Author Topic: Speedo accuracy  (Read 456 times)

Kremmen

  • Approved Member
  • *
  • Posts: 1315
  • Country: england
  • Civinfo interloper
  • Fuel economy:
  • My Honda: MY22 Jazz EX
Speedo accuracy
« on: September 16, 2021, 04:40:58 PM »
The Jazz speedo is a lot more accurate than my old Civic.

I use cruise in the 50mph and 60mph M4 average speed roadworks.

On the Civic I set cruise to 53mph and 63mph respectively which resulted in my phone, dashcam and Garmin to all read exactly 50mph and 60mph GPS.

On the Jazz, to achieve 50mph and 60mph GPS the speedo is reading 51mph and 61mph.
Let's be careful out there!

sportse

  • Approved Member
  • *
  • Posts: 244
  • Country: gb
  • Fuel economy: 63.9 MPG
  • My Honda: 21 EX Platinum White Pearl & Grey Leather
Re: Speedo accuracy
« Reply #1 on: September 16, 2021, 05:22:49 PM »
Mine's also pretty accurate at around just under 2mph out. The loan SR I had was also only about 1mph out.

The worst cars I had for accuracy were 2 Toyota hybrids - they were 10% out so at a real 70mph the speedo would be reading 77mph.

I complained to Toyota and asked if there was any way to calibrate my car better, but was told it was within tolerance and nothing could be done.

It's probably why some Toyota hybrids got a bad reputation for being slow - the drivers were thinking they were at the speed limit, but were actually quite a bit under compared to more accurate speedos.

edam

  • Approved Member
  • *
  • Posts: 285
  • Country: 00
  • My Honda: 2015 1.3 SE CVT
Re: Speedo accuracy
« Reply #2 on: September 16, 2021, 05:45:33 PM »
Speedo accuracy is dependent on tyre wear.
lets try to get this right
When you have new tyres your speedo could be underreading but as the tyres wear then the speedo could be overreading.
Also speedos always heve to overread your true speed but by how much depends on your tyre wear
 
« Last Edit: September 16, 2021, 06:11:00 PM by edam »

Neil Ives

  • Approved Member
  • *
  • Posts: 314
  • Country: gb
  • The day the Internet went down
  • My Honda: Jazz Hybrid EX 2022
Re: Speedo accuracy
« Reply #3 on: September 16, 2021, 06:37:23 PM »
Most speedo's I've checked against a GPS* have over-read by about 10%. Real 70mph typically reads as 77mph on the speedo. I reckon it flatters the cars performance and also saves a few speeding tickets, but it's annoying.

My old motorcycle has a Chronometric speedo. I needed to have it recalibrated after changing the gear ratios so I know it is now accurate.

*GPS is very accurate while the vehicle is driving on a flat road.
« Last Edit: September 16, 2021, 06:41:13 PM by Neil Ives »
Neil Ives

John Ratsey

  • Approved Member
  • *
  • Posts: 2226
  • Country: gb
  • My Honda: 2020 Jazz Crosstar
Re: Speedo accuracy
« Reply #4 on: September 16, 2021, 06:46:25 PM »
I reckon the Mk. 4 Jazz speedo reads +1.5 mph across the whole speed range when the tyres are new. With the system being digital it's easy to add a fixed amount of extra mph (so the driver can't blame the vehicle) whereas with an analogue system it is easiest to adjust the gearing to give a proportional increase. The most optimistic vehicle I've driven was a Kia I bought 20 years ago which was about 10% high. My various Hondas have been less optimistic in terms of speed but the Mk 4 Jazz has the least discrepancy.

culzean

  • Approved Member
  • *
  • Posts: 7349
  • Country: england
Re: Speedo accuracy
« Reply #5 on: September 16, 2021, 07:17:06 PM »
Most speedo's I've checked against a GPS* have over-read by about 10%. Real 70mph typically reads as 77mph on the speedo. I reckon it flatters the cars performance and also saves a few speeding tickets, but it's annoying.

My old motorcycle has a Chronometric speedo. I needed to have it recalibrated after changing the gear ratios so I know it is now accurate.

*GPS is very accurate while the vehicle is driving on a flat road.

Legally the speedometer cannot display a lower speed than the vehicle is travelling, so quite rightly manufacturers allow for possible tyre diameter errors and err on the safe side, nothing to do with flattering car performance.  I would rather have speedo reading optimistic,  just like I would rather have 7 litres in the tank than 1 litre when fuel light comes on.......
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Jeff15

  • Approved Member
  • *
  • Posts: 129
  • Country: england
  • Fuel economy: 70 + mpg
  • My Honda: 2021 Jazz e: HEV EX - Premium Sunlight White Pearl
Re: Speedo accuracy
« Reply #6 on: September 16, 2021, 10:20:57 PM »
On most road vehicles in the UK the Speedo shows 2mph more than you are actually traveling.

Steve_M

  • Approved Member
  • *
  • Posts: 261
  • Country: gb
  • My Honda: 2021 Jazz e:HEV SR
Re: Speedo accuracy
« Reply #7 on: September 17, 2021, 09:17:12 AM »
There were many changes over the past few year that have meant that speedo accuracy had to be improved.

Most of these related to Euro NCAP requirement for speed assist systems.

https://www.euroncap.com/en/for-engineers/protocols/safety-assist/

embee

  • Approved Member
  • *
  • Posts: 301
  • Country: gb
  • My Honda: 2018 Jazz SE CVT
Re: Speedo accuracy
« Reply #8 on: September 17, 2021, 03:32:33 PM »
Remember that what gets displayed on the dashboard need bear no relation to what the vehicle actually "knows" about its speed. With electronic displays it's just a question of selecting a number from a table, take your pick.

Also there is no reason for the speedo and odometer to bear any significant relationship, they come from different calculations, one is total number of counts (odometer) and the other is a frequency (speedo pulses). One might be 5% optimistic, the other might be 4% pessimistic, it's whatever the equation/tables say. It's not like the old analogue instrument driven by a rotating cable, though even then the marks on the dial could be put anywhere the designer liked......... and it was a question of how wildly the needle would swing back and forth.

nowster

  • Approved Member
  • *
  • Posts: 638
  • Country: gb
  • My Honda: 2021 GR3 Jazz EX i-MMD in Midnight Blue
Re: Speedo accuracy
« Reply #9 on: September 17, 2021, 04:55:08 PM »
Even with the Mk2 the "analogue" speedo was electronic. I seem to remember you got the real speed (in km/h) if you went into the diagnostics mode on the radio.

Tags:
 

Back to top