Guy Lowe Posted January 9, 2013 Share Posted January 9, 2013 This may seem a simple question but I just cannot get my head round how worm drive gearing is calculated. My speedo has always over read by about 10% however now it has stopped altogether because the worm drive has gone for a walk down the output shaft. Gone to order a new one and Caterham only list a 7 tooth (start) one, the one I have just removed from my box is an 8 tooth. So if I fitted the 7 tooth would this decrease or increase the speedo reading Guy Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted January 9, 2013 Area Representative Share Posted January 9, 2013 The seven tooth gear will give a lower speedo reading. I've never actually looked at one of these, but it looks like, for one rev of the output shaft, these gear will pick up 7 or 8 teeth of the driven gear. Therefore, the 7 tooth gear will give a speedo reading of 87.5% that of a 8 tooth gear. To confuse the issue further, there have been various driven gears available with 22,23,24,25 teeth..... Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted January 9, 2013 Area Representative Share Posted January 9, 2013 Guy, what year is your car? If it's pre around 2001, then it'll be a mechanically driven speedo. If it's around an electronic speedo, then there may well be switches on the back of the speedo that can be adjusted. My 2002 car has just two settings, but, I understand later cars are adjustable over a much greater range. Link to comment Share on other sites More sharing options...
Guy Lowe Posted January 9, 2013 Author Share Posted January 9, 2013 Richard It is a mechanical speedo and has a BLACK speedo drive gear. Guy Link to comment Share on other sites More sharing options...
zetec Posted January 9, 2013 Share Posted January 9, 2013 Ask james whiting,i know hes got some & understands how it all works etc Link to comment Share on other sites More sharing options...
Guy Lowe Posted January 11, 2013 Author Share Posted January 11, 2013 Thank you Richard, I also had a couple of emails agreeing with you, so I have ordered 7 tooth drive. Guy Link to comment Share on other sites More sharing options...
irrotational Posted January 11, 2013 Share Posted January 11, 2013 Hmm - mine's a 97 car and over-reads by 10%...I wonder if that's what I need.... Link to comment Share on other sites More sharing options...
6speedmanual Posted January 11, 2013 Share Posted January 11, 2013 With all those combos available, just set up a simple calculation spreadsheet of the spiral v the driven gears and find out which one will drive your speedo cable at the desired reduced speed to make it accurate. Peter Link to comment Share on other sites More sharing options...
Guy Lowe Posted January 11, 2013 Author Share Posted January 11, 2013 After speaking to Caterham and R & R yesterday, I'm pretty sure that the black nylon drive together with the 7 start worm drive is the way to go with 3.92 diff and 13 inch wheels. Caterham say they have only ever supplied 5 speeds with the 7 start worms, that's the reason it's the only one they list on their online store. Oh, and one of the receipts I was given when I purchased my car was from Arrowstar for replacing the gearbox only six months after the cars first registration. Think I'll go for the 7 start 😬 Guy Edited by - Guy Lowe on 12 Jan 2013 11:38:18 Link to comment Share on other sites More sharing options...
jackb_ms Posted January 24, 2013 Share Posted January 24, 2013 I had my first gear change to a longer ratio and since then the speedo over reads by 20%+. It's very good a scaring my passenger when they are looking at the speed we are doing but otherwise useless. It's a 1996 car with a mechanical speedo. Is there something i can do to rectify it? Jack Link to comment Share on other sites More sharing options...
Mankee Posted January 25, 2013 Share Posted January 25, 2013 BGH also supplied me with a 7-tooth speedo worm gear, so I reckon it's got to be the way to go in our little cars. Caterham only list 22- and 24-tooth drive gears, presumably for the Sierra 3.62 and 3.92 diffs? But it all goes out of kilter with BMW diffs and when we start messing with tyre sizes anyway. Burton Power list a 23-tooth gear and you can normally find a few kicking about on eBay. Link to comment Share on other sites More sharing options...
Paul Deslandes Posted January 25, 2013 Share Posted January 25, 2013 changing the first gear or any other gearbox ratio, shouldn't affect the speedo reading which is taken off the output shaft. The only changes that will affect the reading are the worm gear and speedo drive pinion ratio, the diff ratio and tyre circumference, plus of course the speedo calibration. Link to comment Share on other sites More sharing options...
jackb_ms Posted January 25, 2013 Share Posted January 25, 2013 I didn`t change any other thing apart from the first gear ratio. Could it be an issue with the cable? Jack Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted January 25, 2013 Area Representative Share Posted January 25, 2013 Quoting jackb_ms: I didn`t change any other thing apart from the first gear ratio. "You" many not have changed anything, but the gearbox will have been in bits to change the ratios. Maybe you havn't actually got as many of your original bits as you think.... Quoting jackb_ms: Could it be an issue with the cable? No. It'll be the speedo drive ratio! The speedo drive is driven off the output shaft of the gearbox, so only changes of diff ratio or wheel overall diameter will affect the speedo reading. Link to comment Share on other sites More sharing options...
Paul Deslandes Posted January 25, 2013 Share Posted January 25, 2013 Speedo cable issues normally lead to no reading at all or a jerky needle. If its steady then it has to be something else. Did you do the gear swap yourself? Changing any gear ratios would have necessitated stripping the box, so its possible that whoever rebuilt it either changed the speedo drive worm and/or pinion gear for some reason. Speedy Cables will recalibrate your speedo. You'll need to do some measuring by jacking the car up, disconnecting the speedo and counting the number of turns of the cable to turns of the rear wheels. Sounds more difficult to do than it is. They'll tell you what you need to do. Paul Edited by - Paul Deslandes on 25 Jan 2013 10:16:21 Link to comment Share on other sites More sharing options...
Guy Lowe Posted February 3, 2013 Author Share Posted February 3, 2013 Well at long last the car is finished and it's not raining so out I go with my son holding a Holux, the result is at 80 mph showing on the speedo the Holux reads 78mph, bloody marvellous So after 10 years of over reading it is now fixed, so should I now wind the speedo back 10% (2,900 miles) to show the cars true mileage 😬 Link to comment Share on other sites More sharing options...
.thumb.jpg.657e9c400f3ec3cc580ef7cc616e7bba.jpg)
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now