How Fast Am I Going?

[sawdust2005]

I'm having toruble with my speedometer. I always thought that the speedometer was reading fast. After installing a GPS in my 70 Z I found out that my speedometer is way off. When the speedometer is reading 59, I am actually going 50.

The tires are the correct size according to the data in the car, 175SR14's.

Are there different rear gears or whole rears that can be used? The rear may have been changed by a previous owner, but I'm not sure.

What can I do?

[Arne]

Before you change anything, check the accuracy of the odometer also. If the odometer error is similar to the speedo error, then re-gearing is a good answer. But if the odometer error is different (or more likely, fairly accurate), then the problem is weak a spring in the speedo itself.

[five&dime]

Does your car have the original 4 speed or a 5 speed? There are many different speedo gears available for Datsuns. They go in the transmission. There are also a number of different rear ends that people use in the Z.

You need to figure out what you have before trying to determine which speedo gear is correct. At least that is the case for the 510's.

[sawdust2005]

The trans is the original 4 speed. I'll check the odomer this weekend.

[Bruce Palmer]

Arne,

Is that problem you mention with the head, a common one? I don't think I've ever heard of a problem with the head altering a read out. Mine have either worked or were solidily on the fritz.

[Arne]

I've seen it on other cars, haven't owned enough old Datsuns to know if these are prone to it as well. But in prior research I gathered that it was a common trouble possibility for any mechanically driven speedo.

[Zs-ondabrain]

You can also get up to 60MPH on the freeway, use a clock (with seconds hand) or stopwatch and do this......

Be at exactly 60 (by your speedo)

when you pass a Mile marker on the freeway, start the clock/stopwatch.

It should take exactly 60 seconds to do 1 mile @ 60 PMH

If it takes you longer to do the 1 mile at 60, your speedo is slow.

If you do the mile faster than 60 seconds (ei: 55 seconds?) your speedo is too fast.

(If it takes 65 seconds to do the mile, your car is actually doing 55 MPH)

Follow Arne's instructions but on the side, you can run her down the freeway and check this way as well.

Another way is.... If you have an emissions station somewhere close, yo can ask them to put it on the roller and check their readout versus your speedo. (I've done it this way, when they are not full of customers or bored)

Edited May 21, 2009 by Zs-ondabrain

[Walter Moore]

Arne,

Is that problem you mention with the head, a common one? I don't think I've ever heard of a problem with the head altering a read out. Mine have either worked or were solidily on the fritz.

The speedometer in my Z came right in (compared to my stop watch and mile markers) once I got the odometer dialed in. But I had an older Impala years ago where the odometer was flat perfect but the speedometer was something like 20% LOW.

Try to explain that to a police officer... luckily the only time I had to, the officer was younger than the car!

(My speedometer actually read 60 and his radar gun read 72!!! Talk about being shocked!) He asked if the car had the correct tire size and I told him the truth, no one had made G78/15 tires in over 20 years.

[JonnyRock]

I recently found out that my tach reads about 300rpm high. Checked it with two different trouble tachs and same result. It was a constant value too, 300 across the whole range.

[Carl Beck]

I'm having toruble with my speedometer. I always thought that the speedometer was reading fast. After installing a GPS in my 70 Z I found out that my speedometer is way off. When the speedometer is reading 59, I am actually going 50.

The tires are the correct size according to the data in the car, 175SR14's.

Are there different rear gears or whole rears that can be used? The rear may have been changed by a previous owner, but I'm not sure.

What can I do?

If you find that your odo matches your speedo - - ie that your speedo is OK.

If the speedo is reading 59 at an actual 50... then you need an 15% decrease in the speed at which your speedo cable is turning - to make the speedo. indicate your true speed.

59 mph x 0.15 = 8.85 mph

59 mph - 8.85 mph = 50.15 mph

The stock rear end for a 4 spd. 240-Z is 3.364:1 It calls for a speedo gear of 16 teeth (yellow)

So IF THAT IS WHAT YOU HAVE - you would need a speedo gear with 15% more teeth.

16 teeth x 1.15 = 18.4 teeth - - So you need an 18 tooth speedo gear to correct the error.

A 18 tooth speedo gear happens to be the speedo gear called for use with a 3.7:1 ratio rear-end.

On The Other Hand:

If your speedo was reading 60 mph when you were doing an actual 50... then you need a 17% decrease in the speed at which your speedo cable is turning - to make the speedo. indicate your true speed.

60 mph x 0.17 = 10.2 mph

60 mph - 10.2 = 49.8 mph

16 teeth x 1.17 = 18.72 teeth - So you need a 19 tooth speedo gear to correct the error.

A 19 tooth speedo gear happens to be the speedo gear called for use with a 3.9:1 ratio rear-end.

3.364 rear gear calls for a speedo gear of 16 teeth (yellow)

3.7 rear gear calls for a speedo gear with 17 teeth (Blue)

3.900 rear gear calls for a speedo gear of 19 teeth (white)

Take a friend to watch the GPS and do the recording - so your eyes can stay on the road.

Run the car at 60 mph and 70 mph according to your GPS - then record what you speedo is indicating. That way you can more accurately dial in the % correction you really need. That is to say that difference between the 16 tooth, the 17 tooth gear and 18 tooth gear will be large enough to overcome rounding errors... and relative positions of the speedo indicator needle etc.

Then simply get the correct speedo drive gear and put it in the transmission. The bonas is you will have a good idea of what rear gear you have as well.

NOTE - pull the present speedo gear out and count the teeth FIRST....Make sure it is a 16 tooth gear - - You may simply have the wrong gear in there to begin with... so you may need to apply different correction factors...

FWIW,

Carl B.

[Jeff G 78]

Like Arne said, check the odo first. I checked mine with a GPS too and mine is way off just like every Z on the road. They ALWAYS read high. Use this tool to calculate your speed. It agrees perfectly with GPS on my Z.

http://www.geocities.com/z_design_studio/

[five&dime]

Good write-up Carl. I even plan on getting the speedo gear that will throw my speedo way off so that if I do get a ticket, I can throw that in and get the calibration done which may get me dismissed in court