I recently leased a 2006 G35 5AT and was using the manumatic mode of shifting to hold the gear constant and determine the speed at various engine RPMs. I was trying to determine if the rear differential has been changed in this car versus the 2005 G35 5AT. The dealership said there are no changes of this sort advertised by Infiniti. However, I obtained the following speeds and RPMs at the various gears, suggesting that either a rear differential gear change has occurred, or the tachometer is reading high by 14-15%:

Gear / Gear Ratio/ Engine RPM/ Speed / Back calculated Rear Diff Ratio
2 / 2.246 / 6000 / 53 mph / 3.95
3 / 1.471 / 3800 / 50 mph / 4.04
5 / 0.834 / 3000 / 70 mph / 4.02

The formula relating speed, engine rpm, gear ratio, and rear differential ratio is

v (mph) = rpm*C/(88*GR*RR)

where C = tire circumference in ft (6.89 ft for either the 215 or 235 tire options for the G35)
GR = transmission gear ratio as stated in the specs for 5AT (3.54, 2.246,1.471,1.00, and 0.834 for 1st thru 5th)
RR = rear differential ratio

The advertised rear differential ratio for the 2005 G35 is 3.357, and 3.538 for the 6MT and G35x. If my car had a 3.357 differential, then my tachometer should have read 14-15% lower at the speeds and gears indicated. That means that at 70 mph in 5th gear, I should be reading 2500 rpm, not 3000.


Has anyone else noticed any difference in this respect with the 2006 models?

 

It's hard to be sure with the numbers from 3rd and 5th but the numbers from 2nd show a definite problem - the torque converter clutch would not be engaged. This means your engine and transmission speeds are not 1:1.

I recommend taking readings around 2,000 rpm with no acceleration for several seconds. Do not however let it coast, that can screw you up too.

 

I had the dealership test the calibration of the tachomoter and they found it was reading 12% low (i.e., at 3000 rpm by visual inspection, the computer was reading 2650).

I also asked what reading was used by the fuel cutoff algorithm and the infiniti technican said it was the feedback from the visual tachometer (???). This means the fuel cutoff had been occuring in 2nd gear as I approached 59 mph (had noticed that a couple of times), instead of the correct 66 mph value. So I had them recalibrate the tachometer, and I now see 2600-2700 rpm in 5th gear at 70 mph.

Thanks for any suggestions provided so far.

 

whoa this is way advanced. Should all check this out? I have an 05 5AT sedan.

 

An easy way to determine the rear gear ratio in your ride is to raise it on a hoist where the both rear wheels are free. Leave the tranny in N (engine off), now rotate the rear wheels slowly exactly 1 revolution while counting the revolutions of the drive shaft.

If you have a 3.35 diff., the driveshaft will rotate 3.35 times.

Likewise with a 3.54 diff., it will rotate 3.54 times.

C.

 

Thanks, Msedanman.

That does sound like the most fool proof way of checking this out without any other parameter affecting the calculation. I'll report back when I get a chance to do this.