P38A Speedo inaccuracy - P38 V8 4.0 auto

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kermit_rr said:
275/45R20 is an odd size but not massively different diameter vs standard.are you sure your GPS is accurate and working properly? not affected by the heated screen etc
you could try it the old school way and follow someone doing a predetermined speed on a particular stretch of road
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When we picked it up I thought it seemed very sluggish but my wife confirmed she was doing roughly the GPS speed in the other car. And both have the heated screens, so I'd hope it would affect the the same. Main part of the route where we were checking speeds is tree free dual carriageway, so should be reasonably interference free.
 
Scotiawhiskers said:
And we're still interested.

Yes, previous twit put 20" Stormer copies on there, with (IIRC) 275/45R20s on them. I measured and they come out at the right rolling radius so shouldn't affect more than +/- 1%. And I'd expect consistent error throughout the range.

I'd originally just assumed that it was slow throughout and by the same %age / margin, but obviously it has a mind of it's own! And if it was slow by around 25% first thing I would have been looking for was a loose / excessive gap / missing sensor meaning it was only reading 3 wheels.

There is a slight ABS / brake error which I hadn't had time to search for, but I think that will be to do with the powered side from the way it behaves. Shouldn't affect speed count.

ABS / brake warning lights behave correctly at ignition / set off from start. But they both illuminate for a second or 2 when first applying brakes, then go out again. As fluid level is good, I was guessing that was some failure on the power side and then clears as the pressure from the pedal / servo builds above a level. Typically it doesn't match any of the behaviours listed for the system in RAVE!
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So there are faults showing. That's the starting point. I guess a wheel-bearing might be on the way out and that is enough for the gap to get too large and lose count sometimes. Braking might exaggerate that. Any flickers going around sharp corners or any whining other than the wife or kids?
 
dubbleRR said:
mine is the same :eek: but if i put 255 60 18 on its bang on ,,,So i get 3mph more than standard ,and at 30 mpg i now get 33 mpg :rolleyes: :D:Do_O..
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Show off:rolleyes:...
IMG_20210511_130812.jpgat 65mph.. Go @RisforRacingnotReverse :D
 
My speedo always reads about 8-10% higher than GPS. At 50 (through M4 roadworks) I usually sit at 54mph on the speedo, and GPS reads 50mph. Consequently if I obey the speedo, then I will not trigger cameras !!

I have compared Android HU, iPhone & Snooper Radar detector, and they all agree within <1mph most of the time. Waze on Android is the least accurate, but that's due to the Chinese operating system software !!
 
Grrrrrr said:
So there are faults showing. That's the starting point. I guess a wheel-bearing might be on the way out and that is enough for the gap to get too large and lose count sometimes. Braking might exaggerate that. Any flickers going around sharp corners or any whining other than the wife or kids?
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There is a shimmy from the steering at 55or so, which feels to me like a flatted trackrod end or ball joint. Haven't had time to have a good pry around yet to find which. But that doesn't trigger the ABS light. Nor does cornering. Free time is a bit scarce at the moment, so I was going to throw it at the trusty local mechanic to give it a once over and service when he is free. That might clear some stuff up. And I'll get him to do any worn joints like that then to get a good baseline.
Any electrical or similar stuff I plan to tackle myself if possible though - he's fab with mechanics but a bit shabby when it comes to trim replacement without creating a jigsaw. Stuff like the boot release - going to browse through RAVE and wiring diagrams and see if I can come up with a sensible workaround and the like.
 
Scotiawhiskers said:
There is a shimmy from the steering at 55or so, which feels to me like a flatted trackrod end or ball joint. Haven't had time to have a good pry around yet to find which. But that doesn't trigger the ABS light. Nor does cornering. Free time is a bit scarce at the moment, so I was going to throw it at the trusty local mechanic to give it a once over and service when he is free. That might clear some stuff up. And I'll get him to do any worn joints like that then to get a good baseline.
Any electrical or similar stuff I plan to tackle myself if possible though - he's fab with mechanics but a bit shabby when it comes to trim replacement without creating a jigsaw. Stuff like the boot release - going to browse through RAVE and wiring diagrams and see if I can come up with a sensible workaround and the like.
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the shimmying could be a few different things. There’s some videos on YouTube that goes through them. Usually a worn steering damper or the tyres.
 
romanrob said:
Scotia - have a read of this https://rangerovers.pub/topic/898-calibrated-speedos?page=1#pid27063 - you can recalibrate the speedo by adjusting two pots on the instrument cluster - both slope and offset, so you may find the right combo. My speedo (unadjusted) is pretty consistently 5mph below google maps GPS, although I haven't paid much attention to the lower speed ranges
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Cheers for that - interesting reading and if it didn't come back into line with the GPS at about 70mph I'd say tweaking the slope would look like a good route to go down. I think I'm going to wait to tweak anything until after the servicing and check over, I'll specifically ask them to look at all the ABS sensors, in case it is a loose / out of spec / worn sensor / trigger / wire on the feed in from the ABS. That could account for an approx 20-25% error quite easily.
I'll bear it in mind if that doesn't throw up an improvement though.
 
romanrob said:
Scotia - have a read of this https://rangerovers.pub/topic/898-calibrated-speedos?page=1#pid27063 - you can recalibrate the speedo by adjusting two pots on the instrument cluster - both slope and offset, so you may find the right combo. My speedo (unadjusted) is pretty consistently 5mph below google maps GPS, although I haven't paid much attention to the lower speed ranges
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Interesting. Given how accurate mine is I wonder if one of his pots is slowly failing?
 
Grrrrrr said:
Interesting. Given how accurate mine is I wonder if one of his pots is slowly failing?
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I'm reckoning you are on to something with that suggestion @Grrrrrr . Took him out for a run today after doing some more minor tweaking and fettling, no speedo at all. Trip and main mileage ticking up correctly though so the cluster is obviously still receiving a valid signal.

I'm guessing that a dead pot could mean no signal to the needle despite the trip / mileage reading being OK? In which case it is dismantle it all and try to get everything back in line / replaced / reconnected as necessary to get it working. Or just see if I can track a secondhand speedo head and swap that into the cluster.
 
Update following the trip to the MOT after 5 months on the drive while we sorted the blown head gasket and assorted other stuff out. Speedo was +/- 1mph to the GPS speedo everywhere between about 20 and 70mph. We'd had both front wheels on and off a few times so I am suspecting an internal cable short to one of the front ABS sensors. Not open circuit so not reading as faulty, but also dropping out one corner of signal to read overall about 25% low previously.
The shimmy at 55ish I think was down to the silly wheels, on standard 235s and 16 inch he is smooth and steady. Might also be that it is 2 stone less weight for everything to control at each corner after the wheel change!
 
Not heard yet. Guy had forgotten to put it on the system when I rang last week, so only got added Tuesday. Probably tomorrow, but he was hoping if someone failed to turn up he could slot it in early. No such luck it appears.
 
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