James.Burton Posted September 4, 2016 Share Posted September 4, 2016 I just picked up my first 7 On the way home, two officers in blue helped me find out that the speedo is reading at least 20% too slow. Luckly in Austria they are humane and it was only a €20 on the spot fine.I guess somebody must have changed the final drive at some stage. Being a 150bhp Euro spec HPC, I guess a previous owner decided acceleration was more relevant than top speed and this was cheaper than upgrading the motor...Is there an easy fix to adjust the speedo?Are there other things I should be concerned of with such a change of the final drive ration? Will it damage or wear parts like the gearbox...?James Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted September 4, 2016 Member Share Posted September 4, 2016 Is the speedometer electronic or mechanical? They can both be changed, but one's easier than the other...What's your current final drive? You can measure that from an accurate speedometer (such as a GPS device, which might be a smart 'phone) and rev counter and tyre size, or by turning the prop shaft by hand and counting.Five or six speed gearbox?JonathanPS: You can also use a GPS device permanently as the speedometer... Link to comment Share on other sites More sharing options...
James.Burton Posted September 4, 2016 Author Share Posted September 4, 2016 Thanks- The speedo is old school mechanical. I bet that's the harder fix.- 5 speed box (ford but the engines Vauxhaul..).- I'll do the gps revs and tyres maths as soon as the rain stops.i know it's a over 20% out as the 510km trip home only brought up 390km on the mileage.James Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted September 5, 2016 Member Share Posted September 5, 2016 Harder, but not insuperable.I'd talk to one of the speedometer repairers, but of course don't do anything until you've decided where you want to end. I have the gear combinations if anyone's interested.In the meantime I'd mark the gauge at the crucial speeds with Letraset or similar and/or stick a smart 'phone on the dash with an app that shows speed. How about Navmii?Jonathan Link to comment Share on other sites More sharing options...
Alastair B Posted September 5, 2016 Share Posted September 5, 2016 James, you've presumed the final drive ratio has changed, this indeed might be correct, but it could also be the driven gear (to the speedo cable) from the gearbox could be the wrong one as there are different versions with different numbers of teeth. A search of the archives will find topics about this. You might be able to compensate for your current final drive ratio with a new speedo drive gear. Link to comment Share on other sites More sharing options...
James.Burton Posted September 5, 2016 Author Share Posted September 5, 2016 Thanks guys. Great tips.I'll check the current ratio, then look into changing the speedo drive gear.Navi as speedo in the meantime. Link to comment Share on other sites More sharing options...
Paul Deslandes Posted September 8, 2016 Share Posted September 8, 2016 Caerbont will recalibrate your mechanical gauge although you obviously have to sent it to them in Wales for it to be done. They need to know the ratio of rear wheel turns to speedo cable turns which is not too hard to detirmine, as follows:Jack up the rear of the car so you can turn both rear wheels,Undo the speedo cable from the back of the instrument and put a piece of masking tape onto the cable inner so it's like a flag.Mark one of the rear wheel rims with a bit of tapeRotate rear wheels, making sure they rotate equally together, counting the cable rotations at the same time, until they have both rotated a complete number of turns.Divide one count by the other, and you have the ratio.Repeat a couple of times to make sure.Tell Caerbont what the counts were.ORBuy a bike or GPS speedoPaul Link to comment Share on other sites More sharing options...
BlackSeven Posted September 9, 2016 Share Posted September 9, 2016 If the final drive ratio had been changed to improve acceleration would that not make the speedo over read rather than under read? Link to comment Share on other sites More sharing options...
James.Burton Posted September 9, 2016 Author Share Posted September 9, 2016 Well a bit of field research shows 4000 rpm, pulling 136kmph, on 205/60R13 A048 => which on my maths comes up to a final drive ratio of roughly 3.1 And I have no idea if that is even possible.BlackSeven - I had assumed any change would be for acceleration. I think you are right. Maybe these are non standard tyre diameters... Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted September 9, 2016 Member Share Posted September 9, 2016 Whence did you get the wheel circumference: wheel size and tyre profile, or a published value? Yokohama quote "Overall diameter 573mm". ..."I've been told the two rare diffs are 3.38 and 3.14"....136kmph"It's borderline on the simulator, Captain. I cannae guarantee that she'll hold up!" ;-)Jonathan Link to comment Share on other sites More sharing options...
James.Burton Posted September 9, 2016 Author Share Posted September 9, 2016 I measured 180cm. Fits with a diameter of 573mm. Hurrah! Link to comment Share on other sites More sharing options...
Paul Deslandes Posted September 9, 2016 Share Posted September 9, 2016 Most accurate way is to put the car on a reasonably flat floor or road, mark the road and tyre where they make contact, roll the car a couple of wheel revolutions, mark the road again and measure the distance between the road marks and divide by the number of wheel revs. Link to comment Share on other sites More sharing options...
James.Burton Posted September 9, 2016 Author Share Posted September 9, 2016 So I could be the happy owner of a rare 3.14 ratio.I think that would explain the 20% difference on speedo readings, as I seem to pick up indications that 3.9 is a more likely standard.I'll see if I can get confirmation...Might be clever to change the diff, than to try and fix the speedo... Link to comment Share on other sites More sharing options...
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