aoie Posted June 15, 2015 Share Posted June 15, 2015 I have a question on the best way to recalibrate my speedo after changing out my spitfire 4.11 differential to a 3.27 ratio. Is it best to change out the transmission speedo pinion gear to match the differentail used, or can I change the gearing in the speedometer? I found a couple of internet threads that claim i just need to change the transmission speedometer drive/pinion gear to a 20 tooth (orange) size, however I'm not seeing anything about the internal worm gear needing to be changed to match. Wiil the worm gear inside the transmission work with differnt drive gear sizes (tooth count changes)? I calculated that my speedo calibration for the odometer would need to be around 1005 using the existing calibration setting (1248) and doing some backwards engineering to find the existing drive shaft turns to 1 cable turn (2.9). But it looks like this only helps me to fix the odometer calibration and not the speedometer. I plan on changing the differential ratio again in the near future, so i'm just looking to see if i can do somthing myself temporarly since the speedo is off by 10+ miles/hour. Andy Sun Parairie, Wisconsin (USA) Link to comment Share on other sites More sharing options...
Millstone10 Posted June 16, 2015 Share Posted June 16, 2015 Cant you just get a GT6 speedo? depends on the gear box ratio's of course but may be easier. If you change the diff later you can sell it on and not cost you anything but postage... Link to comment Share on other sites More sharing options...
Triumph-V8 Posted June 16, 2015 Share Posted June 16, 2015 Inside the speedo there is nothing to change for DIY. Either the transmission inside the gearbox can be changed like you have in mind if the proper toothed wheel is availiable or a transmission inside the speedo cable can be obtained. Speedograph Richfield for example does those little gearboxes. Link to comment Share on other sites More sharing options...
JohnD Posted June 16, 2015 Share Posted June 16, 2015 American cars have for long used a different drive cog on the output shaft of the gearbox to match the final drive ratio. Change that and you only need to change the cog. Good Old British Engineering does the opposite, and the different cog ratio to match the final drive is inside the speedo. So that has to be changed. The only people with stocks of the different cogs and the skills to change them are specialist instrument dealers. Speedograph has been mentioned, also Speedy Cables: http://www.speedycables.com/calibration.html John Link to comment Share on other sites More sharing options...
aoie Posted June 16, 2015 Author Share Posted June 16, 2015 Changing the gear (cog) at the transmission apperas to be the way to go, however i can't get it straight in my mind how the gear will still mesh with the worm gear inside the transmission. Does the different cog tooth size patterns available for the transmision side somehow all match up without changing the internal worm gear driving the cog inside the transmission? just need someone to veryify that I can change the drive cog tooth pattern without touching/replacing the internal worm gear! Andy Link to comment Share on other sites More sharing options...
JohnD Posted June 16, 2015 Share Posted June 16, 2015 Was I not clear, aoie? American cars were designed so that the cog on the gear box could be changed to suit the final drive. UK cars, inc. Triumphs were not. So you can't on a Spitfire. John Link to comment Share on other sites More sharing options...
aoie Posted June 16, 2015 Author Share Posted June 16, 2015 John i guess you just need to bang it into my brain to set me on the correct path! Apparently I’m slow, confused, or missing the concept completely: because what you’re saying conflicts with the information I gathered from the internet! So why do I find all this information on different color triumph gears matching up with known Triumph differential ratios (Yes just like American cars!!!. But with American cars somtimes the different gear colors require different plug-in carrier housings too correct for size differences in the gears (offset corrections). Example below are Triumph transmission speedo gears I can order from Rimmer Brothers, Canleys etc. along with the color and differentail ratio match . Color, Tooth Count, Part number, differential they are supposed to match - Orange 20 219001 diff - 3.27 Green 21 219002 diff - 3.45 Red 22 219003 or 219003A diff - 3.63 Black 23 219004 Blue 24 219005 diff - 3.89 White 25 219006 diff - 4.11 Andy Sun prairie, WI Link to comment Share on other sites More sharing options...
GT6Steve Posted June 16, 2015 Share Posted June 16, 2015 aoie, I likely have the output gear you need in the drawer. I always pull them out when building a race box and have never explored this in detail although I too assumed just as you are it was for calibration. (A Yank too). John's usually right and likely is this time but I'll have a look. I've got a bunch of old speedos as well so see if you can dig up the ID number on the face that suits a 3.27 diff in a GT6... Steve in Las Vegas Link to comment Share on other sites More sharing options...
Nick Jones Posted June 16, 2015 Share Posted June 16, 2015 There are two ways round this and I think Triumph has used both at different times. I may have this back to front, but I think that earlier cars tended to have specific speedo gearing and you'll see that the turns per mile (TPM) figures printed small on the face of the speedo vary between all the different variants of similar cars, whereas later they rationalised the speedo choices and varied the drive gears instead so newer cars all tend to have the same TPM on the speedo (1000 is a common one) As far as I know the same nylon gear in the gearbox output shaft works with a whole bunch of different pinions - not quite sure how..... My cheating method in the past has been to pick the speedo from the Triumph with the same gearing combination I was using - the Vitesse speedo has the internals from a Dolomite 1850 speedo to match the 3.63 diff/1850OD box it used to have. It was spot on then and still is due to the blind luck of the Toyota W58 'box having exactly the same speedo gearing as the Dolomite 'box. Nothing wrong with being lucky once in a while! This is a most helpful link (I find) http://www.dbraun99.com/technical%20index/Calculating%20TPM%20of%20your%20speedometer%20cable.pdf I've used methods 3 & 4 in the past. Nick Link to comment Share on other sites More sharing options...
