 | Home > Technical > Cable driven speedo adjustment |
 
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| Linds Hanson Member Since: 16 Jan 2021 Location: Cornwall Posts: 486  
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They are supposed to be ten percent over by law to allow for tyre wear tolerence etc, can you not just check speed with Waze from your phone?
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28th Apr 2023 9:11am |
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| AMBxx Member Since: 24 Jul 2016 Location: York Posts: 1032 
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10% over is the maximum it can read, not the required level. 20% over is an MOT fail. Anything under is also a fail. My wife's car reads bang on, not tested the Defender.
I'm pretty sure that ignoring a faulty speedo and using a GPS setup instead is also illegal. GPS just isn't that accurate over short distances and can't be relied upon to work all the time (try it in a tunnel). GPS is only useful for testing the speedo at a constant speed over a decent distance. |
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28th Apr 2023 9:14am |
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| TexasRover Member Since: 24 Nov 2022 Location: Paris Posts: 1079  
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Typically on gear driven speedometers you have a few different gearing options, but you might need to swap your speedometer head.
The drive is taken from the transfercase output shaft so the ratio of the transfer case has no impact on the speedometer reading. The differentials and wheel size however do. So LR has a few options to get it right. Inside the transfercase there are (small) gears that drives the output of the speedometer cable. I believe there might be some set ratio's and colours there. You would have to do research to see if you can fit a different ratio. This would obviously be good if you want to keep your existing speedometer. Then the speedometer head is made with different ratio's inside. Regardless the speedometer and odometer are set the same internally. The rating is something like input rotations per mile and is shows as a small number at the bottom of the face. You can buy the speedometer head with the correct gearing for your need. For my 88 Series 3, if fitted the larger 109 7.5 tires which meant the speedometer and odometer were off so I changed the head from a 1504 to a 1408 of a 109. Below the calculations I did at the time: 1376 for tyre 9.00x16 1408 for tyre 7.50×16 (235/85 r16) 1504 for tyre 6.50×16 (205/80 r16) 1536 for tyre 6.00×16 (205/70 r16) size radius circomference rotations/mile factor clock fitted at 100kph: 205/70 r16 0.347 2.180 738.1441143 2.136 1577 1536 102.6481659 205/80 r16 0.367 2.306 697.918277 2.136 1491 1504 99.11924466 7.50xr16 0.385 2.419 665.2883316 2.136 1421 1408 100.9272639 Check of speedo 31 July 2014: tomtom Speedo Speed 54 55 101.9% distance 26.7 26.6 99.6% The calculations Series Land Rover have a 4.7 : 1 reduction differential and a 2.2 : 1 speedometer drive gear reduction from propshaft to speedometer. 4.7 divided by 2.2 = 2.136 which means a 1 : 2.136 step up from rear wheels to speedometer. For every revolution described by the wheels we get 2.136 revolutions in the speedometer. This is a constant relationship (K) that can be use in every speedometer calculation. Here are the calculations for my Imperial speedometer marked 1504 turns per mile. turns per mile divided by K gives the number of turns of the wheels per mile 1504 divided by 2.136 = 704 My Lightweight will do 704 turns of the wheels for every mile clocked up on the speedometer. A mile is, 5280 feet, divide this by 704 wheel turns gives 7.5 feet of travel per wheel turn that is a wheel circumference of 7.5 feet or 90 inches. Dividing this by 3.14 (close enough to Pi) gives the diameter of wheel. 90 inches divided by 3.14 = 28.6 inches. Hence, it is calibrated for 6.50 x 16 Tyres. LWB speedowill have the figures 1408 SWB ones are 1536 or close. Some are 1376 if intended for bigger tyres. |
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28th Apr 2023 9:15am |
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| LandRoverAnorak Member Since: 17 Jul 2011 Location: Surrey Posts: 11324
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The different speedo head for different tyre sizes was a Series Land Rover thing. The arrangement was changed to swappable gears in the speedo drive for later vehicles so that the speedo head is common to all. Darren
110 USW BUILD THREAD - EXPEDITION TRAILER - 200tdi 90 BUILD THREAD - SANKEY TRAILER - IG@landroveranorak "You came in that thing? You're braver than I thought!" - Princess Leia |
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28th Apr 2023 11:51am |
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| TexasRover Member Since: 24 Nov 2022 Location: Paris Posts: 1079
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Interesting, I never played with the later mechanical drives and now I have a Td5 so.
When you say 'speedo drive' I assume this is the small gearset inside the transfercase. That at least provides a fairly easy solution. |
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28th Apr 2023 12:26pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8049  
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Cheers for replies.
Concern is swapping the drive in the t box. Not sure the one in there will comenout easily. So therefore looking for other online add in options or the gps one, legalities aside. Mpn are helpful, thank you. Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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28th Apr 2023 7:38pm |
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| LiftedDisco Member Since: 17 Aug 2014 Location: Towcester Posts: 354
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@jst
What tyre size are you running - there are different drive gears which will adjust the speedo reading. Changing the drive gear can be somewhat challenging, but isn’t too tricky once you get into it. I changed from a green (RRC fitting) to black (Defender) and the old one came out without any drama. To be fair, it’s easier if you have the transfer box out for any reason, but by no means difficult if doing whilst it’s in the vehicle. Hope this helps… |
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1st May 2023 8:45pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8049
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33's
Its a puma t box so presume its the 7.50 gear drive as the 235s on pumas similar od. Don't think they do a 33 gear? Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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7th May 2023 10:02am |
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