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Speedometer calibration

csm1406
Posts: 41
Joined: Sat Feb 13, 2016 6:06 pm
Location: west sussex

Speedometer calibration

Post by csm1406 » Mon Jul 22, 2019 5:05 pm

Hi
I now have a steady speedometer - first time in 39 yrs. I have not bothered about accuracy in the past due to its erratic nature - now I would like to have it recalibrated. At 75 mph it reads 90 - checked against a mobile phone app. Does anyone know how to adjust it ?.
Thanks !
Cameron

classiclife
Posts: 370
Joined: Wed Sep 05, 2018 9:55 am
Location: Ridgewood - East Sussex
Contact:

Re: Speedometer calibration

Post by classiclife » Mon Jul 22, 2019 6:18 pm

Hello Cameron,

It's not a DIY job, if you want it done properly, in my opinion.

Richfield Speedograph do speedo recalibration - in fact I need to get mine done after I have concluded my manual conversion project.

I have attached the guidance form from the company to outline what is required and also their website - you will also see their speedo recalibration tag on the front page.

http://www.speedograph-richfield.com/

Hope that helps ??

Regards.

Richard.
Attachments
Speedo Calibration Form.odt
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1968 Daimler V8-250 Saloon
DLOC East Sussex Area Representative.

Southern Classics Society Events Co-ordinator.
www.southernclassics.org.uk

Ian Slade
Posts: 456
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: Speedometer calibration

Post by Ian Slade » Tue Jul 23, 2019 4:43 am

Depends on your tyre size, 165 R15 will over read 185 R15 will under read, 185/70 R15 is about right. the speedometer works on a magnetic slip via a fixed gear ratio.
Owner since the 70's, Genghis is slightly to my left.

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John-B
Site Admin
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Re: Speedometer calibration

Post by John-B » Tue Jul 23, 2019 6:33 am

Ian Slade wrote:
Tue Jul 23, 2019 4:43 am
Depends on your tyre size, 165 R15 will over read 185 R15 will under read, 185/70 R15 is about right. the speedometer works on a magnetic slip via a fixed gear ratio.
When my speedo registered 134 mph in 1970 downhill on the M4 and the Dart's maximum is 125 mph and I had Avon 185 x 15 x ? 90 tyres which you say under-read, I must have been going faster than 134 mph! :D Amazing. I chickened out at that speed because I was zipping past other traffic and I doubt that they knew I was there.

I sold the car in 1973 but when I bought it back in 2013 the rear tyres were still the 1970 Avons! How's that for longevity.

Ian Slade
Posts: 456
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: Speedometer calibration

Post by Ian Slade » Tue Jul 23, 2019 8:37 am

I was accused by a police sergeant in a Ford van of doing over 85mph on 185 R15 tyres with the rev counter reading 3500rpm or just over 70mph, as he didn't have a calibrated speedo, I argued and walked away without any more action, but he was probably right. I always worked on the rev counter more accurate, 6000rpm is just over 120 mph and only the lighter cars could make that on the flat i.e pre 1962. though an early A spec could make 6500rpm in top, my early B spec could make 7000rpm in third on 185 R15's
Owner since the 70's, Genghis is slightly to my left.

migray
Posts: 123
Joined: Fri Feb 26, 2016 10:31 pm
Location: Bakewell

Re: Speedometer calibration

Post by migray » Tue Jul 23, 2019 8:58 am

There is a useful article about speedometer refurbishment and calibration here http://obswww.unige.ch/~wildif/cars/doc ... repair.pdf

Christopher Storey
Posts: 253
Joined: Wed Mar 16, 2016 6:40 pm
Location: Cheshire

Re: Speedometer calibration

Post by Christopher Storey » Tue Jul 23, 2019 9:49 am

Before you go to a lot of trouble and expense, check the speedo by motorway metre posts, rather than by satnav apps which may not always be accurate . At 60 mph true, = 96 kph , 2kms should take almost exactly 75 secs . If it is more than this, your speedo is overreading , and vice versa

csm1406
Posts: 41
Joined: Sat Feb 13, 2016 6:06 pm
Location: west sussex

Re: Speedometer calibration

Post by csm1406 » Tue Jul 23, 2019 5:58 pm

Hi
Thanks to all for input - I now understand a bit more & as I know speedo is + 15 roughly - I will use this to calculate speed in future.

Thanks again.
Cameron

simonp
Posts: 362
Joined: Mon Mar 07, 2016 9:59 am
Location: Birmingham

Re: Speedometer calibration

Post by simonp » Tue Jul 23, 2019 11:12 pm

Had my SP speedo recalibrated at Speedograph Richfield when the overdrive was fitted. Accuracy is spot on.

SimonP
Daimler SP 250 - "To feel its eager response as you open up is to know a new motoring adventure"(Sales brochure) The adventure continues!

Ian Slade
Posts: 456
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: Speedometer calibration

Post by Ian Slade » Wed Jul 24, 2019 7:12 am

It is reasonably easy to check the real speed by using a digital tachometer and looking up the rolling circumference or diameter of the tyre, or checking by marking the tyre and the ground them moving the car one rev of the wheel, measure that distance in feet, the gearbox ratio in top is 1-1 the differential is 3.58-1. therefore rpm/3.58 x the distance covered in feet will give you ft/min, 30mph is 44ft/sec or 1mph =1.4666' ft/sec, Simples
Last edited by Ian Slade on Wed Jul 24, 2019 11:08 am, edited 1 time in total.
Owner since the 70's, Genghis is slightly to my left.

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