Bobtaylor Posted June 29, 2020 Report Share Posted June 29, 2020 I have a Mk1 Vitesse non overdrive. When driving at 40mph the rev counter is on 3000 (give or take). This means that IF I was to do 80mph I would be red lining way before the theoretical maximum speed. Is this right? Cheers, Bob Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 29, 2020 Report Share Posted June 29, 2020 its not uncommon for rev counters to be out of calibration , suffer the same problems as speedos ( the internals are pretty identical designs it should have 3.55:1 on the face can you check it with anything with a known rpm , or another car. peter Link to comment Share on other sites More sharing options...
RichardS Posted June 29, 2020 Report Share Posted June 29, 2020 Hi Bob I assume that you are talking about top gear? If so, 3000 revs at 40 mph must be wrong although I know nothing about Vitesses specifically. I bought a laser tacho gun for less than £15 on eBay which I use to check tachometers etc. Something like that might be helpful? https://www.ebay.co.uk/itm/DT2234C-Digital-Laser-Rev-Counter-Meter-Non-contact-Tachometer-Rev-Counter-uk/193464331326?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D200495be9ae84930b0c795ce8e1d9134%26pid%3D100675%26rk%3D3%26rkt%3D15%26mehot%3Dnone%26sd%3D181209692912%26itm%3D193464331326%26pmt%3D1%26noa%3D1%26pg%3D2380057%26brand%3DUnbranded&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3Ad4ea48df-b9e3-11ea-a3e5-d6019379fa4d|parentrq%3Aff39ca021720a9bdd3dbc764ffe96628|iid%3A1 Richard Link to comment Share on other sites More sharing options...
Iain T Posted June 29, 2020 Report Share Posted June 29, 2020 Bob, If you want to get the rev counter, or any other, instruments refurbished or calibrated try Speedograph Richfield 0115 926 4235. Iain Link to comment Share on other sites More sharing options...
Colin Lindsay Posted June 29, 2020 Report Share Posted June 29, 2020 Or, if you can, get hold of a little stand-alone rev counter - you can buy them for just over a tenner online - and wire it in paralell as a test of your original unit. Link to comment Share on other sites More sharing options...
Iain T Posted June 29, 2020 Report Share Posted June 29, 2020 Colin, you can do that to test it but it won't solve the problem. I wouldn't have thought the dizzy drive is the issue just the tacho which needs some 'fairy dust' over it. For the ardent diyer I think there is a post on the forum about adjusting the tacho. Iain Link to comment Share on other sites More sharing options...
Colin Lindsay Posted June 29, 2020 Report Share Posted June 29, 2020 Sorry, didn't realise that we had established the tacho as being the problem; was thinking about gearing etc. Link to comment Share on other sites More sharing options...
Bobtaylor Posted June 29, 2020 Author Report Share Posted June 29, 2020 It does say 3.55:1 on the front of the tacho. When you start the car the needle is all over the place, on tick over it does settle down but when driving the needle has a mind of its own as the engine revs are constant the needle does settle down but it seems over sensitive. I unscrewed the cable at the dizzy end and it turns freely, so I am guessing that there is a problem with the taco innards. As a matter of interest what would you expect the revs to be at 40mph in 4th without overdrive, I am not sure how to work it out. As far as I am aware it is a standard diff. Link to comment Share on other sites More sharing options...
johny Posted June 29, 2020 Report Share Posted June 29, 2020 Depends also on your tyres but if everything is standard it should be doing just under 2000rpm at 40mph. Link to comment Share on other sites More sharing options...
NonMember Posted June 29, 2020 Report Share Posted June 29, 2020 8 minutes ago, johny said: Depends also on your tyres but if everything is standard it should be doing just under 2000rpm at 40mph. That would be for overdrive (or a non-OD GT6 in 4th). A Vitesse should have a 3.89:1 diff so in direct 4th at 40mph I'd expect ~2400RPM Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 29, 2020 Report Share Posted June 29, 2020 it may just have some oil on the drive disc in the head i would remove the head . turn the rim to release the glass , take out the rear screws and give the discs a squirt of brake cleaner then re test pete Link to comment Share on other sites More sharing options...
NonMember Posted June 29, 2020 Report Share Posted June 29, 2020 4 minutes ago, Pete Lewis said: turn the rim to release the glass , This step is frequently less easy than it sounds. Otherwise, I think Pete's likely spot on the money. Link to comment Share on other sites More sharing options...
johny Posted June 29, 2020 Report Share Posted June 29, 2020 53 minutes ago, NonMember said: That would be for overdrive (or a non-OD GT6 in 4th). A Vitesse should have a 3.89:1 diff so in direct 4th at 40mph I'd expect ~2400RPM Yes sorry I used the richyrichracing site but left in the wrong diff ratio😳 Comes out 2300rpm on 155/80/13 tyres... Link to comment Share on other sites More sharing options...
Neil Clark Posted June 29, 2020 Report Share Posted June 29, 2020 Is it electronic ignition? Made the rev counter on one of my old cars go bananas until an electronic gizmo was fitted. Link to comment Share on other sites More sharing options...
