Speedometer Readings v. Actuals

[Rabbers]

According to the readings provided by the GPS-based Coyote alert system, which I take to be highly accurate, my speedometer overstates actual speeds

by as much as 11% at lower speeds, with 9% as a general average.   Comparisons are as follows:

50kmh, actual 45kmh   ...  90kmh, actual 82kmh   ... 110kmh, actual 101kmh   ... 130kmh, actual 120kmh   ... 150kmh, actual 139kmh

The differences are significantly high than the 6-7% I estimated for my 2016 RC, which I took to be the norm for Lexus speedometers insofar as they were

closely similar to the figures for the three generations of the IS I previously drove.   As a result I long ago acquired the habit of increasing my indicated

speeds accordingly, secure in the knowledge that there was no risk of getting a speeding ticket.

 

Even though the higher deviations from actual speeds I am seeing in the 2019 RC offer a higher and theoretically more welcome protective margin against

speeding, I must say I find them irritatingly excessive.   I now often find myself accelerating in order to bring my actual speeds up to what the limits are,

usually to no good purpose whatsoever.   In other words, I feel as though I'm going too slow and needing to remedy the fact.   Perhaps this is some kind

of personal protest against what I see as an excess of nannying on Lexus' part? 

[NemesisUK]

All manufacturers speedometers over-read, usually by approximately 10% By law they must not under-read.

I've only checked mine against those roadside indicators and always seems to be within a couple of mph shown.

[malcolmw]

I assume that you have the standard wheels and tyres?

 

[Rabbers]

1 hour ago, malcolmw said:

I assume that you have the standard wheels and tyres

Yes, I almost always have had.  I once had 16" winter tyres and 17" OEMs on an IS200, but although I was generally aware that different tyre sizes

can have a quite dramatic effect on speedometer accuracy, I must say that my perception of it was minimal.

1 hour ago, NemesisUK said:

All manufacturers speedometers over-read, usually by approximately 10% .... I've only checked mine against those roadside indicators                                                      and always seems to be within a couple of mph shown.

The 2019 RC is the only car I've ever owned or (I think) driven that comes anywhere close to 10% speedometer overstatement, certainly in the 

case of a Lexus, half that figure being closer to my experience.   I seem to recall that my last BMW, a 2001 328ix, deviated by no more than 3-4%.

Like you, I based my estimates on comparisons with roadside courtesy speed warnings, placing my greatest reliance, as for most things requiring

precision, on Swiss ones, though I have occasionally ventured into mental arithmetic by way of relieving boredom on long drives.

 

[DanD]

I always have Google Maps showing on my phone while driving... for two reasons:

1. Whilst the Lexus satnav is sufficiently accurate, it does not react as quickly as google maps to changes in traffic conditions

2. You can configure Google maps to show your current speed... which it does accurately. 

[NemesisUK]

37 minutes ago, Rabbers said:

The 2019 RC is the only car I've ever owned or (I think) driven that comes anywhere close to 10% speedometer overstatement, certainly in the 

case of a Lexus, half that figure being closer to my experience.   I seem to recall that my last BMW, a 2001 328ix, deviated by no more than 3-4%.

Like you, I based my estimates on comparisons with roadside courtesy speed warnings, placing my greatest reliance, as for most things requiring

precision, on Swiss ones, though I have occasionally ventured into mental arithmetic by way of relieving boredom on long drives.

Perhaps I should have said "up to a maximum of 10% over-read".

Unless I have modified my car by fitting different wheels/tyres etc. I don't concern myself with the speedometer accuracy. Speed limits are just that, a limit. Not a target.

[Rabbers]

1 hour ago, NemesisUK said:

.... Speed limits are just that, a limit. Not a target.

A wholly admirable principle.  I only wish I could always live up to it.   

[NemesisUK]

22 minutes ago, Rabbers said:

A wholly admirable principle.  I only wish I could always live up to it.   

Me too ... 

[Linas.P]

On 12/31/2019 at 2:07 PM, NemesisUK said:

Speed limits are just that, a limit. 

Exactly, and arbitrary set one... for no good reason. That said, for me any limits exists only to be broken.

Going back on the topic, coming from IS250 where speedo would over-read by 9-10% I now find myself in opposite position from OP as 2016 RC only overreads by ~7%. In other hand I never drive at least 20 over the limit so it barely makes a difference.

[Rabbers]

7 hours ago, Linas.P said:

...  any limits exists only to be broken. ...

... and as a result prospectively generate income from fines.  

[Vlady]

Funny enough, the only accurate speedometer I read about (it was about 20 years ago magazine) was on Chrysler PT Cruiser.

Saying that I always drive based on speedometer, on long journeys I drive to sat-nav speed reading i.e slightly higher than my speedometer shoes.

[NemesisUK]

Wow! Your speedometer has shoes? 

Was that an optional extra or after-market?

[Linas.P]

12 hours ago, Rabbers said:

... and as a result prospectively generate income from fines.  

That is unfortunate reality we are living in. On the other hand I have only ever got one fine for 2MPH over the limit which was on motorways at 4AM. I was doing 52MPH in 70MPH motorway with roadworks limit of 50MPH... and you guessed it - there were not a single worker on the road and barely any cars.

This is off-topic, but for limits to be respected they need to be justified... 30MPH limit next to the school during it's open hours - fine, 30 MPH in narrow residential street.. fine. 70MPH in 4 lane motorways at night - sorry but that is ridiculous. Actually, recently on my way from airport around midnight and I was driving on M11 at 20MPH... - that is what we now get on "smart motorways"..

