4.88 gears 32" tires

[cal]

My speedo is properly calibrated.
I do agree that the 4.0 runs nice & smooth @ that speed, But MPG suffers, I am sure I can pick up a couple of MPG by running it at a lower RPM closer to OEM.

MPG doesnt suffer from the RPM's. With deeper gears, you're using less throttle to get to the same RPM's. If your mileage is suffering, look at other factors.. load, roof rack, bad sensor, tires, driving style, unrealistic expectations, karma, .. but its not a direct relation to RPM's.

 

[garr]

MPG doesnt suffer from the RPM's. With deeper gears, you're using less throttle to get to the same RPM's. If your mileage is suffering, look at other factors.. load, roof rack, bad sensor, tires, driving style, unrealistic expectations, karma, .. but its not a direct relation to RPM's.

So why then do all the OEM setups keep the RPM's as low as possible ?
Usually around 2K or less at crusing speed.
The faster an engine spins the more fuel & more air it sucks in, correct ?

 

[techno1154]

MPG doesnt suffer from the RPM's. With deeper gears, you're using less throttle to get to the same RPM's. If your mileage is suffering, look at other factors.. load, roof rack, bad sensor, tires, driving style, unrealistic expectations, karma, .. but its not a direct relation to RPM's.

I have heard that statement here on many times. My experiances are different. When I was using 30" tires with 3.55 gear ratio, my XJ always consumed a lot more fuel running the hills along I-65, I-75 and I-95 in third gear at 3,000+ RPM's. It is always just me and about 300 lbs of gear in the XJ. Driving between 65 and 75 MPH using cruise control, the AW4 would hunt for the correct gear going up those long hills. To prevent this, I usually select the 3rd position.

 

[cal]

So why then do all the OEM setups keep the RPM's as low as possible ?
Usually around 2K or less at crusing speed.
The faster an engine spins the more fuel & more air it sucks in, correct ?

The engine doesn't suck fuel in. Fuel is pumped in based on throttle position.. that's one of many reasons carburetors have been replaced by fuel injection..

 

[cal]

I have heard that statement here on many times. My experiances are different. When I was using 30" tires with 3.55 gear ratio, my XJ always consumed a lot more fuel running the hills along I-65, I-75 and I-95 in third gear at 3,000+ RPM's. It is always just me and about 300 lbs of gear in the XJ. Driving between 65 and 75 MPH using cruise control, the AW4 would hunt for the correct gear going up those long hills. To prevent this, I usually select the 3rd position.

So when you're climbing hills at 3k rpm, you use more fuel than when you aren't?


There's obviously a point of diminishing return.. but in your instance running the higher rpm's is the result of increased load, therefor increased fuel consumption?

 

[garr]

The engine doesn't suck fuel in. Fuel is pumped in based on throttle position.. that's one of many reasons carburetors have been replaced by fuel injection..

I understand that, I used the term "more fuel & more air it sucks in" Loosly :wave1:

But I have to ask again, Why then do all the OEM setups keep the RPM's as low as possible ?
.

 

[cal]

emissions?

lack of lifts, big tires, or any other need to keep the engine closer to peak torque?

why do xj's come with 3.55's, 3.93's, 4.10's or even 4.56's vs the 2.72's cars came with in the 70's? the delta of gear ration vs overdrive still leaves us at higher rpm's than cars used to have..

I can't tell you why the XJ was built the way it was. I can tell you based on the 5 XJ's I have owned and probably 45 others I have been very involved with, you'll do better with deeper gears and higher RPM's than you will with the opposite.

Your mileage may vary. Although it probably won't.

 

[kastein]

where do you buy those 3.93 gears?

 

[cal]

http://www.precisiongear.com/dana30.htm

http://www.precisiongear.com/dana44.htm

On the 30 is 3.91, on the 44 is 3.92.

Came OEM on early XJ's with some packages.

 

[kastein]

oh yeah, the gas sipper package. Completely forgot about those.

 

[cal]

oh yeah, the gas sipper package. Completely forgot about those.

thats the one.. with the deeper gears, and higher rpm's..

 

[kastein]

thats the one.. with the deeper gears, and higher rpm's..

can't be. That would never save fuel! :laugh2:

 

[ktm racer 419]

So why then do all the OEM setups keep the RPM's as low as possible ?

to lessen the NVH (noise, vibration, harshness)

the general public would complain about the added noise and probably not buy the vehicle. im not saying you get way better milage at higher rpms, but there is something to be said about the engine not working as hard, and cruising at higher rpms the AW4 stays in 4th gear overdrive, not shifting gears on hills or to pass, slipping the torque converter which makes the transmission less efficient, thus using extra fuel.

i just like the extra power with lower gears, and i still get decent fuel economy, and dont mind the noise.

 

[bobnoxious]

So what gear ratio will give me 225hp at the wheels with 20+mpg?

 

[kastein]

2.71s for super high top speed

 

[Voteban]

2.71s for super high top speed

Back when I was buying my first Cherokee I came across one that had 2.72 or 2.93 or something like that. The VIN # pulled up a "highway package".