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Speedometer appears to be under-reading, who can fix it?

  • 15-03-2015 01:47AM
    #1
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭


    This is going to sound like a pile of shíte but hear me out...I've been done for driving at 59km/h in a 50km/h zone last Saturday. It's Cork Street in Dublin and those that know it as well as I do know it has a Garda speed van on it at least twice a week. Since this was only 7 days ago and I drive my car maybe twice a week I've a full memory of the event, the van's location etc and am full sure that like pretty much every time I drive down that road I had the cruise control on at 50km/h to avoid a heavy foot getting me some points. I made no attempt to slow down, after all I was doing 50 :confused:

    So I'm gutted I got the notice yesterday but figured something was amiss. I took the car out today and with my dad using a GPS app on his phone I had some interesting results: When I'm doing 40kph on the speedo it's really 48, when doing 50 it's 58/59, 60 => 68, 80 => 88, 100 => 109; I didn't do 120, this was just an N road :pac: We repeated the test several times to be sure and the difference was fairly constant. I'm running stock wheels so there's no trickery on that front. Clearly something's wrong with it under-reading so much but does anybody know somebody who can fix it?

    Before anybody thinks I'm working some sort of angle here I don't think I'll bother appealing the points - it doesn't change the fact I was speeding and while 3 points is sickening risking 5 for an appeal isn't worth it IMO. I'm just looking not to get collared again for speeding when I think I'm fine; eventually I'll forget to shave 10kph off my current reading in my head and this'll happen again.


«1

Comments

  • #2
    Special Circumstances
    Closed Accounts Posts: 7,540 ✭✭✭Special Circumstances


    What car.
    What size wheels.
    What size tyres.


  • #3
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    E46 M3. 19" OEM CSLs. F:225/40/19 R: 255/35/19


  • #4
    challengemaster
    Registered Users, Registered Users 2 Posts: 16,925 ✭✭✭✭challengemaster


    http://forum.e46fanatics.com/showthread.php?t=999740


  • #5
    ironclaw
    Registered Users, Registered Users 2 Posts: 7,994 ✭✭✭ironclaw


    If you had a bit of cash, I'd be interested in seeing how this could be challenged. If the car was absolutely stock, and you were willing to swear this legally, and have it independently assessed and again, sworn legally that the fault was unknown to you (e.g. ECU Error) Would you have a case? There is no onus on the State for their equipment to be proved calibrated, accurate or in working order. Therefore, can it be expected that the accused has all their equipment in the right order? Its quite possible to buy a car, and unknown to you, have a dodgy speedo. Granted we'd all have a fair idea of 30km/h versus 80km/h, but 30km/h vs 50km/h would be harder to know. 35km/h on the Quays would be enough to have you nabbed.

    I'm not read in law but there is probably a case if you had enough moola to pursue it. I know for a fact cases like this in the States are heard widely.


  • #6
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    I hear what you're saying but spending a lot of cash and risking 5 penalty points in front of a judge isn't something I'm up for. The way I see it is it was 59km/h so I can't argue on the grounds that I wasn't speeding and arguing on the grounds that I didn't know I was speeding seems doomed to failure. I found out last night that some cars have their needle slip and the E46 has a cluster test mode that will show your speed according to the ECU. I can use that to compare to my needle reading to ECU and see if there's a discrepancy or if this is an actual internal fault that I need to take out a hexeditor or go to BMW to fix.


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  • #7
    Skatedude
    Registered Users, Registered Users 2 Posts: 2,667 ✭✭✭Skatedude


    just download a gps speedometer for your phone, it will confirm your speedo readings. best done on a straight road.


  • #8
    visual
    Closed Accounts Posts: 3,265 ✭✭✭visual


    Car speedo was set for stock size wheels changing the diameter effects both gear ratio and speed reading.

    Bit like wearing high heel shoes makes you taller


  • #9
    visual
    Closed Accounts Posts: 3,265 ✭✭✭visual


    Car speedo was set for stock size wheels changing the diameter effects both gear ratio and speed reading.

