Speedometer problem, help needed

[Milenko2121]

I'm having huge difficulties with my speedometer.

 

I'm running the factory 195/70/14 in the rear, and 175/70/14 in the front at ~30 PSI.

 

When I purchased the car it had a white speedo cog with 19 teeth in the transmission so I assumed it had a 3.9 differential.

GPS was the same 100% of the time, and it said my speed matched what it should be for a 3.9 differential.

 

So obviously, I believed I had a 3.9 differential.

 

Well, I converted to turbo so I wanted longer gears, so I bought a 3.545 differential, and popped off the back to be sure it had 39:11 stamped on it, and the ring gear had 39 teeth, and the pinion gear had 11 teeth.

 

I also ordered a black 17t cog from courtesy nissan to make sure I got the factory part.

 

I dropped my differential, and popped off the back out of curiosity, and it had 39:11BB stamped on the ring gear. This had me confused because that says it's 3.545 already. I counted the teeth on both gears, and yup 39:11. I already had a 3.545 differential.

 

The new one was in better condition so I swapped it in anyway just to be on the safe side.

 

Well, GPS says my speedometer is wrong. GPS says my speedometer is reading too fast. 

Example,

at 2500 RPM my speedometer says i'm going about 60 which a 3.545 should be saying with a 0.864 OD.

and at ~3100 RPM my speedo says i'm going 77 which is correct...

 

but the GPS says i'm going 55 at 2500rpm which is correct for a 3.9 differential with the same OD.

and at ~3100 RPM it says i'm going 70 which is correct...

 

Can someone please explain what could be going on here? I am highly confused. I've swapped two transmission into my car(for other reasons, not for speedo being wrong) with the same outcomes in speed readings. 

 

Before I put the 3.545 speedo cog in, the white speedo cog was giving me the correct speeds that match my GPS.

 

I'd really like to know what exactly is going on at this point.

 

Thanks,

Edited June 17, 2013 by Milenko2121

[zohanisback]

There is a reason no one else has answered this... It's kind of a tough one.

 

First off, I read with horror when you said you had 39:11 and bought a 3.54 diff...    But it's an easy mistake if you forget diffs are all about ratios.

 

To answer your question, there are only a few things that could cause the problem:

 

The cog in transmission, the diff itself, or the speedometer gauge. I absolutely have NO idea why you had good readings before, but my bet is that if you change that white cog out for a 3.54 on, you should be pretty golden.  It's an inexpensive test, anyway. Also remember that over time, older cars read faster than what they are actually going.  That's my experience with nearly every older car I've driven.

[Milenko2121]

Yeah, this is very tough.

 

I've tested my GPS in a 2004 Acura TL and other cars and it was 100% accurate to their gauge.

 

I tested the 3.545 black cog and it read too fast, did not match GPS. I also had a friend go 60 and when I went "60" he was going faster than me.

 

The 3.9 white cog matches GPS 99% and kept up exactly with my same friend.

 

And even in a transmission calculator, it matches up to what it should be for a 3.9 differential.

 

 

I'm going to just swap it out for the new differential I bought once I figure out how to fix the backlash. It's too loose, and it clicks when I turn it once.

 

I'm just pissed off because I was excited to have a 3.545 finally so that first gear was actually useful with my turbo.

 

 

I also just put a new transmission in yesterday because mine grenaded on me going 70. That was fun.

Same results with the 3.9 speedo. So this is 3 transmission saying the same thing so far.

Edited June 25, 2013 by Milenko2121

[zohanisback]

Keep in mind that the transmissions actually don't mean anything though, because the cog is paired with the diff only. Any transmission can be used with any diff and its corresponding cog and it should read accurately. Thus, I have only a few things I can come up with:

 

1. Your speedo reads slow like all old cars. how far off was it at 60mph? My datsun read 6mph slow at 60mph. So I would routinely drive 73 to keep up with traffic going 65. With old speedos, the faster you go, the wider the variance. So that guy that says he did 130 in his old Chevy nova ... probably needs a reality check.

 

2.Tire size is going to change the reading a LOT! You can even out the old speedo by raising tire size a bit. Will let you take control of that turbo more, too! 

 

3. Finally, don't rely on the color of the cog. Count the teeth number to be sure. The fact that a black cog on a 3.54 had good speedo readings seems to me that maybe it took advantage of the older speedo to even things out. Not sure if that logistics of the gearing works (I'm too tired to figure it out).

 

Here are the teeth number, courtesy of one of my favorite websites: http://datsunzgarage.com/engine/

 

YELLOW is the 16 tooth for the 3.36
BLACK is the 17 tooth for the 3.54
BLUE is the 18 tooth for the 3.70
WHITE is the 19 tooth for the 3.90
RED is the 20 tooth for the 4.11
PURPLE is the 21 tooth for the 4.38

[Milenko2121]

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Edited March 21, 2014 by Milenko2121