Ashleigh Posted October 22, 2023 Share Posted October 22, 2023 Hello, I have a question and wondered if someone could help. I have a discovery 2 td5 running 285/75/16. Currently the speedo is out by a few mph obviously due to tyre size. I am in the process of having some diffs built by Nige at xcess 4x4. We have discussed changing the CWP to 3.75 ratio. The question I have will this alter the speedo and make it out by more? Is the speedo run via the abs ring on the hub? Thanks for your help Ash. Quote Link to comment Share on other sites More sharing options...
Ed Poore Posted October 25, 2023 Share Posted October 25, 2023 Short answer yes. The speedo takes its measurement from the back of the transfer box so it "doesn't know" about any changes downstream of it - basically tyres and diffs. Larger tyres should make the speedo "over read" a little as for the same revolution of the transfer box the vehicle travels a little further. Going from a 3.54 to 3.75 means that the transfer box how has to turn the propshaft a little bit more to achieve one revolution of the wheels so should cause the speedo to "under read" slightly. If you want more precise answers it's some basic maths. 1 Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted October 25, 2023 Share Posted October 25, 2023 Just to add - you can get different speedo drive gears for the transfer box so you can (usually) swap that and bring the speedo "close enough" to being correct. Item #16 on this diagram: https://www.lrworkshop.com/diagrams/land-rover-defender-transfer-box/transfer-box-lt230/rear-flange_53758#18 FRC3310 - Gear-speedometer drive, Blue, 20 Teeth FRC3311 - Gear-speedometer drive, Green, 21 teeth FRC3312 - Gear-speedometer drive, Yellow, 22 teeth FRC3313 - Gear-speedometer drive, Red, 23 Teeth etc. 1 Quote Link to comment Share on other sites More sharing options...
geoffbeaumont Posted October 25, 2023 Share Posted October 25, 2023 3 hours ago, Ed Poore said: Larger tyres should make the speedo "over read"... Going from a 3.54 to 3.75...should cause the speedo to "under read" slightly. Other way round, surely? 1 Quote Link to comment Share on other sites More sharing options...
bishbosh Posted October 25, 2023 Share Posted October 25, 2023 Note that I think in an automatic disco 2 the speed information is done by electrickery in the gearboxes somewhere as there is no sensor on the output housing of an auto D2 transfer case. 1 Quote Link to comment Share on other sites More sharing options...
Ashleigh Posted October 25, 2023 Author Share Posted October 25, 2023 4 hours ago, Ed Poore said: Short answer yes. The speedo takes its measurement from the back of the transfer box so it "doesn't know" about any changes downstream of it - basically tyres and diffs. Larger tyres should make the speedo "over read" a little as for the same revolution of the transfer box the vehicle travels a little further. Going from a 3.54 to 3.75 means that the transfer box how has to turn the propshaft a little bit more to achieve one revolution of the wheels so should cause the speedo to "under read" slightly. If you want more precise answers it's some basic maths. Hi Ed Thanks for the reply. Are you sure about this on a discovery 2, I was searching the other day trying to get answers and it says the speedo is run of the hubs and abs on a d2. I know the discovery 1 200tdi and 300tdi are run from the transfer box and you can change for different drive gears. Quote Link to comment Share on other sites More sharing options...
Ashleigh Posted October 25, 2023 Author Share Posted October 25, 2023 3 hours ago, FridgeFreezer said: Just to add - you can get different speedo drive gears for the transfer box so you can (usually) swap that and bring the speedo "close enough" to being correct. Item #16 on this diagram: https://www.lrworkshop.com/diagrams/land-rover-defender-transfer-box/transfer-box-lt230/rear-flange_53758#18 FRC3310 - Gear-speedometer drive, Blue, 20 Teeth FRC3311 - Gear-speedometer drive, Green, 21 teeth FRC3312 - Gear-speedometer drive, Yellow, 22 teeth FRC3313 - Gear-speedometer drive, Red, 23 Teeth etc. Hi FridgeFreezer, I was lead to belive this was just on the old discovery's and defenders as the speedo runs off the transfer box but apparently discovery 2 td5 and not the same. I was searching the other day and it I belive it is through the Abs on the hub so I'm hoping it won't make any difference changing the CWP to 3.75. Quote Link to comment Share on other sites More sharing options...
Ed Poore Posted October 25, 2023 Share Posted October 25, 2023 1 hour ago, geoffbeaumont said: Other way round, surely? Yes. I was thinking in terms of revs rather than speed. 1 Quote Link to comment Share on other sites More sharing options...
landroversforever Posted October 27, 2023 Share Posted October 27, 2023 On 10/25/2023 at 11:59 AM, bishbosh said: Note that I think in an automatic disco 2 the speed information is done by electrickery in the gearboxes somewhere as there is no sensor on the output housing of an auto D2 transfer case. My memory agrees with that…. D2 uses the ABS system for road speed. 1 Quote Link to comment Share on other sites More sharing options...
Daan Posted October 28, 2023 Share Posted October 28, 2023 The speedo aside, 3.75 is not much of a change, about 6%. I would suggest 4.1 ratios, to make a difference, as the TD5 is not very torquey at the bottom revs. 1 Quote Link to comment Share on other sites More sharing options...
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