind. Suppose you have a rubicon with 4:10 gear ratio, would you get better gas milage with 35s or 33s. or what about with a 3:73 gear ratio. If you have one of these gear ratios and your rig has a 4 inch lift, tell me mods and gas milage, beacuse i am stuck on what to get. I live on the coast of Louisiana (sea level mostly) and i need to figure out what jeep i am going to buy for this summer. This jeep will see only the rocks in my driveway and maby a lil mud here and there, but i want it to be built to go through whatever i want when i need to.

I have a Rubi with 35's with stock 4.11's (3" lift). Pretty good gas mile hit going to 35's. Dropped from ~17-18mpg, to ~14mpg. It really depends on how you drive. On the highway if there is any incline or wind, overdrive is useless (I have an automatic). I need to regear.

33's will get you better gas mileage, but honestly its a jeep. Gas mileage is going to suck.

I drive about 60 miles round trip each day, with about 60% "Chicago" highway (stop and go about 1/2 the time). I actually picked up a 2001 Civic to stop the wear and tear / bad mpg's. My JK was offically promoted to a toy!

So there are a lot of factors that go into fuel economy, tire size, and gear ratios. Also as a mechanical engineer having worked for GM and done engine research for my Masters you can trust me on what I say :wink:

In all the largest factor in your fuel economy is the driver. The faster you accelerate and faster you cruise on the highway the more fuel you will use (assuming all else, transmission, tires, and speed are equal).

With that said the second largest factor in fuel economy vs. tire size and gear ratio is your transmission. If you have a 6-speed you will have much better fuel economy potential than an automatic. I say this because the automatic JKs have a tendency to want to downshift on the highways if you are going fast or climbing grades because of the lack of sufficient power at low RPMs with the 3.8. If you have the 6-speed you can make it stay in gear longer to keep the AVERAGE engine RPMs lower and thus keep the fuel economy up.

Finally assuming you know you want an auto or a 6-speed (so assuming you drive the exact same way and have the exact same transmission between two vehicles with varying gear ratio and tire size) the rule of thumb is the lower the AVERAGE engine RPMs the better your fuel economy.

So what this means is that THEORETICALLY if you have a JK with 3.73 gears and 37" tires would get great fuel economy compared to one with 3.73 gears and 32" tires or one with 5.13 gears and 37" tires (because the average engine RPMs will increase THEORETICALLY by decreasing tire size or increasing the gear ratio). BUT we live in a world where hills, aerodynamic drag, and other parasitic losses are a large factor.

This is where the general statement that larger tires decreases fuel economy comes into play. Larger tires increase both the power required to turn your tires and the aerodynamic drag of your JK on the highway because it makes the aerodynamic "profile" of the vehicle larger, requiring more power to push the larger wall of air down the highway at speed in layman's terms. So to maintain speed up hills or to increase speed you need to downshift more often with larger tires (assuming EVERYTHING else is constant) to increase the power output from the engine. The results from downshifting more often is that you increase your average engine RPMs and decrease your fuel economy. So even though your potential engine RPMs are lower with larger tires and the same gear ratio, by downshifting more often these theoretical gains are not realized due to the real-world considerations such as drag and parasitic power loss from larger tires.

So in all the optimum balance between fuel economy, tire size, and gear ratio exists at the magical point that keeps your AVERAGE engine RPMs at its lowest possible point at a given speed. For you at sea level with mostly flat roads, you could easily run 33" tires on an auto with 3.73 gear ratio or in a 6-speed with 3.21s and you would get great fuel economy (low to mid 20s on pure highway driving). If you go past 33" tires I would say to get the next gear ratio down (4.1 for an auto and 3.73 for a 6-speed) and you could run upwards of 35s on flat roads at sea level with those gears (and they come stock so no need for the added $1000 cost of re-gearing).

