Speedo issues

[rayaapp2]

So I have an RB25det under the hood of my 260Z and its actually capable of going fast. I have a corrected gear drive for my speedo that matches my rear gear set as well. Ive been driving down the freeway and noticing that I was driving rather fast to stay with the flow of traffic. Ive also noticed that the freeway spd sign(the radar based ones) have been saying Im going much slower than my speedo says. So to prevent any tickets I decided to test my setup. Initially the speedo was calibrated on a dyno(as at least verified up to 90mph) and it was good. 2 years later I am noticing changes. So I installed a garmin GPS unit to see how far off my speedo is now. Its off! at 35mp its off by 3-5mph. At 55pmh its off by 8-10mph. At 70mph its off by 15mph. At high speed 134mph it says 160mph.

 

I know these speedos arent really good for high speed already.

I believe its going out of calibration simply by taking up to high speed. It seems to be getting worse the faster I take the car. My plan is to eventually go electronic and use the skyline speedo in the 260z speedo housing.

 

Im not really trying to state I have an issue or a correction for that issue here. Im just stating what I have noticed. I think the faster I take the speedo the less calibrated it seems to be. Anyone else have this issue or is my speedo about to die?

 

I would think at normal street driving the speedo should stay close to the actual speed, but it just keep degrading.

Edited March 4, 2011 by rayaapp2

[ezzzzzzz]

Stock speedo? What transmission? Assuming it is a stock speedo and transmission, have you pulled the speedo drive gear to see if the plastic gears are still in good condition? Has the speedo cable been lubricated since it came off the assembly line?

[neotech84]

Not to sound harsh but you could try not taking it over normal speeds.

 

The choices above are a great place to start. Or you could try swapping it out for a known good one.

[rayaapp2]

Well normal speeds up to 70mph read 85-90mph.

It didnt before. And the car sees the track so speeds in excess do happen. Ive had the car up to 137.6mph in the straights with the GPS. Speedo is kicked over past the 160mph mark.

 

As for transmission its the stock rb25 trans. The gears and cable are 2 years old and were new. They were lubed up prior to assembly 2 years ago. 2 years old and ~9500 miles on them. Im sure they are still good. Im fairly sure the fault is the speedo and the magnetic coupler. I would also expect wear or a lack of lube to put drag on the components that would slow the whole thing down giving me a lower speedo reading not a faster one.

 

I have other speedos I could install, but at that point I think I would just switch over to the electic speedo and VSS.

 

What strikes me as odd is that it seems to be the act of taking it up to higher speeds that seems to be degrading the coupler. When we initially tested everything the speedo was accurate within 5mph at 90mph on the dyno. I was expecting it to be off at that speed though. These speedos just arent calibrated for those speeds. I didnt think holding it at speeds around that would make the situation worse though. So I was thinking this might be a sign that the speedo is giving up the ghost. It doenst jump around like Ive seen other Z speedos do so I didnt really notice the difference until traffic seemed to be driving 85-90mph in 70mph zones. 80mph is pretty typical fast lane driving around here, but not much above that.

 

To keep the speedo reading accurate I would be limited to 35mph zones or less so that doesnt make much sense.

Edited March 4, 2011 by rayaapp2

[BLOZ UP]

Sounds like your counter spring is worn. VSS > GPS > Eddy current speedos.

[Oddmanout84]

Not sure if you're willing to drop money on gauges, but the Speedhut speedometers (GPS and "traditional" VSS) are pretty nice. I have one that is/was hooked to a 240sx VSS in my transmission. All you have to do is calibrate it by driving a known distance of 2 miles, usually between mile markers on the highway. The internal workings of the speedometer will count the pulses and formulate the pulses per mile at the conclusion of the test. Then you're good to go!

[Newtz]

Have you changed the sizing on your wheels/tires? because i know on my lincoln navigator i have 23" rims and it sets it off by 10% Example- speedo reads 70 mph = actually 77 mph,50=55 and so forth

[beermanpete]

Speedo accuracy can be wrong due to changing drivetrain parts such as your rear tire size, differntial gear ratio, transmission, or the speedo itself could be bad. Here is a method to determine which it is:

 

1) Disconnect the speedo cable from the speedo and route the free end so you can see it while moving the car.

 

2) Make a pointer out of tape on the end of the cable to count the turns as you move the car.

 

3) Measure out and mark a distance of 44 feet on a flat and level area in a driveway or parking lot.

 

4) Roll the car 44 feet and count the turns of the speedo cable as the car move through this distance.

 

The cable should turn 10 times if the drive ratio is correct. This is becasue the speedometer needs to spin at 1200 RPM to indicate 60 MPH. This works out to 10 turns every 44 feet.

 

If the cable turns more than 10 turns the sppedo will read too fast. This is a percentage error, meaning if the cable turns 11 times the speedo will read 33 at 30, 55 at 50, 77 at 70 and so on. Less than 10 turns will have the opposite effect and result in a slow reading.

 

If the speedo reading is not in agreement with the cable speed error as described above your speedo needs to be serviced.

 

To correct the cable drive ratio you can get different speedo drive gears from the dealer This may require removing the transmisson for installation as one gear goes on the output shaft (inside the trans). Another method is to buy a little gearbox devices to install in-line with the speedo cable to correct the drive ratio. Meter companies such as Stewart-Warner sell these. You tell them the number of turns your cable turns in 44 feet and they set the internal gearing for the necessary correction factor.