240Z Speedometer not Accurate.

[Rmr32990]

When driving the speed shown on the speedometer is very different then actual road speed. When i go about 40 it says 60. when im going 70 it says about 100 or 105. I,ve checked the cable and it doesn't seem to be damaged. I checked the gears in the speedometer and the are slightly damaged, but i don't know if that would cause it to read high. Can anyone help?

[Zs-ondabrain]

The needle could have slipped. Is it a stock (unmollested) speedometer? I ask because I have white face gauges in my 70' and the speedo was off by 20 MPH after I did the face swap.

This is where you need a friend to follow. Have your friend do EXACTLY 30 MPH. when he is doing that 30 MPH, have him put his hand out the window to let you know he's doing 30. Look at your guage and mark the speed your needle is at. roughly find out exactly how much it's off. now go home.

If you can take out the gauge (2 wingnuts, the speedo cable, and the 4 bulbs) remove the 4 side screws, seperate the black housing from the metal gauge assembly.

Use a flat head screw driver to push down on the small wire that stops the needle at 10 or 20 MPH, so that the needle can move past it's stopper wire.

Note where the needle rests on it's own. Pull the needle straight up to remove it. Then put it back on about 20 MPH counter-clockwise.

When I did mine, I had to put it back on so it would come to a rest half-way between the MPH font and the 160 font.

When it's push on and resting where you think it should be, push the small wire back down again so you can slip the needle past it. let up on the wire and pull up on it a little so it's back where it should be. The speedo needle should now rest on the 10 or 20 MPH mark like usual.

get back in the car, screw on the speedo cable without actually installing the gauge or reassembling the whole gauge. Take it for a test drive with the gauge sitting loose in the speedo hole. Have your friend do the 30 MPH again and see if you got it or if you need one final adjustment.

The answer may be a whole lot easier than this but this is what I had to do to fix my own mistake. But like I said, the needle may have slipped and needs to be put back on correctly, by you.

Good luck.

Dave.

By the way, what year Z is it. or is it even a Z?

[WarrenZ]

Hi

You don't say if has always been like this or only just started doing it.

Has the gearbox been changed ? Perhaps it has the wrong speedo drive in it.

Warren

[Zs-ondabrain]

Hi

You don't say if has always been like this or only just started doing it.

Has the gearbox been changed ? Perhaps it has the wrong speedo drive in it.

Warren

I thought about that too but 20 to 30 MPH off, not usually a condition on gearing or swaps. 5 to 10 MPH maybe.

And I just reread the title "240Z speedo" Duh, sorry.

anyways, this is what I meant about the resting needle placement.

[DougN]

You can also use a GPS instead of a friend if you have access to one :classic:

[Rmr32990]

I'll try reseting the needle. and let you know if it works. The car is a '72 240Z. Thanks

-Ryan

[Rmr32990]

I tried reseting the needle like you said, but my gauge starts at 0 not at 20. The car has the original gearbox. Also the reading is not off by a constant number. The faster the car goes the more inaccurate the speedometer becomes.

thanks

-Ryan

[MikeW]

I tried reseting the needle like you said, but my gauge starts at 0 not at 20. The car has the original gearbox. Also the reading is not off by a constant number. The faster the car goes the more inaccurate the speedometer becomes.

Does it have the original differential? Changing that without changing the speedometer gearing would cause the symptoms you describe. Running really small tires would also cause this problem.

[Rmr32990]

Does it have the original differential? Changing that without changing the speedometer gearing would cause the symptoms you describe. Running really small tires would also cause this problem.

It has the original differential and i'm fairly sure i'm running regular tires on it, they're 195/70R14.

[FairladyZS30]

haha i have this same problme except mine is off my 10 but i have my gps to help me

[JimmyZ]

This is clipped from an earlier post. I found it handy when doing tranny swaps. Go to this page about midway down and check out the colored gears for the speedo. (Tranny gear)

http://www.geocities.com/zgarage2001/z.html

(mid way down the page "Performance tips" link)

Sounds like you should start with trying a different speedo gear. The gear that drives the plastic gear remains the same. All you have to do is determine how many teeth you need on the plastic gear. There are several threads touching this topic. Hope this is what it is for ya.

Jim

[Rmr32990]

This is clipped from an earlier post. I found it handy when doing tranny swaps. Go to this page about midway down and check out the colored gears for the speedo. (Tranny gear)

http://www.geocities.com/zgarage2001/z.html

(mid way down the page "Performance tips" link)

Sounds like you should start with trying a different speedo gear. The gear that drives the plastic gear remains the same. All you have to do is determine how many teeth you need on the plastic gear. There are several threads touching this topic. Hope this is what it is for ya.

Jim

thanks, but the differential is the original, so not sure why anything would have changed with the gears.