    Bit like wearing high heel shoes makes you taller


  • #10
    TitianGerm
    Registered Users, Registered Users 2 Posts: 30,536 ✭✭✭✭TitianGerm


    Could you look to put a car on a rolling road ? This would be able to give more accurate speed measure than a phone GPS


  • #11
    CiniO
    Registered Users, Registered Users 2 Posts: 16,041 ✭✭✭✭CiniO


    visual wrote: »
    Car speedo was set for stock size wheels changing the diameter effects both gear ratio and speed reading.

    Bit like wearing high heel shoes makes you taller

    How can changing wheel size affect gear ratio? :confused:

    Gear ratio depends on amount of teeth on cogwheels inside gearbox.


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  • #12
    TheChizler
    Registered Users, Registered Users 2 Posts: 9,559 ✭✭✭TheChizler


    emmetkenny wrote: »
    Could you look to put a car on a rolling road ? This would be able to give more accurate speed measure than a phone GPS
    As long as you're keeping a constant speed on the flat the GPS derived speed will be correct.


  • #13
    TitianGerm
    Registered Users, Registered Users 2 Posts: 30,536 ✭✭✭✭TitianGerm


    TheChizler wrote: »
    As long as you're keeping a constant speed on the flat the GPS derived speed will be correct.

    I always found the GPS on phones to be a little off or inconsistent between different phones. I tried it before where we were using an iPhone 5 and a Samsung S4 at the same time and both were 3/4 KPH different in there speeds.


  • #14
    ironclaw
    Registered Users, Registered Users 2 Posts: 7,994 ✭✭✭ironclaw


    emmetkenny wrote: »
    I always found the GPS on phones to be a little off or inconsistent between different phones. I tried it before where we were using an iPhone 5 and a Samsung S4 at the same time and both were 3/4 KPH different in there speeds.

    Actually depends how the programmer worked with the data being fed from the GPS itself, and how accurate they were willing to accept. This, and rounding, can greatly affect accuracy and hence the numbers you see.

    Either way, on average, it will be fairly spot on.


  • #15
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    Thanks to the powers of the internet it turns out that the E46 has a cluster test that displays the speed in KPH as read by the ECU. So naturally enough I went out again today for more testing.

    We put a GPS app against the kph readout from test mode and found they were constantly within 1 of each other. I've got reasonable confidence in the accuracy of the GPS speed and the cluster's test mode. No idea why my needle is off by 8/9. I've no idea about that 8.9% its supposed to read over the ECU speed according to challengemaster's link either.

    I'll call a few of those odometer places tomorrow to see about getting it tested/fixed


  • #16
    visual
    Closed Accounts Posts: 3,265 ✭✭✭visual


    CiniO wrote: »
    How can changing wheel size affect gear ratio? :confused:

    Gear ratio depends on amount of teeth on cogwheels inside gearbox.

    The final cogwheel is the wheel it's self.


  • #17
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    visual wrote: »
    The final cogwheel is the wheel it's self.

    I hear what you're saying but this is stock wheels & stock tyres though. This isn't wheel or tyre variance, we are talking almost 10km under real speed and 20km under what the speedo is supposed to show (8.9% over real speed).


  • #18
    challengemaster
    Registered Users, Registered Users 2 Posts: 16,925 ✭✭✭✭challengemaster


    TouchingVirus wrote: »
    I hear what you're saying but this is stock wheels & stock tyres though.

    It's not really though. Stock wheels for an E46 M3 are 18's. Sticking 19" CSL wheels on it (despite being OEM) means there'll be a difference. The speedo will have been set up for the original alloys.

    Same as if you stuck CSLs on a E46 that came from factory with 16's. The speedo would be way out.


  • #19
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    challengemaster wrote: »
    It's not really though. Stock wheels for an E46 M3 are 18's. Sticking 19" CSL wheels on it (despite being OEM) means there'll be a difference. The speedo will have been set up for the original alloys.

