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It is normal for the speedometer to read fast. The specified tolerance is +5% -10%. So at actual 60 it can read 57 to 66 and still be acceptable. If you'd like better precision, calibrate the speedometer. Or if your tires or gears are non-stock size, read on about changing the speedometer readout.
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Overview
Check the odometer first. The gears are to make the odometer correct, than if needed the speedometer can be adjusted.
There are basic reasons for speedometers reading fast or slow:
- Factory speedometer tolerance allows it to be off slightly
- Different size tires than original, changed diff ratio, or transmission has been changed
For the latter, there is a mechanical solution of changing to an appropriate speedo-drive gear at the back of the transmission.
For the former, where you have the "right" size tires, final gear ratio and stock transmission -- the speedometer itself may need to be calibrated.
Factory speedometer tolerance is usually 5% fast to 10% slow (this is in general, I don't know what Nissan's specs are). 1200 Top Speed discusses this. One magazine test noted the 1200 speedometer in the test car was 7% slow. My 1200s have been between 7.5% - 10% slow (with stock running gear include tire size). Also see speedo off
Then again all MPH speedometers may be off from the factory. The outer markings for the kph ranges, for example 100kh (62mph) starts at about 64mph
Cable Speed Correction
In cases where the Odometer is reading incorrectly, changing the gears is the solution.
NOTE: when the odometer is correct, but the km/h (mph) is off, the speedometer mechanism itself must be corrected. Do not do this by changing the gears
The traditional solution is to change the pinion gear using Datsun factory parts. (about $15)
Another solution is a speedometer ratio adapterᴳ (about $140).
Clark Brothers speedometer adaptersᴳ
Correction Adapter — read about it at stevemckelvie's blog
Odometer Calibration
Before messing with the mechanism, make sure the odometer (distance counter) is correct. If it is not, different transmission-to-speedometer cable pinion gears are called for.
Get on the highway and drive. As you pass the mile markers note the indicated milage. A 10 mile drive is good for accuracy. If the odometer reads 10.0 miles it is perfect. But if it read 11.0 mile it is running 10% off.
- Change the pinion gear
- Or, change tire size. For example if you have 13" tyres, choose a low profile so the diameter is close to the stock 22.7"
- Or, use a #speedometer ratio adapter
Once you get the odometer reading correctly, then you can to confirm the speed needle readings.
Mechanism Calibration
Where can you go to get a speedo calibrated? Can it be done at home? Assuming your odometer reading is now verified correct, proceed to calibrating the needle reading.
You can:
- Change the pointer position. This is good if the speedometer is off by say 2mph at all speeds.
- or
- Change the magnetic tension. This will adjust the scale across the board. Be careful as it is easy to ruin it
For ideas, see:
Bench Test
You can turn the speedometer cable with an electric drill. As long as you have a way to adjust it to run at a steady speed, you can test it versus the odometer.
It says "60 MPH = 1024 RPM" on the bottom edge.
Parts needed:
- reversible electric drill to drive the speedo cable
- 500W dimmer switch to vary constant drill speed
- 60W light bulb (dimmer requires this)
- Extension box
- Outlet plug
- Electric wiring
NOTE: Danger. Use caution and properly grounded and boxed connections. Do not attempt at home. Use professional equipment.
A typical reversible electric drill may run at 2500 RPM. The speedometer is 50mph=1024 rpm, so turn the Dimmer down about 1/3 from full to start.
Pull trigger fully to run drill at constant speed. Then vary the dimmer switch to show 60mph.
With a stop watch, time exactly 60 seconds. If the odometer moves exactly 1.0 miles it is good. If you are not sure, run for three minutes. If it shows 3.0 miles it is perfect. Otherwise record the reading.
Repeat for 30 mph and 90 mph.
Calculate the MPH difference at each speed.
Road Test
- Compare against roadside radar box
- Timing test. Go out on highway and drive at steady mph while you time the mileposts.
- 10 miles 60 mph should take exactly 10 minutes
- 5 miles 30 mph should take exactly 10 minutes
Compare both a high and low speed.
Adjustment
- If all reading are off by a fixed MPH, carefully move the needle to correct it.
- If the readings get worse the faster it goes, the mechanism needs calibration.
- NOTE: haven't tried this yet. Don't know how well it works
- Adjust pointer so 30 mph reading is correct
- If 90 mph reading is now incorrect, adjust cup drive
Changing Scale
You can easily change the scale to make it read correctly, or to make a 180 kph scale. The downside is that the odometer (distance counter) will be inaccurate.
- Scale Adjusted for low gearing, looks good printed on photo paper and bolted to speedometer.
- Use a light, removable contact cement. Otherwise if you just tack the edges or a places, the photo will eventually buckle and warp and interfere with the needle movement.
- I modified mine to change the scale since I didn't want to get under the car and change the plastic speedo gears.