NonMember Posted June 30, 2020 Report Share Posted June 30, 2020 Electronic ignition can do weird things to some of the 1970s electronic tachos but the Vitesse has a mechanical one. I can't see how EI would affect that, short of going EDIS and losing the distributor, but if the OP had done that I think we'd have been told Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 30, 2020 Report Share Posted June 30, 2020 as its got 3.55 on the face its pretty good that its .......mechanical apart from oily drive discs i have seen the needle spindle bearing worn out that allows disc to disc contact as it waves around inside not a diy fix , its replace or send to speedy cables etc for a refurb Pete Link to comment Share on other sites More sharing options...
Bobtaylor Posted July 9, 2020 Author Report Share Posted July 9, 2020 Thanks for all the advice. I decided to buy a recon tacho which I have now fitted and all is well. I now have a spare on which I will try the above advice to fix it. Cheers Bob Link to comment Share on other sites More sharing options...
euan douglas Posted July 13, 2020 Report Share Posted July 13, 2020 Quote I've been reading this topic with interest so perhaps someone could enlighten me regarding my car, MK3 GT6. When going at 60mph in overdrive 4th, the tacho reads 2000 rpm. The tacho has 3.55 on it and the speedo has 6211/OOS, or 005, 980. What the diff ratio is I have no idea but here's the question- Is the speedo reading correctly? Is the speedo the wrong one for the O/D - perhaps the car did not have O/D fitted as standard, who knows? I'd be grateful for any thoughts, however I'm not likely to do anything about it, just curious! Running on standard wheels and tyres. Euan Link to comment Share on other sites More sharing options...
euan douglas Posted July 13, 2020 Report Share Posted July 13, 2020 Sorry, pushed the wrong button last time I tried to post! I've been reading this topic with interest so perhaps someone could enlighten me regarding my car, MK3 GT6. When going at 60mph in overdrive 4th, the tacho reads 2000 rpm. The tacho has 3.55 on it and the speedo has 6211/OOS, or 005, 980. What the diff ratio is I have no idea but here's the question- Is the speedo reading correctly? Is the speedo the wrong one for the O/D - perhaps the car did not have O/D fitted as standard, who knows? I'd be grateful for any thoughts, however I'm not likely to do anything about it, just curious! Running on standard wheels and tyres. Euan Link to comment Share on other sites More sharing options...
johny Posted July 13, 2020 Report Share Posted July 13, 2020 Hi Euan, the speedo reading is unaffected by the OD as it takes its drive from the output of the gearbox. All that OD does is drop the engine revs at any given speed but obviously your speed is unchanged. A different ratio of the diff will change your speed at any given revs and so give an incorrect speedo reading. On the underside of the diff there should be a number hand stamped lengthways on the front casing. It starts with two letters and, if the internals haven't been changed, indicates the ratio and model of car it's from... Link to comment Share on other sites More sharing options...
johny Posted July 13, 2020 Report Share Posted July 13, 2020 It does sound like there's an error somewhere as I think a standard GT6 should be doing 42mph at 2000rpm. Even with the highest ratio diff available it would still only be 50mph.... Link to comment Share on other sites More sharing options...
JohnD Posted July 13, 2020 Report Share Posted July 13, 2020 Indeed. The engine to road speed ratio for the available diffs is as follows: 4.55:1 (Courier only, AFAIK) 14,5mph/1000rpm (there was a 4.875:1 on very early Heralds) 4.11:1 (Herald 1200, Vitesse Mk1, Spitfire Mk 1,2,3) 16mph/1000rpm 3.89:1 (Vitesse Mk2, Spitfire Mk 4, GT6 Mk3) 17.1mph/1000rpm 3.63:1 (Spit 1500) 18mph/1000rpm 3.27:1 (GT6 Mk1, 2 non-overdrive) 20.15mph/1000rpm 3000rpm at 40mph is 13mph/1000rpm, so somethings not right, somewhere. John Link to comment Share on other sites More sharing options...
NonMember Posted July 13, 2020 Report Share Posted July 13, 2020 15 minutes ago, JohnD said: 4.55:1 (Courier only, AFAIK) 14,5mph/1000rpm (there was a 4.875:1 on very early Heralds) I believe the 4.55:1 was also used on 948 Heralds when fitted with twin carbs (some coupes/convertibles?). 17 minutes ago, JohnD said: 4.11:1 (Herald 1200, Vitesse Mk1, Spitfire Mk 1,2,3) 16mph/1000rpm Vitesse 1600 only. Mk1 2L was 3.89:1 And for bonus pedantry, the US market 1500 Spitfire used a 3.89:1 (I forget whether the US Mk4 1300 had a unique diff) Link to comment Share on other sites More sharing options...
JohnD Posted July 13, 2020 Report Share Posted July 13, 2020 I KNEW, I just KNEW, that someone would call me out for not getting the list of models and their diffs not absolutely, perfectly and completely correct! I COULD NOT CARE LESS! Goodness knows why I did it, I struggle NOT to give way to OCD when working on the car - that tiny blemish on the underside of the exhaust silencer will NEVER be noticed. By anybody. Here, I was only concenred to supply people with the ratios of speed to rpm. Lesson to self. DO NOT provide more information than is needed! EVER! John 2 Link to comment Share on other sites More sharing options...
RichardS Posted July 13, 2020 Report Share Posted July 13, 2020 21 minutes ago, JohnD said: I KNEW, I just KNEW, that someone would call me out for not getting the list of models and their diffs not absolutely, perfectly and completely correct! John Dontcha just love internet forums? 😉 Richard Link to comment Share on other sites More sharing options...
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