JohnD Posted June 16, 2015 Share Posted June 16, 2015 (edited) aoie, Andy, Well knock me down with a feather - I might be wrong! Sorry if I misled you. I've never seen those different coloured gears before, either, so what do I know? But I don't see them now, just a white one on Rimmer's: http://www.rimmerbros.co.uk/ItemSearch--search-Speedometer-Gear--srcin-1. That's the 'worm that goes on the output shaft, and thnk that's a constant. I did find this page: http://www.rimmerbros.co.uk/ItemSearch--search-Speedo-Drrive-Pinion--srcin-1 but only the brick-red, bright-red and white are for Spitfires. Where did you find the list you posted? John Edited June 16, 2015 by JohnD Link to comment Share on other sites More sharing options...
aoie Posted June 16, 2015 Author Share Posted June 16, 2015 John The list I posted came from a forum of TR7 and TR8 owners (The Wedge Shop) that apparently use the same Triumph gear part numbers as the Spitfires and GT6. What set my down this path was an old listing/post selling a GT6 3.27 differential along with an orange 20 (P/N 219001) gear to adjust your speedo! Doing a google part search for 219001 shows it's obsolete but I can purchase a used one from Canley's. Not sure the price plus shipping was worth doing without first verifying it would work. Since I plan to upgrade the differential to a slightly higher ratio R160 LSD in the near future. I was only looking for a cheap temporary solution for the speedo to avoid a speeding ticket. If purchasing a ratio adapter from a speedometer repair shop works out cheaper I’ll try that route. Alternatively if someone else is following this discussion and knows what it takes to upgrade my speedometer to a digital Smiths (classic style) were I just push a button at 60 MPH to calibrate whenever I choose to alter my drive train, I would really be open to that discussion. Andy Link to comment Share on other sites More sharing options...
JohnD Posted June 17, 2015 Share Posted June 17, 2015 I found the Wedge Shop site - not easy to search for parts as it doesn't split searches into models or even marques. I got all sorts from Starter Rings to clutch sets in response to "Speedometer drive gear". And the Wedge Shop Forum name is now for sale and not accessible from the Shop's own site. A quick, dirty, cheap and temporary solution would be to mark the glass of your speedo with the new scale, perhaps on a ring of tape. Or less cheap, to use a small GPS speedo. What model Spitfire is yours? And what mods? AND, doing some research into this, I find that Minis also used pinion gears, colour coded to indicate the number of teeth, to match different final drive ratios. What I said above, about US cars using this system but not UK, was complete bollocks! No idea where I got it from - apologies all around JOhn Link to comment Share on other sites More sharing options...
aoie Posted June 17, 2015 Author Share Posted June 17, 2015 John My Spitfire is a 72 MK4 USA. 4.11 differential that had to go! In addition to the pinon gear info from the Wedge Shop forum I also found a nice white paper on Smith/Jaeger Speedo repair and calibration. http://www.triumph.daveola.com/NOTES/Speedo_Repair.pdf Your suggestion to mark my current speedo would be best for now (free). I believe some of my local club members have GPS rally equipment I hopefully can borrow for accurate markings. Other drive train mods:1) 3.27 differential, this was a freebie that came on a GT6 frame I purchased for the Roto-Flex and front brake/suspension parts to upgrade my Spitfire rear end (current project), and my front suspension and brakes (done). 2) 14x5.5 Miata daisy Rims with 4x100 bolt pattern. This would also affect my speedo but it appears my tire choice is only ½†bigger in diameter than the original 13†wheels (on paper). Current wheel diameter is 22.5â€. I’m happy with my current tire choice so I think this will not change in the future. 3) (Near Future) upgrade to a R160 LSD with a gear ratio in the 3.70 vicinity. 4) (Way Future) upgrade transmission to a 5 Speed AndySun Prairie, WI Link to comment Share on other sites More sharing options...
Spit131 Posted June 17, 2015 Share Posted June 17, 2015 (edited) . Edited June 17, 2015 by Spit131 Link to comment Share on other sites More sharing options...
JohnD Posted June 17, 2015 Share Posted June 17, 2015 Aaaaah! 14" wheels. Check your calcs with the Mighty MintyLamb Tyre Compar-o-rama: http://www.mintylamb.co.uk/?page=tyre.htm John Link to comment Share on other sites More sharing options...
zetecspit Posted June 17, 2015 Share Posted June 17, 2015 Do a search on here for GPS speedo. Chris and somebody else have bought them (from the USA I think!) Speedhut? Link to comment Share on other sites More sharing options...
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