    Same as if you stuck CSLs on a E46 that came from factory with 16's. The speedo would be way out.

    Oh? Jaysus, I bought it with 19" non-CSLs :( Would the difference of 1" cause it to go from overeading by 8.9 percent to underreading by 9km?


  • #20
    pippip
    Registered Users, Registered Users 2 Posts: 2,246 ✭✭✭pippip


    TouchingVirus wrote: »
    Oh? Jaysus, I bought it with 19" non-CSLs :( Would the difference of 1" cause it to go from overeading by 8.9 percent to underreading by 9km?

    Ask here - http://www.boards.ie/vbulletin/forumdisplay.php?f=380


  • #21
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    So I thought about it a bit more, it can't be the wheels. The engine speed estimation shown by the cluster test mode was calibrated to 18" wheels if you're right challengemaster. If it says I'm doing 60, the GPS says 60/61. So the wheels might account for that 1km difference, but the needle is way back at 52kph when it should be at 64/65 given it is supposed to overestimate. Correct me if I'm wrong :)

    Edit: Using 2*Pi*r as the circumference of the wheel says there's just over a 5% difference. So yeah :/


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  • #22
    challengemaster
    Registered Users, Registered Users 2 Posts: 16,925 ✭✭✭✭challengemaster


    TouchingVirus wrote: »
    Oh? Jaysus, I bought it with 19" non-CSLs :( Would the difference of 1" cause it to go from overeading by 8.9 percent to underreading by 9km?

    Actually it seems like the 19's were an option, don't think I've ever actually seen them before though. 18's are certainly more common and would be the base option for M3s.

    I wonder if they were calibrated from factory for the base. I doubt it would cause that much of a change though.


  • #23
    Special Circumstances
    Closed Accounts Posts: 7,540 ✭✭✭Special Circumstances


    I'd say some offset fault on speedo or someone was dicking around with it previously.


  • #24
    ongarite
    Registered Users, Registered Users 2, Paid Member Posts: 8,864 ✭✭✭ongarite


    Based on this site and the standard 18" size of 225/40 your speedo will be 4% out with those 19" CSL wheels.
    http://tire-size-conversion.com/tyre-size-calculator/

    CSL alloys, if they are OEM not replicas should be running 235/35/19 front and 265/30/19 rear.


  • #25
    Bongalongherb
    Closed Accounts Posts: 3,766 ✭✭✭Bongalongherb


    What's it now, €30 to hook up the machine to the engine control unit ? if the results received show an error with the speedo then that will probably quash your penalty points if you present this information in court or maybe before court. It seems reasonable enough to pay €30 for the information regarding any errors found as it will remove the 3 penalty points.

    The above comment is just an idea as I am not sure if an ECU check will actually show an error from the speedo.


  • #26
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    ongarite wrote: »
    Based on this site and the standard 18" size of 225/40 your speedo will be 4% out with those 19" CSL wheels.
    http://tire-size-conversion.com/tyre-size-calculator/

    CSL alloys, if they are OEM not replicas should be running 235/35/19 front and 265/30/19 rear.

    So I just double checked, looks like i was wrong last night. I'm running 235/35 on the front, 255/35 on the rear. The CSLs are OEM.

    I'll bring it to the garage and get the entire setup looked at. Could be a sensor, bad cluster or a heap of other things so I'll let the pros sort it out and check for codes etc. Cheers for all your help


  • #27
    OldMrBrennan83
    Closed Accounts Posts: 26,611 ✭✭✭✭OldMrBrennan83


    This post has been deleted.


