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Nissan and UpRev final drive tables.

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I have a Nissan tuned with UpRev, and have changed the final drive gears which has resulted in some complaints from the ECU, and has disabled cruise control. The ecu error is saying it is receiving miss matched speed data from the transmission and the speed sensors on the wheels. Thats cool and makes perfect sense.

Where I am stuck is there are tables for the final drive ratio in the ECU that can be changed via UpRev. But I don't know how they work becasue the numbers in them make no sense. EG, the stock ratio is 3.13 to 1, and the table value is 0.393479. I looked at another example, 3.357 gears and a value of 0.352707 in the table.

There are also tables for wheel radius and tire effective radius, so I thought maybe the value was related to how far the wheel rolls per drive shaft revolution. But using the default values of these I get a value of 0.36575, not 0.393479. So I am unsure if that is how this is worked out.

Does anyone have any ideas or know how these tables work to give the ECU correct data on the final drive ratio?

I swapped the final drive on my 2009 370z (MT) to 4.36 and had no problems with the cruise control.

My 2 guesses would be:

1. You changed axle stubs when swapping the final drive and the new ones have a different number of teeth than the original ones. Some of the axle stubs for Z's and G's have 46 teeth and other have 48. If the shop that changed your final drive needed to fit a shorter axle stub to install an LSD, you may now have mismatched axle stubs or axles with a different number of teeth than original.

2. Your 370z is a sport model with SRM and has additional inputs from the transmission that my 370z Non-Sport did not have. Since the SRM adds additional sensors in the transmission, it is possible that the ECU performs additional verifications using those data points.

For #2, simply turning off SRM temporary and testing if the cruise control still generates a fault may be a easy test to perform.

The parameter in uprev probably have more to do with the expected pulse width generated by the axle stubs (the width of the teeth) or a ratio between the drive shaft rotations and axle stub teeth. In other words, even if kept the identical FD ratio, if the number of teeth on the axles are different, the ECU will see a giant difference in the wheel speed between the front and rear tires all of a sudden. It is understandable the would cause it to freak out a bit when asked to maintain a cruising speed of X when it "sees" the front wheels are going 120kph and the rear at 130kph.

Hi Paul, thanks for the reply. I also went to a 4.363 ratio, however this is an auto. how did you find the gears? I am actually heading to the track tomorrow to try it out properly for the first time :)

I think the cruise control is another problem and that it was just a coincidence that it happened as soon as I swapped the final drive so led me down the wrong track. The car has problems with the break assist system, the radar from that is used in cruise control, as it was turning on but not working for the first few days, and then wont turn on at all. Unfortunately it means the ECU is ignoring the cruise control button's and I can't swap maps on the fly. When logging in Uprev there are 3 speed columns that can be turned on, 2 of them report values very similar to each other, one is in steps of 2kph and one accurate, the 3rd reads 0, but it used to read a similar value, so I think thats the cruise control radar speed signal that gave me the error code now no longer working.

The stub axles came from an identical car (these are Nissan Fuga's known as Infiniti M45's in your part of the world) but I might take a look at the tooth count anyway just to be sure.

But I think I mostly solved the value of that table. Take 1 divided by the ratio of the top gear and divided by FD ratio, in my case 0.834, so 1 / 0.834 / 3.133 = 0.383 so almost the 0.393 of the ECU table. And similar results of a few other rom's I tried from UpRev rom page, none exact but all very close.