my 6 speed manual is producing 2500rpm @75 mph. Does this seem correct?

 

I don't know about your 6 speed, but my 5 speed is like 3000+ at at that speed.

 

The gearing on the auto with a 3.73 axle should be more economical than a manual w/3.91 axle. When you last checked, were you cruising or accelerating in a lower gear under throttle?

 

Technically at 75mph, it should be at 2128rpm, using a gear calculator. 3.727 diff gear ratio, 0.716 fifth gear tranny ratio (automatic), 1:1 transfer case ratio, 31.6" tire diameter (265/70/17).

 

^^^ what he said ^^^

 

Then how does the auto acheive better mpg. According to your explanation, the manual with 3.91 axle at 2500rpm should get better mpg.

 

Not sure how you came to that conclusion, but it is incorrect.

The maunal technically should be doing 2490rpm at 75mph in 6th gear. Using the following figures: 3.909 diff gear ratio, 0.799 6th gear tranny ratio, 1:1 transfer case ratio, 31.6" tire diameter (265/70/17)

75mph in top gear:
Auto 2128rpm
Manual 2490rpm

Auto is doing less rpm's, hence the better mpg especially on the highway. Also remember, there will be some variance in what your tach says, depending on the tire size or inflation, how accurate the tachometer is, whether the torque converter has locked up or not, etc....

 

It makes sense now. originally, the first responce to my question from "getitdone" was that he was making 3000+ rpm at 75mph. Obviously, not in top gear--which now makes more sense.

 

i suppose it depends on your truck... mine has 4.56 gears in it stock from the factory. and im in his boat... zippin down the interstate at 75+ is often 3K+ in 5th gear. in my case on flat ground i run out of gear long before i run out of engine.

 

Yea, I was under the impression that I have 4.10s and in 5th gear with my stock tires I remember the rpms at around 2800 when cruzing at 60. I would really have to push the motor(and go down a hill) to go 75, and the tach read over 3000 rpms.

 

Ummm... This is the FJ Cruiser section, so probably talking about an FJ Cruiser here.

 

Well... you know... saw the thread in the RSS and just started posting away.

 

Because the manual FJ is a 6 speed, and the 6th gear should be higher than the 5th on the auto.

But auto is 2WD, which uses less fuel than the manual's AWD.

However, I read that in pure highway driving the manual will be better than the auto in fuel economy. That makes sense if the RPMs are lower.

 

I guess You didn't read my posts above, the 6th gear in the manual is not "higher" than the 5th in the auto, they do not share the same gear ratios with an extra gear added to the 6 speed.

The 4wd is available in auto or manual.

The manual will not get better fuel economy in pure highway driving.

Read my posts above which have the FJ Cruiser diff gear, and tranny top gear ratios listed.

 

that depends on the grade though. The 6th gear has a lower gear ration than 5 in auto, so it doesn't need to down shift as often & will maintain its speed, unlike the 5th in auto that needs to downshift for more power.