08JKsilv

Just fugured out that i am getting almost 15 MPG city and highway mixed. I think thats pretty good with stock gears 3.21 and 35" tires. So if i was to gear it maybe 4.88's could i expect to see better MPG. Just curious if anyone else has figured what they are getting. I know its a jeep and i dont expect to get 30-40 i didnt buy it for the gas mileage. I was just wondering what others are getting with gears and 35" tires.

jcrowell

there are tons of threads on fuel consumption with different tire size and gearing.. do you have an auto or a stick?

vdimemmo

Wth the higher axle ratio (4.88s) your MPG will go down due to the higher engine RPM's that will be required to attain vehicle speed. Think about peddling your bike 1st gear versus 5th.

Conversely, you attain the benefit of increased acceleration as the engine can build-up RPMs faster due to the mechanical efficiency of the higher axle ratio.

I'm sure others can express this more eloquently.

PS:According to my car's dash readings, I get anywhere from 14.8 - 15.3 MPG running 91 octane fuel. I have 35's, 3.73:1 axle gears, and an auto tranny. I also keep my foot firmly planted on the accelerator most of the time.

Big D2

Yes with a regear you might see some better numbers. In theory, you wont since the engine will have higher rpms and thus burn more gas. However, in real life, you will see a benefit.

Torque is what moves the Jeep. What you want to be in the best possible position when it comes to the torque curve. So, if you have 3.21 with 35's, on the highway, your engine rpm will be probably around 1800. Now, if you look at the torque curve of the engine, that's near the absolute bottom. So, one little hill and the tranny will have to shift. With 4.88s or 5.13s, your RPM's will be stock or slightly higher. So hopefully this will put you in the ideal torque curve for highway speeds.

So although it doesn't make sense on paper, you will see some real world increases.

Rubicon Rando

so... if i m getting this correctly... the higher rpms in the proper torque range will net better gas mileage?

i am going to be regearing in the next few weeks, i was told 5.13's, but i think that is too short, 4.88's maybe?

what do you think? 4.88's or 5.13's?

seer1

A couple of things here; although I still haven't figured out this 3.21 thing except maybe Superior paid someone at Jeep off so they could sell a boatload of gears to folk, but my optimal curve with 4.10's and MPG is 1200-1800 rpm. Secondly, vdimemmo why are you running 91 octane? Our JK's our quite happy running 85-87 octane. Mine has never pinged at 6000-10,000 feet, you're just throwing your $$$ away with that stuff.

Warlord2

if you have manual, 4.88's will net you best mpg
if you have automatic, 5.13's are expected to net a little better mpg.

ducatijr

Just keep in mind, if you havent regeared your speedo for 35's than your dash reading of MPG will be wrong. you would actually be getting better millage. and remember that your tach would be off too.

Old_School

Huh? Why would the tach be off?

ducatijr

That's how speed is calculated. multiply the current RPM by what gear you're in by what's the tire diameter... Right?