510 Speedometer fix for Ka24de & Sr20det swaps

[DanielC]

At freeway speeds, a GPS speedo does a reasonable job.  As slower speeds, not so much.  The "consumer error" is a constant distance, and at slower speeds, the error is a greater percentage of the actual distance over time, and thus, the speed.   I would suggest you are more likely to get a speeding ticket for 3 MPH over in a 20 MPH school zone, than for 3 MPH above posted freeway speeds.

 

GPS speedometers use two different methods to calculate speed.  One method is a point-fix method.  The GPS locates itself, and a little while later locates itself again.  It knows how long it took from the first point, to the second point, and how far apart the points are, in a straight line.  Then it uses the distance, and time to calculate the speed.  The speed it WAS going the last time it got a fix on a point.

 

The other method GPS uses to calculate speed is Doppler shift of the RF signal it is receiving.  But this requires a satellite directly in front or behind you, low on the horizon.  The satellites directly overhead, or off to the right, or left, you have no relative movement to or away from them, to create a Doppler shift.   But a satellite low on the horizon has a weak signal, because of the additional distance of atmosphere the RF signal has to travel through.

 

 

However, you can use a GPS to check calibration on a manual speedometer.   If you are travelling in a straight line, at a constant RPM, and with a constant load, you can assume you are going a constant speed, especially with a manual transmission.

 

But before you correct the speed, you must check the odometer first.  The odometer is a solid geared connection in our older Dtasuns.  once the odometer is corrected, then you can go to a speedometer shop, and get the spring tension on the speedometer needle adjusted to correct speed reading errors.

[DISLEXICDIME]

I wonder if they still sell the boxes that mount in between the cable and the transmission that you could use to adjust your speedometer

[datzenmike]

Possibly. Keith Law on The Realm has one but it would have been bought a long time ago and wasn't cheap then. It has swappable brass gears sets inside. He's running a 4.625 rear differential and the stock 510 is 3.90? This is an 18.5% difference.*

 

The factory plastic  pinions are about $20. They range from 16 to 22 teeth (roughly) and each tooth represents 5-6% correction (roughly). This amounts to 2.5 to 3 MPH at 50 MPH. (I save every pinion I come across)

 

If your speedometer is reading fast then a pinion with more teeth is needed to slow the speedometer cable down.

 

If your speedometer is reading slow then a pinion with less teeth is needed to speed the speedometer cable up.

 

* The 510 uses a 17 tooth pinion I believe so a 20 tooth is a 17.6% decrease in speed and is as close as possible. One % is .5 MPH at 50 MPH.

[q-tip]

I have the red gear in my 510 with a 3.90 rear end and it is pretty damn close. I also have the speedo pinion assembly from an early 90's r32 it I doubt it would be different.

[LS_Wagon]

Thanks for the info. Just ordered the cable and gear assembly.