Results 1 to 16 of 16

Thread: My speedometer is over by 4 - 8 km/hr

  1. #1
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    My speedometer is over by 4 - 8 km/hr

    Hi All:
    With the new school year recently started here there are quite a few radar speed devices showing the car's speed. I find that my speedometer over reports my speed by 4 - 8 km/hr. As an example the radar says I'm going 44 km/hr but my speedometer says I'm going 50 km/hr. I've calibrated the radar with my 928GTS and it is spot on, i.e., radar say 50 km/h and my digital readout is 50 km/h.

    Now I do have after market 18" rims with Micheline Pilot Sport A/S 3+ with a diameter of 25.7". I'm not sure if there is a difference in the diameter versus the stock 16 rims.

    I am assuming that the speedometer measures the driveshaft rpm and then via some cables and gears are used to translate that rpm to km/h. If so perhaps I've got the wrong diameter/number of teeth in the gear that results in an overstating of the speed by 4 - km/hr. I would also assume that the mileage on the counter too is over reporting.

    Any suggestions?


    Many thanks,

    Joe
    1992 500E Euro
    1993 928 GTS

  2. #2
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Joe, first thing is to determine if it's always a fixed amount off, i.e. indicated 24kph = actual 20kph, and indicated 104kph = actual 100kph? If so, the speedo needle would need to be re-located.

    However, if the error is a linear percentage across the range (i.e., off 4 at 40, off 8 at 80, etc) then you have an issue with the speedometer head itself (changed?), differential gearing, tire size, or some combination of the above. Stock tire size of 225-56-16 will be roughly a 25.9" diameter so you are pretty close. The speedo is electronic and reads from an inductive 2-wire sensor at the rear of the transmission housing (not a Hall sensor).

    If it's a linear error, you can use an electronic correction module like YellowBox or Dakota Digital to adjust. I've found the speedo is best case accurate within 1-2% at speeds up to 100mph / 160kph; above that it seems to get more optimistic vs GPS readings.

    Dave M.
    1997 E420 (Bugeyes)
    1994 E420 (Blondie)
    1994 E500 (Q-ship)
    1992 500E (Mach 5)
    1987 300D (Sportline Stage 2)
    Click here for my website
    Click here for my YouTube channel

  3. #3
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by gsxr View Post
    Joe, first thing is to determine if it's always a fixed amount off, i.e. indicated 24kph = actual 20kph, and indicated 104kph = actual 100kph? If so, the speedo needle would need to be re-located.

    However, if the error is a linear percentage across the range (i.e., off 4 at 40, off 8 at 80, etc) then you have an issue with the speedometer head itself (changed?), differential gearing, tire size, or some combination of the above. Stock tire size of 225-56-16 will be roughly a 25.9" diameter so you are pretty close. The speedo is electronic and reads from an inductive 2-wire sensor at the rear of the transmission housing (not a Hall sensor).

    If it's a linear error, you can use an electronic correction module like YellowBox or Dakota Digital to adjust. I've found the speedo is best case accurate within 1-2% at speeds up to 100mph / 160kph; above that it seems to get more optimistic vs GPS readings.
    Thanks Dave for the post and useful information.
    I certainly won't try to check 100 km in the school zone but rather will find a radar detector/speed indicator on the highway and will check if the error is linear or a fixed amount. I do know from past experience that if I set the cruise control to 100 km (speed limit 90 km) I get passed quite often.
    Once I have some data I'll report back.

    Would the actual mileage is lower than what's shown on the odometer because of the overage on the spedo?


    All the best,


    Joe
    1992 500E Euro
    1993 928 GTS

  4. The Following User Says Thank You to JCM1 For This Useful Post:

    TerryA (5 Days Ago)

  5. #4
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Easiest method to check the speedo is with a GPS on the freeway.

    You need to check the odometer separately using markers on the highway, preferably over at least 10-20km's.


  6. #5
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by gsxr View Post
    Easiest method to check the speedo is with a GPS on the freeway.

    You need to check the odometer separately using markers on the highway, preferably over at least 10-20km's.
    I have done some checking with both the GPS speed indicator ap as well as roadside radar. The results are:

    Speedometer Reading GPS/Radar
    40 33
    50 44
    60 49
    80 72
    90 84
    100 90

    The GPS was difficult to read and repeat so I am estimating it is +/- 3 km/h .

    Was not able to check the odometer.

    Any idea on what is going on?

    Many thanks,

    Joe
    1992 500E Euro
    1993 928 GTS

  7. #6
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Your data points are very erractic, variance between 7% (90/84) and 22% (60/49)!!. Do you get the same result from both GPS and radar?

    The odometer data will be very helpful, I would try to find a road with KM markers so you can check that next.

