Speedometer and/or Odometer 'Accuracy'..

[LRNAD90]

(Okay, inaccuracy would probably be more accurate, no pun intended)

So 1995 NAS D90.
Cable driven speedo.
Original Tire Size 265/75-16, current tire size 285/75-16.

Speedometer reads about 5 mph high, when compared to GPS speed reported by WAZE app on a phone..

What is odd to me, is that it is consistently 5 mph high:

Indicated 30 mph, 25 mph on gps..
Indicated 40 mph, 35 mph on gps..
Indicated 50 mph, 45 mph on gps..
Indicated 60 mph, 55 mph on gps..
Indicated 70 mph, 65 mph on gps..
Indicated 75 mph, 70 mph on gps..

I would think its 'inaccuracy' would be a percentage, so the actual difference would vary with speed. I would also think that rolling on larger tires would cause the speedometer to read lower than it did on stock tires, so it read like 10 mph fast on stock tires?

What am I missing?

This also leads me to the question of mileage/odometer accuracy. If it reads faster than actual speed, then it is racking up more miles that actually travels as well, right? But if the error is not a percentage of speed/distance, how would I 'estimate' how many more miles are reported by the odometer than it has actually traveled?

 

[Mirrajumper]

With 255/85/16 my row Tdi gauge is 5 mph off also, but the reverse of yours. My gauge 70 is gps 75. It was accurate with 7.5x16 size

 

[javelinadave]

Sounds like you needy to change the gearing to your speedo.

How to replace the speedo drive gear on a Land Rover - Land Rover Blog

I have been without a speedo for sometime now, and been relying on the GPS to tell me what speed I was travelling. So, unable to put up with it any longer, I got a new speedo cable and spent 30mins replacing it last week. When it didn’t fix the problem (speedo was still not …

www.land-rover-blog.co.uk

 

[Mirrajumper]

Any idea what colors correlate to what tire size? For the green and red?

 

[javelinadave]

First step would be to see what color gear you have now. I believe more teeth will slow down your speedo (registering to fast now) and less teeth will do the opposite. It's been a while since I worked this issue.

 

[pmatusov]

This also leads me to the question of mileage/odometer accuracy. If it reads faster than actual speed, then it is racking up more miles that actually travels as well, right?

This is a loaded question.
First, the speedo may read high simply because the clockspring behind the needle is tired. In my 95 Classic with 387kmi, the speedometer starts reading high after about 65-70 mph (and the needle can fall down onto the top peg). If that's the case, it has no bearing to the odometer (as it is in my case - the odometer racks up mileage about 5% slower than actual, due to 7.50R16 vs 235/70s).

The needle may also have shifted on its axis. It would mean that at zero it should read 5 mph, but the low-end of Land Rover speedos doesn't have a stellar precision.
Speedometer gears may also be part of this, but judging by what's been posted earlier, there isn't one lower than 20 teeth for taller tires.

 

[LRNAD90]

With 255/85/16 my row Tdi gauge is 5 mph off also, but the reverse of yours. My gauge 70 is gps 75. It was accurate with 7.5x16 size

That's weird, larger tires should make it read slower than smaller tires

First step would be to see what color gear you have now. I believe more teeth will slow down your speedo (registering to fast now) and less teeth will do the opposite. It's been a while since I worked this issue.

Based on the PNs listed above, and the associated descriptions, it appears larger tires require a gear with fewer teeth. That is how I interptret it anyway.

Pretty sure the NAS Defenders came with the largest tires fitted to production Defenders, so always assumed there was not a different option available for larger tires. But the speedo read fast when stock, so I figured increased tire size was only likely to bring it closer to true speed, not further away. And it likely has, though I don’t recall if it was as much as 10 mph fast when stock, as that was 27 years ago..

This is a loaded question.
First, the speedo may read high simply because the clockspring behind the needle is tired. In my 95 Classic with 387kmi, the speedometer starts reading high after about 65-70 mph (and the needle can fall down onto the top peg). If that's the case, it has no bearing to the odometer (as it is in my case - the odometer racks up mileage about 5% slower than actual, due to 7.50R16 vs 235/70s).

The needle may also have shifted on its axis. It would mean that at zero it should read 5 mph, but the low-end of Land Rover speedos doesn't have a stellar precision.
Speedometer gears may also be part of this, but judging by what's been posted earlier, there isn't one lower than 20 teeth for taller tires.

I don’t think it is an issue with the spring, as the needle doesn’t really seem to mimic what you describe, and it only has (about) 50K miles on it. Shifted on its axis, maybe, that would explain why it is off by 5 mph no matter what the speed, instead of being off by a percentage, but not really sure how that would have happened..

Agree, don't think there is a gear option available from LR that would make it any better. Guess I just need to go to 35's.. (but then I'll need to re-gear and then it was all for nothing, so) LOL

 

[Dipodomys]

You might consider upgrading to the VDO speedo kit offered by Rovers North. It eliminates all of these mysteries and also eliminates the breakage-prone and bouncy speedo cable. Recalibrating the unit after tire or gear changes is a breeze. I love mine, and the speedo is dead accurate.

I also included their three-gauge kit that changes the fuel, temp, and voltage gauges. An excellent kit that includes everything you need. You'll now have a temp gauge that tells you in degrees the temperature of your coolant (rather than the nebulous factory gauge) and a fuel gauge that can be calibrated to be dead accurate for your specific tank. The factory gauges are pretty lame, and this is really a nice upgrade.

The whole setup is a little pricey, but I am really pleased with mine.

https://www.roversnorth.com/parts/rndskit2_digital_speedometer_kit_for_defender&type=4

 

[Z.G]

You might consider upgrading to the VDO speedo kit offered by Rovers North. It eliminates all of these mysteries and also eliminates the breakage-prone and bouncy speedo cable. Recalibrating the unit after tire or gear changes is a breeze. I love mine, and the speedo is dead accurate.

I also included their three-gauge kit that changes the fuel, temp, and voltage gauges. An excellent kit that includes everything you need. You'll now have a temp gauge that tells you in degrees the temperature of your coolant (rather than the nebulous factory gauge) and a fuel gauge that can be calibrated to be dead accurate for your specific tank. The factory gauges are pretty lame, and this is really a nice upgrade.

The whole setup is a little pricey, but I am really pleased with mine.

https://www.roversnorth.com/parts/rndskit2_digital_speedometer_kit_for_defender&type=4

Normally I push them, but we haven't quite nailed it down yet for ECU vehicles. Still working on it

 

[DaveG]

VDO sells a GPS receiver that is the senser to the speedometer eliminating issues when changing tire sizes. Mine is on order but hasn't arrived yet, so I can't offer any first hand experience.