Originally posted by OffroadX:
The 4.9:1 ratio is new for the 2002 non-SC SE models only, the 2000 V6 either has 4.636 (most common) or 4.363 (on the base XE V6).
Just as a slight correction to this statement, the 4.900 is not actually "new for the SE models only", but debuted as the "optional limited slip differential" fitted to 2002 naturally aspirated XE-V6 as part of the Sport Package, along with the change to 16" rims. It was, however, as you state, also the "standard" diff fitted to 2002 SE with 16" rims. If one opted for the "LSD upgrade" on an SE, the vehicle was then fitted with 17" rims and 4.636 final drives. However, the diameter of the 17" tires was still around 30.5", due to their lower profile.
They should be consistent from vehicle to vehicle no matter what. If it says you're doing 65 MPH in 4th with the torque converter locked up, the tach should read just under 2500 RPM, period. Variances in tires and axle ratios will throw off the speed indicated from actual, but the RPM should read the same at the same indicated speed.
Agreed, if one assumes that the speedometer drive ratio hasn't been changed to reflect the higher actual speed. (Take a ludicrous case where we fit a 2.45 rear end - half of 4.900, but don't change the speedo drive to reflect this. We'd be barrelling along at an actual 130mph with the speedometer showing 65mph and the tach reading 2431rpm, in the case of an automatic, or 2842rpm in the case of a 5-speed. If we change the speedometer gears to twice their numerical ratio, the speedometer would now read the "correct" speed of 130mph, while the tach still reads the actual engine speed.)
This variation between indicated and actual was the point of my closing remarks in my previous post when I said "I would caution you about trying to read any similarity between indicated and actual road speed...".
Usually, when manufacturers change the rear end ratio for various models in a production run, the speedometer drive gear pair (both the helical one on the output shaft and the pinion gear on the speedometer drive - be it a cable or encoder) are selected to give a more realistic readout of actual vehicle speed. Usually this is based on the actual speed with new OEM tires, so as the tires wear, the actual speed drops a little from the indicated. In most cases, with electronic speedometers, this gearing is selected to give 8000 output pulses per mile, since this very closely matches 5000 pulses per kilometer, making a change in units simply a change in the divider value.
My comment was that, in Nissan's case, they don't appear to have gone as far as to change the speedometer drive gears. Otherwise the speedometer and tach wouldn't read the same relative values in vehicles with similar transmissions, but different rear-end ratios.
So every "similarly transmission"-ed V6 out there seems to be doing the same
relative engine speed at any
indicated vehicle speed, regardless of tire size or axle ratio.
Finally, for those looking for a set of 4.900 axles (with an LSD rear) out there, just look for any 2002 non-S/C XE-V6 with a 3-pak - it'll have them...
Regards,
Richard.