Jon White Posted January 25, 2008 Share Posted January 25, 2008 Has anyone got any idea how I can calculate which speedo drive gear I need? Anyone written a wizzy spreadsheet etc for it, or know how to calculate it?? I've got 4.75 ring and pinions on 33" tyres. Speedo is a standard series 3 one from a LWB but I cant rememeber off the top of my head how many turns per mile it is, but I can easily look when i get home! Any help will be duly appreciated! Cheers Jon Quote Link to comment Share on other sites More sharing options...
DaveSIIA Posted January 25, 2008 Share Posted January 25, 2008 IIRC the series speedo drive uses a ratio of 2.2:1 and correction for tyre size is done at the speedo head. For your set up, the effective gearing increase due 33" vs 31" tyres and 4.75 R&P is about 5% (4.7/4.75x33/31) over a standard series diff with 4.7 R&P. This suggests as speedo drive ratio of ~2.1 would be right. The LT230 in a RRC uses a 23 tooth drive gear (red) as standard. I'd try a 21T (green) drive gear - FRC3311. Alternatively, the 110 uses a 20T (blue) gear - FRC3310. A bit more info on alternative drive gears can be found here 1 Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted January 25, 2008 Share Posted January 25, 2008 A finger of fudge for the first person who tells me the correct gear for my truck with 35" tyres and 5.99:1 diffs Quote Link to comment Share on other sites More sharing options...
Jon White Posted January 25, 2008 Author Share Posted January 25, 2008 Thanks Dave - thats easy enough then! So is the LT230 speedo drive ratio:- 2.0:1 with a 20 tooth 2.1:1 with a 21 tooth 2.2:1 with a 22 tooth etc If so that makes it pretty easy to sort out then! With the series gearbox in it it was always reading 10 -15% low with this set up. Cheers Jon Quote Link to comment Share on other sites More sharing options...
Jon White Posted January 25, 2008 Author Share Posted January 25, 2008 JU, Yours is 12% different. By my (admittedly simple) maths you'd need to use either the 20 or 21 tooth pinion, BUT you'll need to swap the speedo head for one from a SWB thats calibrated for 29" tyres. If I've understood how it all works correctly............. Jon Quote Link to comment Share on other sites More sharing options...
western Posted January 25, 2008 Share Posted January 25, 2008 you'll find the different drive gears & part numbers/tooth count here Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted January 25, 2008 Share Posted January 25, 2008 Cheers Jon! Is my brain correct in thinking to go for slightly safe over-reading I'd need the 20-tooth one? Quote Link to comment Share on other sites More sharing options...
TomG Posted January 25, 2008 Share Posted January 25, 2008 How handy! was going to ask the same thing for my 4.75 R&Ps & 35" tyres. Probably a dumb question as i don't know much about Series LRs but can the series speedo head be stuck in a defender dash and is so is the speedo cable connection compatible? For the LT230 speedo drive gear i take it more teeth mean a slower reading speedo Quote Link to comment Share on other sites More sharing options...
western Posted January 25, 2008 Share Posted January 25, 2008 Don't think the cable end fittings are compatible, looking at these, the Series 3 cable looks to me like it screws onto the back of the speedo, the defender one clips on. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted January 25, 2008 Share Posted January 25, 2008 My Series one pushes on, very similar to a defender one. In fact I have a Series and defender speedo head and a series speedo cable kicking about, if I can find them I'll try to make one fit the other. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted March 1, 2008 Share Posted March 1, 2008 Right, I am now confused of Gosport (OK that's not hard) However I have checked and the Series speedo cable definitely doesn't fit the Defender speedo head, so at least that's definite. The LT230 speedo drive gear works on the output of the LT230 so the gear ratio of the transfer box is irrelevant, which means some calculations kicking about are incorrect, and also means that (I think) I can't actually get a drive gear & speedo combo off-the-shelf that's low enough for my gearing. Doing some basic maths, if you take a Defender on small tyres: 3.54:1 Diffs x 29" tyres = 24.3mph / 1000rpm Then take my gearing: 5.99:1 diffs x 35" tyres = 18.3mph / 1000rpm Which (I think) means I'm looking to buy an aftermarket speedo that I can calibrate to my gearing. Or a set of 50" tyres Quote Link to comment Share on other sites More sharing options...
muddy Posted March 1, 2008 Share Posted March 1, 2008 John you may aswell save a load of hassle-50" tyres Quote Link to comment Share on other sites More sharing options...