One thing with these overstated speedometers which bugs me are on A roads in UK you are often in situation where there are no opportunities to overtake or you just want to cruise along at 60MPH... suddenly you catch-up with multiple cars doing 50MPH, for one because their speedometers are indicating higher and their drivers thinking "limit is the limit not target". This is not helped on motorways either by non-existent lane discipline and the attitude "I am doing the limit so I don't need to move over", whereas actually they are in the 4th lane doing 62MPH.

[Nicholas128]

As pointed out, the indicated speed on almost all cars is an over-estimate. Trucks on the other hand have a much more accurate indicated speed.

I find my RC 350 overestimates by about 6%.

Nicholas.

 

[m4rkw]

My RX over-reads consistently by about 5mph. I have an aftermarket cruise control system with two persistent memory buttons on the stalk, so I've programmed them to 50mph and 70mph true speed (according to gps) as these are the most common speeds I'll cruise at on the motorway.

[ColinBarber]

This is the acceptable range for the series II RX - not actually sure what region as they all have different regulations but appears to be consistent with European guidelines. American speedos are typically closer to the actual speed.

I think these are based on full tread depth tyres (i.e. new tyres) inflated to the correct pressure. Once tyres are worn to the legal limit you have up to 1 or 2 percent deviation.

[RadicalCoupe-Fuji]

The first day I had my RC-F I got randomly overtaken in 30mph zones twice in the same journey, i used a gps app to learn the real speeds i need to see on the dash. ie - 30mph is 33mph on the dash.

[Illogan]

When I'm just cruising around town, or even on the motorways, without a deadline, I find my speedometer over reads by an estimated 9% at 30mph, based on roadside read-out cameras, and by 7% at 60<>70mph (based on GPS, (iPhone).

 

However, when I'm concentrating to make an appointment, caught in traffic, or underestimated my journey time, the speedometer over-reads by at least 28%.

Edited March 24, 2023 by Illogan
Poor grammar

[DavidCM]

Apparently the speed indication on a GPS/satnav is the most accurate indicator..apart from a calibrated speedgun or sped camera.

Using Waze,the speed reads 28 ish mph when the car speedometer reads 30mph.

[Rabbers]

10 hours ago, RadicalCoupe-Fuji said:

The first day I had my RC-F I got randomly overtaken in 30mph zones twice in the same journey, i used a gps app to learn the real speeds i need to see on the dash. ie - 30mph is 33mph on the dash.

Based on my OP about my 2019 RC300h’s speedometer’s over reading of actual speeds by 9-11%, my advice would be to confirm the relative figures for one’s own car and then memorize them with a view to taking full advantage of the protective margin they provide, small as it is, against the chances of getting a speeding ticket.

[RadicalCoupe-Fuji]

Ive never had this issue in any other car, obviously it was enough to annoy people behind me into overtaking, that has never something ive experienced before & definitely not in a 30 zone. I thought i was doing 30mph. I believe my old Golf R was almost spot on, perhaps 31 in a 30 would still flash a 30mph thank you on a safety sign. 

[Linas.P]

On 3/25/2023 at 12:05 PM, RadicalCoupe-Fuji said:

Ive never had this issue in any other car, obviously it was enough to annoy people behind me into overtaking, that has never something ive experienced before & definitely not in a 30 zone. I thought i was doing 30mph. I believe my old Golf R was almost spot on, perhaps 31 in a 30 would still flash a 30mph thank you on a safety sign. 

It is required by law, so no standard car is ever spot on. They overstate by as little as 3% to as much as 10%, up-to I believe 250KMH/155MPH, after that they ca overstate by more. If you had for example car with bigger then standard wheels then it is possible the speedometer was about right or even understate the speed, would be quite unusual for normal car but is quite common for off-roading community where fitting significantly larger off-road wheels is common. 

Most of Lexus cars in my experience always been on the upper end ~7-10%.

[Las Palmas]

On 12/31/2019 at 8:46 AM, Rabbers said:

According to the readings provided by the GPS-based Coyote alert system, which I take to be highly accurate, my speedometer overstates actual speeds

by as much as 11% at lower speeds, with 9% as a general average.   Comparisons are as follows:

50kmh, actual 45kmh   ...  90kmh, actual 82kmh   ... 110kmh, actual 101kmh   ... 130kmh, actual 120kmh   ... 150kmh, actual 139kmh

The differences are significantly high than the 6-7% I estimated for my 2016 RC, which I took to be the norm for Lexus speedometers insofar as they were

closely similar to the figures for the three generations of the IS I previously drove.   As a result I long ago acquired the habit of increasing my indicated

speeds accordingly, secure in the knowledge that there was no risk of getting a speeding ticket.

 

Even though the higher deviations from actual speeds I am seeing in the 2019 RC offer a higher and theoretically more welcome protective margin against

speeding, I must say I find them irritatingly excessive.   I now often find myself accelerating in order to bring my actual speeds up to what the limits are,

usually to no good purpose whatsoever.   In other words, I feel as though I'm going too slow and needing to remedy the fact.   Perhaps this is some kind

of personal protest against what I see as an excess of nannying on Lexus' part? 

Difference in wheel height when tyres are new and close to being no longer legal make the wheel 10 - 20 mm smaller giving another way to get speedometer show incorrect.

[Linas.P]

As well it is often important to consider the largest wheels size that was available for the car. For example if Lexus IS250 came with 16", 17" and 18" wheels, then speedometer will be adjusted for 18" wheels (as manufacturer cannot legally sell car which indicates less speed than it is actually doing, thus they have to use largest wheel as reference) at which point it will probably overread by 5%, but if you get lower spec car with 16" wheels then the difference is likely to be 10%. 

[RadicalCoupe-Fuji]

I guess it's more noticeable when you have a digital dash displaying figures rather than a needle hovering over numbers on a gauge.