GPS speedometer ap accuracy

[Randalla]

I've been wondering for some time how accurate the speedometers are on the Z and the Roadster since I have no idea if I have the correct speedo driver in each transmission, and the tires/wheels are far from factory stock. Tonight Lisa went with me to record the numbers at 50, 60 and 70 mph (on the speedos). Both cars were within 1 mph at all those speeds, prompting the question of how accurate these aps are. The one on my phone is just called "Speedometer." The ap gets 4.5 stars.

[Turboman280]

All my vehicles read a bit high according to the roadside radar signs. The DigiHUD GPS phone app I use has similar readings, so that would seem to be confirmation to me.

I imagine the accuracy improves at steady speed, as GPS detection and calculation takes a wee bit of time to happen.

[Randalla]

Thanks for responding Carl. Honestly, I thought I may have been off by as much as 5mph before checking, so I'm actually pleased it's that close.

[Turboman280]

Thanks for responding Carl. Honestly, I thought I may have been off by as much as 5mph before checking, so I'm actually pleased it's that close.

I'll have to ride with you some time and we can compare apps.

[Jeff G 78]

My 280Z with the correct OD tires reads about 10% high compared to GPS, but interestingly, the odometer is very accurate. I checked both on a road trip several years back using a GPS and the mile marker signs on a toll road. After 100 miles, the odometer was within 0.1 miles total even though the speedometer read way high. I made a speed chart to keep in the car so I know how fast I'm going compared to the speedometer.

[Randalla]

Now that's a bit of a conundrum. Weird that the two would not sync. I'm sure there must be a logical explanation.

[Jeff G 78]

I am no speedometer expert, but I'm guessing it proves that the mechanical part of the system is working. The speedo gear in the trans, the cable, and the odometer are are in sync, but the speedo has no direct drive. The faster the cable spins, the higher the speedo reads and I believe this is done with a magnetic pickup. I'm not sure if they degrade over time, or if the Datsun speedos were just never very accurate even when new. I've never taken the time to disassemble one to see if there is anything that can wear or become dirty leading to inaccuracy. When I made the move, I don't even think I kept any of the half dozen sets of gauges I had.

[cgsheen]

I'm not sure if they degrade over time, or if the Datsun speedos were just never very accurate even when new.

Although they were kinda sealed up, at their age the seals aren't great and sometimes missing. They definitely crud-up (industry term) even if there aren't critters building nests/laying eggs inside. So they are both: never completely accurate (cuz era thing) AND will degrade over time.

They aren't that difficult to take apart and clean & refresh and if you repaint the inside with flat white paint, it's amazing how much brighter you can get your gauges looking even without LED "bulbs"...

[Turboman280]

A place like Dick's speedo tach should be able to calibrate.