Guest diesel_jim Posted March 1, 2008 Share Posted March 1, 2008 Which (I think) means I'm looking to buy an aftermarket speedo that I can calibrate to my gearing. Or a set of 50" tyres VDO do an 80mm electronic programmable speedo some installation info on the same speedo you just push a button and drive for exactly a mile, job done, so ideal if you regularly swap tyres over that are different sizes. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted March 1, 2008 Share Posted March 1, 2008 I was looking at those, then I found this: Which has many features, doesn't cost too much (£165) and is waterproof. Hmmm. Quote Link to comment Share on other sites More sharing options...
Jon White Posted March 2, 2008 Author Share Posted March 2, 2008 John, No - I did take account of the fact that the transfer box ratio is irrelevant! If you use a series speedo they are calibrated different - hence my origional post! There are a number of different series speedos available with different turns per mile listed - they should have written on them what the differences are! Jon Quote Link to comment Share on other sites More sharing options...
minivin Posted March 2, 2008 Share Posted March 2, 2008 Guys, remember this thread: http://forums.lr4x4.com/index.php?showtopi...speedo&st=0 just having to do calculations for 101 speedos, some thwack smashed mine and I'm hoping I can use a SWB speedo head as the only thing that is out from my calculations at present is the speedo drive I will shortly try and do the calc and see what comes up J&J Quote Link to comment Share on other sites More sharing options...
minivin Posted March 2, 2008 Share Posted March 2, 2008 A finger of fudge for the first person who tells me the correct gear for my truck with 35" tyres and 5.99:1 diffs from my calcs, a 35" tyre gives a circumference of appr 109.956" 63360" per mile means the wheel rotates 576.23 rev/mile with 5.99 diffs the gearbox output shaft rotates 3451 rev/mile based on a S3 LWB speedo needing 1392.7 rev/mile, this would mean your speedo drive would need to be 2.478:1, therefore with the standard 9T drive gear, a 22.3T driven gear would be needed. So 22T if I've sussed that right Quote Link to comment Share on other sites More sharing options...
minivin Posted March 2, 2008 Share Posted March 2, 2008 For the LT230 speedo drive gear i take it more teeth mean a slower reading speedo Yes, as the gear is driven, it will rotate slower Quote Link to comment Share on other sites More sharing options...
minivin Posted March 2, 2008 Share Posted March 2, 2008 JW, I concur with dave with the following calculation: 33 tyres 103.67" circm. 611.155 rev/mile of wheel 2902.986 rev/mile of gearbox o/p shaft based on LWB speedo, 2.084:1 speedo drive needed, meaning 19T. Therefore an 20T gear out of a RRC 3 speed LT230R will do the job as best as possible, if I got that right........ Quote Link to comment Share on other sites More sharing options...
minivin Posted March 2, 2008 Share Posted March 2, 2008 Oh yeah, forgot to say, the 4.7 diff, 235/85x16 tyres and 20T driven gear in my LT230R has resulted in my speedo being only about 2% slow! shweet Quote Link to comment Share on other sites More sharing options...
Jon White Posted March 3, 2008 Author Share Posted March 3, 2008 Ah cool!!! Thanks Rob! Its got a 22t in it at the moment but I've not taken it down the road yet! Cheers Jon 1 Quote Link to comment Share on other sites More sharing options...
minivin Posted March 3, 2008 Share Posted March 3, 2008 nay probs Quote Link to comment Share on other sites More sharing options...
mhemingway Posted November 27, 2016 Share Posted November 27, 2016 I'm in the midst of a project rebuild. I've got a 200tdi in a Series 88 that I put an R380 in with an LT230 (1.2 Ratio). I've got 4.11 Differentials in the series axles. I'd like to use the Series speedometer if at all possible. Recommendations are welcome on what speedo gear I would need. Quote Link to comment Share on other sites More sharing options...
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