 | Home > Technical > LT230 Transfer box speedo gears - Your experiences ? |
 
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| Hufflepuff Member Since: 25 Oct 2014 Location: Hampshire Posts: 727   
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Hmm that's odd. I had the Ashcroft 1.4 -> 1.2 ratio change put into my transfer box (by a garage), and as far as I know the speedo gearing wasn't changed - and it still reads fine.
I thought/assumed the speed reading came off the back of the transfer box - ie. the speed with which the prop-shafts are rotating, which is independent of the gearbox/transfer box gearing. Was your speedo reading correctly before the change? No changed made to the differential ratios? 2005 Td5 90 XS 1989 V8 110 CSW |
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12th Feb 2019 1:24pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311  
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The speedo was reading high before which is why i asked for a bigger speedo gear. The diffs are standard 1.54.
I have got it right though haven't I ? I mean if i put in a smaller Blue gear, the speedo would read even higher wouldn't it ? I'm beginning to think the Speedo unit itself might be at fault. Might have to test that out Thanks Huff, |
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12th Feb 2019 1:31pm |
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| Martin Site Admin Member Since: 02 Apr 2007 Location: Hook Norton Posts: 6605  
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For 235/85R16 tyres (or 265/75R16) you want the blue gear FRC3310 (20 teeth).
You've gone the wrong way with the number of teeth 
Edit: https://www.defender2.net/forum/topic39592.html?highlight=frc3310  1988 90 Td5 NAS soft top
2015 D90 XS SW |
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12th Feb 2019 1:43pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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hmm ok, I'll order one and see, i must admit i cannot see how that can be though.
Maybe the logic is different with a worm gear arrangement. I'm thinking larger driven gear on a push bike is slower, smaller gear is faster ? Anyway thanks, i'll report back when I've installed the Blue Gear. |
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12th Feb 2019 2:13pm |
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| LandRoverAnorak Member Since: 17 Jul 2011 Location: Surrey Posts: 11324
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Your logic is backwards. For any given speed and speedo reading, smaller tyres rotate faster than bigger ones and visa versa. The drive wheel compensates for that variation rather than vehicle speed. 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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12th Feb 2019 2:36pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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Aha.... that moment when the penny drops is a nice feeling. Yes you are right !
As I understand it now, for every 360 degree revolution of the speedo gear, Red turns it 23teeth along and the Blue 20teeth, Hence why the Blue is slower. Tell ya i still get Gross and Net wrong...... Thanks guys, it's been doing me head in. Sorted! Blue gear is ordered and one more problem solved. I'll have 2 red gears and 1 yellow gear up for grabs if anyone wants/needs them. |
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12th Feb 2019 2:53pm |
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| Martin Site Admin Member Since: 02 Apr 2007 Location: Hook Norton Posts: 6605
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Think of the rear propshaft, which is effectively what the speedo drive gear connects to directly.
Imagine with "normal" wheels and tyres. To drive a mile the prop will do a certain number of turns (for argument's sake call it 1000). The speedo drive cable does (say) 50 turns by using a 20 tooth drive gear. Now imagine fitting bigger tyres with twice the circumference; each turn of the wheel goes twice as far. So the prop only needs to turn 500 times to cover the mile. With the same drive gear as above, the speedo drive cable does only 25 turns - and shows half speed/distance covered. So in the example above we need to double the number of turns of the speedo drive cable (or pulse generator) to still show 1 mile covered. We would do this by fitting a speedo drive gear with HALF the number of teeth. 10 teeth gives 50 turns of the speedo drive cable. Getting back to the Defender, going bigger with tyres means the distance covered per turn of the wheel is greater, so the prop turns slower at the same speed, the speedo drive is slower and it needs speeding up. So fewer teeth 1988 90 Td5 NAS soft top
2015 D90 XS SW |
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12th Feb 2019 3:06pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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Aha but my problem is my speedo is reading much higher.
Reducing the teeth will make it faster.... but i want it to go slower. In fear of sounding stupid, have you missed that my speedo was reading too high ? It's reading 40mph when i'm actually doing 30mph according to my GPS ? I want to slow the speedo down 33% roughly and from your analogy less teeth = faster ? |
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12th Feb 2019 3:53pm |
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| Martin Site Admin Member Since: 02 Apr 2007 Location: Hook Norton Posts: 6605
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Was the speedo reading high even before you had all the transfer box done?
1988 90 Td5 NAS soft top
2015 D90 XS SW |
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12th Feb 2019 3:54pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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yes, the previous tyres were 275's or something close to that, it's reading higher now, i assume because the 235/85's i have are smaller.
Is it worth taking a known good Speedo out of a friends 200tdi and testing it in mine to see? Damn, i've just ordered a blue cog anorl.... |
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12th Feb 2019 4:02pm |
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| Martin Site Admin Member Since: 02 Apr 2007 Location: Hook Norton Posts: 6605
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Yes swap the speedos over to check it isn't a speedo issue
1988 90 Td5 NAS soft top
2015 D90 XS SW |
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12th Feb 2019 4:04pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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Update***
Installed the blue gear this last weekend and the speedo is reading even higher. than previously with the larger red gear. I must have a faulty speedometer then i guess. |
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25th Mar 2019 10:59am |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8032  
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Typically a blue pinion on 255.85s (33s) Will read bang on with GPS. It's specified for smaller 235.85s but then over reads by just under 10%
Are your diffs not 3.54? Not come across 1.54s typo I guess? Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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25th Mar 2019 3:14pm |
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| windy81 Member Since: 14 Mar 2018 Location: North Wales Posts: 311
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everything is standard.
1.54 is a typo. I'm just perplexed by it at the moment, becasue i have a feeling the speedo is fine and it's something else. |
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25th Mar 2019 4:20pm |
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