I can say that I have the "dreaded" 6-speed with 32" tires and 3.21 gear ratio and I personally LOVE it. I live ~2,000 feet above sea level and my average highway fuel economy with the AC on full-blast, crossing through some steep highway grades in West Virginia, and fully loaded with people and luggage was 22 mpg this past summer. Alone without AC during the spring and fall I average 24 under normal driving and can push it as high as 25 if I set the cruise control at the speed limit and don't accelerate to pass anyone.

Hope this helps clarify why there are lots differing view points on gears. It is a balancing act that some see an increase in MPG by regearing while some see a decrease. It is because if you drive it hard and put new gears in you will downshift less, increasing your fuel economy. If you drive it nicely in flat areas where you are in OD most of the time, if you regear and increase your engine RPMs in OD you will decrease your fuel economy.

:beer:

I put 35" tires on mine and it killed the power and mileage. So I figured I could change the gears to make up for this. I got the power back but no mileage improvements. I was told this is because of the extra rotational mass of the large tires.

I gained about 2mpg when I went from 4.10s to 5.13s on 35s. up to 17mpg :ya::ya:

Thats great. Maybe I drive to hard

Had to read this like 3 times :crazyeyes:. Alright i think i learned something, i live and drive on flat ground, southern louisiana doesn't own hills. So the higher number in gear ratio you go, the lower the ratio is(3:73<4:10) is that right?:dontknow2: To clarify things, i do want a 6 speed transmission and my target fuel milage is 17-18[trying to get better mpg than my silverado(16 average)] I suppose im going to get a rubicon or a jk with tow package(3:73 gears) it all just depends on the better price(i know rubi is more exp, just saying if i can find one at a good price), and what other things make mpg better EX. computer programmer, cold air intake, exhaust, list more cause im kindof new to the world of jeeps and i know possibilaties are endless.

Is your MPG gain by the computer or hand calculated at the pump? If it is by the computer it could be due to not recalibrating your speedometer/odometer when you put larger tires on. By doing that your Jeep "thinks" you have gone less distance than you actually have because the same number of rotations of the transmission shaft will take you a longer distance after putting larger tires on (making the computer calculated MPG go down). Then when you put higher ratio gears in it will require more rotations of the transmission shaft to rotate the the tires the same number of times making the Jeep's computer "think" you have gone a longer distance again (making the computer calculated MPG go back up).

The higher the number in the gear ratio, the more rotations it takes to rotate your axle one time (and the "lower" the gearing is called). You should be able to get 17 - 18 out of any trim level 6-speed JK with tires up to 33" easily. :beer:

thx for input. i have decided to go with 33's so that i wont have to get new tire mounting bracket

There is a huge difference in fuel mileage depending on the terrain you roll over. Rural state highways at 50-55 mph are quite different than Interstate driving at 65-70mph. Aero drag is the big killer, nearly doubling from 50 to 70mph. That's a big part of what makes your horsepower/rpm requirements and why TPM, Planman and myself probably have very different needs.

I know Planman is rolling at 70-75 mph or even faster at over 3,000 feet elevation. Mountain passes with 3+ degree sustained grades makes it a reality to twist at higher rpms and hold in OD or 6th. TPM is likely in a similar situation minus a couple thousand feet and likely 10-20mph slower highway speeds, which is a huge part of pulling better efficiency numbers. As for myself, I'm cruising at 70mph highway speeds on flat ground at 600 feet.

Three very real and different scenarios and probably none of them match exactly what you need to cope with. For me, and sustained speeds of 70mph, I need to pull 2200+ rpm to maintain speed over the 3 degree grades of overpasses that I see every couple miles. If I'm at a lower rpm I lose speed, easily picked up on the negative grade side but not necessarily endearing myself to the other motorists. At 60mph, not an issue even at the reduced rpm.

Anyway, just wanted to illustrate that there isn't an elastic pants answer to your question. You want better fuel mileage, slow down as you're going to be moving a barn through space and air. As TPM stated, an automatic transmission with frequent downshifts and gear hunting is going to be the greatest hit on efficiency. You want to move the responsibility of maintaining speed from the transmission to the throttle body. The rpm range is really a tertiary concern after answering the questions on terrain and speed.