  • #28
    9935452
    Registered Users, Registered Users 2 Posts: 3,143 ✭✭✭9935452


    TouchingVirus wrote: »
    This is going to sound like a pile of shíte but hear me out...I've been done for driving at 59km/h in a 50km/h zone last Saturday. It's Cork Street in Dublin and those that know it as well as I do know it has a Garda speed van on it at least twice a week. Since this was only 7 days ago and I drive my car maybe twice a week I've a full memory of the event, the van's location etc and am full sure that like pretty much every time I drive down that road I had the cruise control on at 50km/h to avoid a heavy foot getting me some points. I made no attempt to slow down, after all I was doing 50 :confused:

    So I'm gutted I got the notice yesterday but figured something was amiss. I took the car out today and with my dad using a GPS app on his phone I had some interesting results: When I'm doing 40kph on the speedo it's really 48, when doing 50 it's 58/59, 60 => 68, 80 => 88, 100 => 109; I didn't do 120, this was just an N road :pac: We repeated the test several times to be sure and the difference was fairly constant. I'm running stock wheels so there's no trickery on that front. Clearly something's wrong with it under-reading so much but does anybody know somebody who can fix it?

    Before anybody thinks I'm working some sort of angle here I don't think I'll bother appealing the points - it doesn't change the fact I was speeding and while 3 points is sickening risking 5 for an appeal isn't worth it IMO. I'm just looking not to get collared again for speeding when I think I'm fine; eventually I'll forget to shave 10kph off my current reading in my head and this'll happen again.

    Personally i dont think its the tyres/wheels that is causing this.
    Logic says if the outside diameter of the wheel/tyres is 10% bigger than standard, you should do 10% more at 50k and and 10% more at 100k, which is not the case.
    The speedo is reading 8/9k out at all speeds.
    Most likely there is an offset (of sorts ) that is out.
    Its something i would talk to a BMW specialist about , they would have more info.
    As regards using this as a excuse in court i wouldnt say it would fly. It would open an aweful loophole . Everyone would start using it as an excuse in court.
    but that said if it could be proved that the car was supplied day one from bmw like that you would have a case i court


  • #29
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    9935452 wrote: »
    As regards using this as a excuse in court i wouldnt say it would fly. It would open an aweful loophole . Everyone would start using it as an excuse in court.
    but that said if it could be proved that the car was supplied day one from bmw like that you would have a case i court

    I've no interest in taking it anywhere near a court but thanks, I'm agreeing with you; it's not like the cluster has a warranty sticker on it that can prove I wasn't near it. And that becomes relevant when you read the following...

    So I just did a cluster test lights & gauges test, I think the needle is off. During the test all the gauges go right the way around - petrol, oil, engine-temp, rpm and speed. The clock ends at 185mph but the speedo needle never makes it there, stopping short around 179 or so. On closer inspection it does appear to be off - the last mark on the gauge is 5mph but if you take the distance from 10mph to 5mph and infer where the needle should be for zero it's not there but a bit further back. The more I think about it as the morning passes the more I'm thinking this is the problem - it certainly explains the reason for the speedo being constantly out by 8/9kph rather than a percentage. I'll see if I can remove the needle and refit it so it's just shy of the 5mph and do some GPS tests to make sure I'm back over-estimating.


  • #30
    VeVeX
    Registered Users, Registered Users 2 Posts: 1,395 ✭✭✭VeVeX


    TouchingVirus wrote: »
    So I just double checked, looks like i was wrong last night. I'm running 235/35 on the front, 255/35 on the rear. The CSLs are OEM.

    I'll bring it to the garage and get the entire setup looked at. Could be a sensor, bad cluster or a heap of other things so I'll let the pros sort it out and check for codes etc. Cheers for all your help

    E46 M3 OE sizes are -

    235/35R19 87Y & 265/30R19 89Y
    or
    225/40R19 93Y & 255/35R19 96Y


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  • #31
    TouchingVirus
    Registered Users, Registered Users 2 Posts: 8,581 ✭✭✭TouchingVirus


    VeVeX wrote: »
    E46 M3 OE sizes are -

    235/35R19 87Y & 265/30R19 89Y
    or
    225/40R19 93Y & 255/35R19 96Y

    It's not the wheels or tyres, but thanks :)


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