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Speedometer calibration
Posted: Mon Jul 22, 2019 5:05 pm
by csm1406
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
Re: Speedometer calibration
Posted: Mon Jul 22, 2019 6:18 pm
by classiclife
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.
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 4:43 am
by Ian Slade
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.
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 6:33 am
by John-B
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!
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.
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 8:37 am
by Ian Slade
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
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 8:58 am
by migray
There is a useful article about speedometer refurbishment and calibration here
http://obswww.unige.ch/~wildif/cars/doc ... repair.pdf
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 9:49 am
by Christopher Storey
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
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 5:58 pm
by csm1406
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
Re: Speedometer calibration
Posted: Tue Jul 23, 2019 11:12 pm
by simonp
Had my SP speedo recalibrated at Speedograph Richfield when the overdrive was fitted. Accuracy is spot on.
SimonP
Re: Speedometer calibration
Posted: Wed Jul 24, 2019 7:12 am
by Ian Slade
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