I just got this Xterra (2000, 2WD, V6, auto Trans) and I was wondering what RPM I should be turning @ 55 mph. Right now it's turning 2500 to 2700 RPM @ 55. HELP?

Thanks


Aif

That's what you should be turning at 65-70 MPH at a lazy cruise in 4th (overdrive on). At 55 MPH the only time you should be seeing those RPMs is under load, climbing a hill, or otherwise asking for more than 1/4 throttle, in which case it's running with the torque converter unlocked, or you have the overdrive turned off and effectively locking your tranny in third gear.

Brent

Manual or auto? That sounds about the same as my manual. At 65mph I'm at 3000 RPMS in 5th gear.

We're going to have to do some math. Try and stay with me a while...

The engine speed is calculated by the following formula:

RPM = (Vehicle Speed) /60 x (tire revs per mile) x (differential ratio) x (transmission ratio)

I'm assuming that you're in 4th and the torque converter is in "lock-up". The "divide by 60" part is to convert miles per hour into "miles per minute".

I'll guess at some numbers as I don't have all the data:

- I assume you have 265/70R16 tires: these are about 30.7" in diameter and therefore will turn at 660 revs per mile
- Also I assume you have a 4.900 differential (look on your driver's door frame sticker under "AXLE": HG46 is 4.636, and HG49 is 4.900)

Therefore your actual engine speed at 55mph in 4th gear lock-up will be:

55 / 60 x 660 x 4.900 x 0.694 = 2057RPM

However, I would caution you about trying to read any similarity between indicated and actual road speed, or the indicated and actual engine RPM. From my investigations, (and also by comparing my 5-speed XE S/C with my friend's 3-pak XE, and "test driving" some of the local dealer's vehicles), it appears that Nissan has chosen one setpoint for the tachometer and speedometer calibrations, such that they read "close enough" regardless of the tire size or differential ratio fitted to your particular vehicle. There is "only" a 5.5% difference in the 4.636 and 4.900 diff ratios, and much less than that in tire diameters (they're all around 30.5", regardless of rim diameter, due to the selection of the tire profile). This way Nissan only need fit one calibration of tachometer and one speedometer encoder drive across the entire V6 range, and still be within the manufacturers' allowable limits ...

I originally started looking for answers when I discovered that my tach and speedometer with a 4.636 diff read identically the same as my friend's 3-pak XE with a 4.900.

Normally one would never notice a "spread" of 3.5mph at 65mph, or that the tach was reading 3000RPM instead of 2850RPM. Maybe I'm just a little too "anal" about this sort of thing...

Regards,
Richard.

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).

To even factor the vehicle speed, tire diameter, or axle ratio is overkill. The only thing that matters is the transmission gear ratio and engine RPM when it comes to what speed is indicated. 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.

Brent

Thanks for all the info. I'll try a trans. service first, then see how it does. This is a used vehicle and may not have had the trans. serviced when it should have. Wish me luck on this one.


Aif

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.

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.

Actually, I mis-typed. The 4.9 is new for 2003 as far as I knew. If it was available in 2002 models, that's news to me. Where do you get your info? I'm impressed. If the 2002 XE V6 w/ LSD had 4.9 gears, I'm in luck if the one I'm looking to help part out is so equipped!

Brent

From what I've seen, I'd guess it's been around since at least March 2002. The XE-V6 I rented in Phoenix in late May this year, and subsequently drove 1938 miles in 9 days throughout Arizona, Nevada and Death Valley, was an automatic with 4.900's (build date 03/02).

The girlfriend's 5-speed XE-3pak (build date 05/02), which she bought a few weeks later, also has an HG49.

In fact, at that time, all the XE-V6 3-paks we saw on the dealer's lot, (there were about eighteen of them, but only one 5-speed, which she bought) were HG49s. They all had build dates between 03/02 and 06/02.

Here's a thought - why not start a survey amongst 2002 V6 3-pak non-S/C owners? They could give the build date and axle type and we could then see when the switch to the 4.900 exactly occurred...

Regards,
Richard.

Well this thread just answered my thoughts about my RPM's, because I thought they were high for the speed.

My Axle has HG49 and was made 01/02 and it's an SE with auto.

Hmm, so at least for most of the 2002 MY, the XE and SE trim had 4.9 gears with the LSD...

Yeah, let's do the poll thing...

Brent

I'll bet they changed in the 02 at the mid year change, that is when the axle vents changed too.