Marcus Aurelius wrote:Measuring against a known standard calibrated speedometer is the only way to tell how accurate your speedometer actually is.
Timing over a known distance will give you a sufficiently accurate indication of a speedometer's accuracy.
Yep.
Until people give me some info about my question, and we have some facts to go from, I won't go further on this, but yes, checking on known distances will give you calibration.
GPS devices will suffice. Many (most?) cars these days have Satnav and they show your vehicle speed. Even if it's not dead on accurate, it's better by far than a car's speedo.
Marcus Aurelius wrote:Measuring against a known standard calibrated speedometer is the only way to tell how accurate your speedometer actually is. If you’ve got particularly large aftermarket rims, and haven’t compensated by reducing your tyre’s profile, that will cause the speedo to read incorrectly as well.
This is correct. GPS doesn't measure distance on hills particularly well because of the lengthening effect of a diagonal, but should be "good enough" on a flat road with good satellite acquisition See item 19 Speedometers (link) for chapter and verse In practice my speedos (not the swimming trunks) have over-read by 3-4 mph on several cars across the speed range from 30mph upward Regards tim-b
Marcus Aurelius wrote:Measuring against a known standard calibrated speedometer is the only way to tell how accurate your speedometer actually is. If you’ve got particularly large aftermarket rims, and haven’t compensated by reducing your tyre’s profile, that will cause the speedo to read incorrectly as well.
If you have a car, as per standard new, still under warranty and nothing changed from the spec, and you compare the speedo to GPS, as well as the distance too ......................... how do they compare?
Our car is spot on for distance even though it is six years old and on new tyres after renewing a coupe of times ........ but is consistently 10% high on the speed and has been forever maybe.
I'm asking for people to give me their findings.
Thanks. Thought you were angling for some sort of speeding defence [emoji23]
Don’t have figures to hand but cars I have owned generally lead 2mph under across the range going by both GPS and roadside speed indicators.
Hi, Useable 6-7 mm of tyre tread will have a dramatic effect on true reading on your speedo! Only cycling needs to be accurate
Sat navs dont even go there...............
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope You'll Still Find Me At The Top Of A Hill Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
The digital speedo on my car agrees to within 1mph with the radar speed thingies the police litter the sides of the road with. Tyres are fairly new too - so in theory it would read higher for the same speed when they're worn but it's not allowed to over read...
(It has an analogue dial too but I never bother looking at that.)
Modern systems are all programmable so it's worth querying the calibration with the dealer if it's miles out. Could be you have a wheel option package that's not been programmed into the cars brain
On mine (BMW) the 'analog' dial is over actual speed by 3% give or take but the hidden digital display is bob on.
Tangled Metal wrote:Our SEAT seems to run at 2-3MPH greater than actual.
Percentage or linear? What about 60 or 70?
It was at the 30mph zones I noticed that 3mph difference. At higher speeds I don't really know. I assume it'll be something like the same percentages so 70mph on the Speedo I'll possibly be doing 63mph actual. At that speed I'm likely to be on motorways or dual carriageway with the same 70mph limit. I very much doubt at that speed many drivers keep to below 70mph anyway. I do find if I do I get passed a lot by cars and even trucks. That could mean they're speeding or I'm actually doing that 7 mph less than actual or a combination of the two.
I'm really only any good at keeping a steady speed at 70 when driving the van. Both car and van have cruise control but I've only used it on the van because it was easy to see how to use it. The SEAT has unusual buttons and switches to operate it so I've not trusted it. Plus there's less chance of seeing those speed awareness sensor signs on a motorway. I'll have to set up my satnav when I'm next driving a road with a higher speed limit to check? Probably when my van comes back from the conversion company.
Gut feeling is it's not linear though, suspect it's less than 10% at higher speeds.
I'm off Down West tomorrow morning down the A30 to Penzance and back. 70odd miles each way. I'll be taking my TomTom (not for navigation purposes, but for showing ETA.
I'll do some experimenting 30/40/50/60/70 and report back. Hopefully, by then some of you could have done a similar test.
I've checked the odometer in the past using GPS and marker posts. Odometer, marker posts and GPS all agree, meaning that the odometer is correct. I will check again tomorrow that TomTom distances match with odometer distance.
Going to Penzance to complete my Cornish parish churches quest. Only 14 to do out of the original 226. I'll also ride on some of the (few) Cornish roads that I've not been on yet.
54miles to ride. Just less than a 2hr drive each way, plus 5hrs of riding.
thirdcrank wrote:It's interesting that on speed awareness courses people should be told that speedometers generally over-record speed. I should have thought that it's an area where a bit of blissful ignorance would keep them on the strait and narrow. Is it because people bleating that they were only 1 mph over the limit would have been driving at speed shown faster than that on their speedo if they flashed a camera?
(Edited to remove a blinding error)
It came up as the discussion got quite heated.Basically people on the course were opposing what was being said(you can't do this).I have been told different things on different courses though.I've found that they are quite inconsistent if you get the instructors talking "off script"! I'm booked on one next month.......but that's a rant for a separate thread(and not because I've got to go on one!)
“speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph. So if you're going 40mph, your speedometer may read up to 50.25mph - but it can never read less than 40mph.”
When the earth becomes sick the animals will begin to die. When that happens The Warriors of the Rainbow will come to save them.
Navara wrote: ... It came up as the discussion got quite heated.Basically people on the course were opposing what was being said(you can't do this).I have been told different things on different courses though.I've found that they are quite inconsistent if you get the instructors talking "off script"! I'm booked on one next month.......but that's a rant for a separate thread(and not because I've got to go on one!)
Would it be rude to ask how many speed awareness courses you have attended and over what period? With alleged speeds / relevant speed limits if I may make so bold.
Navara wrote: ... It came up as the discussion got quite heated.Basically people on the course were opposing what was being said(you can't do this).I have been told different things on different courses though.I've found that they are quite inconsistent if you get the instructors talking "off script"! I'm booked on one next month.......but that's a rant for a separate thread(and not because I've got to go on one!)
Would it be rude to ask how many speed awareness courses you have attended and over what period? With alleged speeds / relevant speed limits if I may make so bold.
Not rude at all. I've been on three,next month will make four since 2014.Same camera every time National speed limit 60mph (average speed cameras) this time it is allegedly 73mph over 2.2km.I take the exact same route to/from work.Every time I deviate to go somewhere I get caught as I pass through two cameras rather than turning off between the two on my usual route I contest it every time and I get offered a SAC every time.
I contest it every time and I get offered a SAC every time.