    You could also pull the instrument cluster and look for the "K" number. I only know the USA K-numbers, but another forum member may be able to share the number for kph speedos.


  8. #7
    Senior Member
    Join Date
    Nov 2014
    Posts
    108

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by gsxr View Post
    Your data points are very erractic, variance between 7% (90/84) and 22% (60/49)!!. Do you get the same result from both GPS and radar?

    The odometer data will be very helpful, I would try to find a road with KM markers so you can check that next.

    You could also pull the instrument cluster and look for the "K" number. I only know the USA K-numbers, but another forum member may be able to share the number for kph speedos.

    Any competent speedo shop can change the k number. They need to know the GPS speed and the indicated speed at two different rpms in the same gear. And they need the cluster.


    Sent from my SM-G955U using Tapatalk

  9. #8
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Thanks! When I pull the instrument cluster where would I find the "K" number? The car was imported from Japan.

    All the best,

    Joe
    1992 500E Euro
    1993 928 GTS

  10. #9
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Thanks atg!
    I'll collect the rpm data as well GPS and indicated speed and see if I can find a speedo shop in Vancouver.


    All the best,

    Joe
    1992 500E Euro
    1993 928 GTS

  11. The Following User Says Thank You to JCM1 For This Useful Post:

    atg (4 Days Ago)

  12. #10
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by JCM1 View Post
    Thanks! When I pull the instrument cluster where would I find the "K" number? The car was imported from Japan.
    K-number will be on the paper tag on the rear of the speedo. USA E500E cluster shown in attached photo.

    Attached Images Attached Images
    Dave M.
    1997 E420 (Bugeyes)
    1994 E420 (Blondie)
    1994 E500 (Q-ship)
    1992 500E (Mach 5)
    1987 300D (Sportline Stage 2)
    Click here for my website
    Click here for my YouTube channel

  13. #11
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Thanks Dave!
    Is the odometer driven by the same system as the speedometer? If so and if the speedo is reading xx% faster than actual would my actual mileage/kilometers also be more than actual?

    All the best,

    Joe
    1992 500E Euro
    1993 928 GTS

  14. #12
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Yes, the odo & speedo will be driven together. If the speedo is reading higher than actual, the odometer is probably high as well. Of course, it's also possible someone swapped in a different cluster and the odometer reading is completely off. You'll know more when you pull the cluster and check the K number. If it's wrong, you've got a "true mileage unknown" situation.

    Have you checked the differential housing to see if your 500E has stock 2.82 gears?


  15. #13
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Where would I see if the marking on the differential? I'll jack the car up and look. Would a different gear set explain the difference in indicated and measured speed?
    1992 500E Euro
    1993 928 GTS

  16. #14
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Location
    Boise, ID, USA
    Posts
    21,175

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by JCM1 View Post
    Where would I see if the marking on the differential? I'll jack the car up and look. Would a different gear set explain the difference in indicated and measured speed?
    Photos attached.If the ratio was numerically higher (i.e., 3.06) it would cause the speedo/odo to read higher. The K-number needs to match both the gear ratio and tire size.

    Attached Images Attached Images

  17. #15
    Senior Member
    Join Date
    Nov 2014
    Posts
    108

    Re: My speedometer is over by 4 - 8 km/hr

    Quote Originally Posted by JCM1 View Post
    Thanks atg!
    I'll collect the rpm data as well GPS and indicated speed and see if I can find a speedo shop in Vancouver.


    All the best,

    Joe
    North Hollywood speedometer did mine. Captruff mentioned another place Down here also,if you do not get any traction locally

    Sent from my SM-G955U using Tapatalk

  18. #16
    Member
    Join Date
    Jun 2016
    Location
    Horseshoe Bay, BC
    Posts
    72

    Re: My speedometer is over by 4 - 8 km/hr

    Thanks! I'll see what can be done here first. Your help is appreciated!
    1992 500E Euro
    1993 928 GTS

Similar Threads

  1. Is it possible to use a ASR speedometer head with Non ASR Faceplate?
    By JC220 in forum Electrical, CAN and Wiring
    Replies: 10
    Last Post: 07-17-2018, 10:50 AM
  2. WTB: 500E Speedometer
    By yayyap159 in forum Wanted to Buy
    Replies: 8
    Last Post: 05-16-2018, 07:19 AM
  3. Speedometer conversion.
    By Jim in forum Electrical, CAN and Wiring
    Replies: 24
    Last Post: 08-13-2016, 08:20 PM
  4. Help! My speedometer is off by 15-18 miles (higher)
    By Gmoneys in forum Troubleshooting and Diagnostics
    Replies: 2
    Last Post: 12-09-2015, 07:45 PM
  5. Special UK Speedometer
    By Christian_K in forum Interior Topics
    Replies: 0
    Last Post: 03-25-2012, 04:34 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •