How accurate is speed limiter?

[JEC]

I use speed limiter a lot to ensure avoiding tickets! When I set to say 50 mph the car speedo will show 49 and there will be a procession of cars passing. (I know many drivers confidently drive faster than speed limit by a bit) If I set the limiter when I am doing 50 mph the limiter will often show 52mph. If anyone knows Does Toyota build in a mis match? I appreciate this may not be the most important issue in the world!  Love the car. Rav4 PHEV design 2022

[philip42h]

I don't use the limiter, but do use the cruise control - I'm going to assume that the speed setting stuff is the same for both.

When you first press 'set' it sets the speed / limit to the current speed of the car. When you press + or - it adjusts the set speed / limit to the next higher / lower multiple of 5 mph.

It appears to keep the speed with +/- 1 mph of the set speed / limit - that's about the limit of accuracy you can expect.

Note also that the set speed / limit is given in terms of the speedo reading rather than actual speed along the road. The speedo will over-state your actual speed (it would be illegal for it to under-state your actual speed) by around 5% - the difference will be dependent on tyre type, wear, pressure and temperature.

So, if you set the speed to 50 mph it is perfectly possible that you are actually travelling at 45 mph, and it would be perfectly legal for other drivers to pass and still be within the limit.

HTH

[Roy124]

Regarding cars passing when you set 50 that is because they are doing 50 true.  You may be going as slow as 45 true. 

Other than that I find the limit set is the samecas indicated. 

[philip42h]

1 hour ago, philip42h said:

if you set the speed to 50 mph it is perfectly possible that you are actually travelling at 45 mph

... which is just about true but not what I meant to write!

In previous cars I have found the difference between speedo speed and GPS speed to be around 10% so that an indicated 50 mph is a true 45 mph. The speedo in my current HEV reads closer to true so the difference is around 5%. So, if I set the speed to 50 mph, my actual speed is likely to be around 47.5 mph.

In a motorway speed 50 mph speed restriction section, I'll set the cruise control to 55 mph - meaning that I'll actually be travelling at around 52 mph when traffic allows. That's slow enough to be 'observing the speed limit' and fast enough not to be unreasonably holding up other motorists who want to get through the restriction as quickly as possible. (And much of the time I'll wind-up following other traffic at 45 - 49 mph ... )

On an unrestricted motorway I simply leave it set to 70 mph (so a true 66 / 67 mph) which is fast enough to get me where I need to be without burning too much fuel, and leaves a bit of leeway for those who wish to travel faster.  

[Roy124]

With the new Satnav and Smart Connect it displays the average speed between two Specs cams. I am now sure if this a a true average derived from GPS. 

I need to do a bit more driving to evaluate this. 

[JEC]

really appreciate the tme given to reply to my question. Sounds Like I can set limiter to say 52 in a 50 mph zone and be fairly confident I am near the speed limit

 

[Roy124]

JEC, one thing you can do it to run through the speed signs that give your accurate speed.  I did this today with my new car and got a 2-3 mph difference at 50.  My previous car was 5 mph or the full 10%.

[Flatcoat]

I find my speedo seems to be consistently 2-3mph adrift irrespective of the speed. In practice I tend to set my cruise at an indicated 5mph over the limit so probably about 2mph over….. 

[ernieb]

Similar to my PHEV, 3 to 4 mph error. I tend to use AA and use the speed there especially when setting ACC. This uses the car to keep telling me I’m over speed but I’m happier knowing my speed is accurate. Every car seems to have errors but agree the RAV seems to be a constant regardless of the actual driving speed.

[JEC]

To conclude now driving at say 32 or 33 mph in 30 mph zone and driving at same speed as everyone else! Also in 50 mph zone travelling at 53 or 54 to match everyone else. Now annoying fewer other drivers. Thanks for the replies.

[Roy124]

JEC, less annoyance true, but I liked one road sign stating the speed limit was a limit and not a target.

I all honesty, how much time will someone loose travelling at 28 rather than 30 when passing through a village?  Also a 30 sign is the speed that you should be doing as you pass it rather than a suggestion that you should slow down.

Regarding Specs 50 zones, a bit different as they often give you a legal chance to overtake many cars on a dual carriage way.

[Sealiedog]

It seems that the regulations allow the Speedo to overestimate by a huge amount according to what I've read.

"UK law - which is based on a European Union standard -  UK speedometer regulations also outline how accurate speedometers must be.

The regulation states that speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph".

So your Speedo could show 50.25 mph when you're actually doing 40, and that would still be legal. Thankfully most of them are a bit more accurate than that! 

[Centaur]

A few years ago, I had an aftermarket bluetooth device fitted to a VW Touareg.  Amongst other things, it allowed additional diagnostic information to be displayed on the dashboard, including the vehicle's speed.  That was always exactly the same as the speed as measured by a GPS device and 2-3mph less than was displayed on the speedometer.  The car, therefore, knew exactly how fast it was going but was programmed to over-read.  Interestingly, the recorded mileage appeared to be linked to the 'true' mph, not the 'displayed' mph.

[nlee]

7 hours ago, Centaur said:

A few years ago, I had an aftermarket Bluetooth device fitted to a VW Touareg.  Amongst other things, it allowed additional diagnostic information to be displayed on the dashboard, including the vehicle's speed.  That was always exactly the same as the speed as measured by a GPS device and 2-3mph less than was displayed on the speedometer.  The car, therefore, knew exactly how fast it was going but was programmed to over-read.  Interestingly, the recorded mileage appeared to be linked to the 'true' mph, not the 'displayed' mph.

Did the car have a built in Sat Nav? Maybe the bluetooth device picked the speed signal up from that rather than the speedo feed? 

[philip42h]

As stated above: "A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph".

Manufacturers still use rotations of the wheels as the means to measure speed and distance - so the measurement will be affected by the make of tyre fitted to the rim, its state of wear and inflation pressure. Since the speedo must never show less than the actual speed manufacturers design the system to report a speed somewhere in the middle of the allowed range with an OEM tyre at the correct pressure. So and overstatement of the actual speed of around 5% is to be expected - what it is in practice will vary from manufacturer to manufacturer and the state of the tyres ...

[ernieb]

Just use AA or Car Play and the display will show the speed pretty accurately on the display. I’ve calibrated my phone/speed display in the car against an external GPS module, no issues there.

As has been said the car Speedo is designed to never show a speed below the actual speed, can you imagine the claims against the manufacturers if it was in error that incurred a speeding ticket?

[nlee]

2 hours ago, ernieb said:

Just use AA or Car Play and the display will show the speed pretty accurately on the display. I’ve calibrated my phone/speed display in the car against an external GPS module, no issues there.

As has been said the car Speedo is designed to never show a speed below the actual speed, can you imagine the claims against the manufacturers if it was in error that incurred a speeding ticket?

The GPS speed can also be displayed on the screen saver of some of the Nextbase dashcams that have a screen. Mine does, but it's positioned so I can't see it! But I get regular speed warnings from the wife!

[Roy124]

My Navigon with POI Warner was worse than SWMBO.  Brunhilda would announce "Slow down, you are exceeding ze speed limit" 

[Nick72]

Everything is set over the speed that it is actually going at. Adaptive cruise, limiter, speedometer. It's a percentage. At a speedometer value of 70mph you're actually only going 66mph. Have confirmed many times off two different GNSS (includes GPS) devices which is the most accurate way of monitoring speed versus anything on the car. And one of the most accurate methods in general. At 30mph speedometer value you're doing about 28mph.

As a rule I use and set the cruise to 33mph in a 30. I set it to 76mph in a 70 (I'm doing real world speed of 71 or 72 in that case).

Each car may however be slightly different given a whole lot of additive tolerance differences but the above seems consistent with many replies.

I think what Toyota has done is work out the worst cast scenario and put a percentage of speed offset function in to account for it. So you're always going to under the speed limit if the Speedo says you're on it.

Just opinion and deduction only.

[ernieb]

The difference I've noticed with the Toyota system is that the offset appears to be constant regardless of the indicated speed, 3-4 mph, this obviously means the error is less as the speed increases. On previous cars I've seen an offset proportional to the indicated speed, small difference at low speeds and higher difference as the speed increases.

[Marcusthehat]

I have deliberately and repeatedly  checked my indicated speed against those roadside displays that provide this information, and yes, as to be expected, the car consistently reads a few mph high.

So I set the SL at 32 or 33, or 43/44 (but often drive slower regardless, the SL is only there as a backstop) and the CC is generally set 5 mph faster than the 50/60/70 limit, though I mostly pootle about at 45-50, and seem get there just a fast .

Marcus

[Nick72]

On 10/13/2022 at 7:32 AM, ernieb said:

The difference I've noticed with the Toyota system is that the offset appears to be constant regardless of the indicated speed, 3-4 mph, this obviously means the error is less as the speed increases. On previous cars I've seen an offset proportional to the indicated speed, small difference at low speeds and higher difference as the speed increases.

Interesting. Definitely increases for me versus GNSS. Not by much but a couple of mph more when at higher speed vs low. Wonder what's happening in the system.

 

[ernieb]

9 minutes ago, Nick72 said:

Interesting. Definitely increases for me versus GNSS. Not by much but a couple of mph more when at higher speed vs low. Wonder what's happening in the system.

 

It depends Nick on what you and I would class as higher speeds? 😀😇

[Oscarmax]

On 9/3/2022 at 9:49 AM, JEC said:

I use speed limiter a lot to ensure avoiding tickets! When I set to say 50 mph the car speedo will show 49 and there will be a procession of cars passing. (I know many drivers confidently drive faster than speed limit by a bit) If I set the limiter when I am doing 50 mph the limiter will often show 52mph. If anyone knows Does Toyota build in a mis match? I appreciate this may not be the most important issue in the world!  Love the car. Rav4 PHEV design 2022

Silly question how do you operate the speed limiter, I have picked up the car this morning, looked through the manual, completely confused.