Speedometer accuracy observations

[MaineFusion]

So on a long road trip a couple weeks ago, I came to an observation thanks to the TomTom app on my smartphone. What I discovered is that my FFH's speedometer was registering one mph slower than what my GPS was recording and this was consistent through a range of speeds. For instance if I set my cruise control at 55mph, my GPS was consistently reporting a speed of 54mph. Along these lines, prior to the PCM update I could set my cruise control at 63mph and still reliably go into EV mode.

 

Of course the problem with all of this is it is based on what my cruise control is keeping the car at. The speedometer itself is an analog dial that can be hard to read with great precision.

 

Being a computer geek, I'm kind of obsessive about precision, and the thought of setting the cruise control at say 55mph and to be really doing 54mph is an annoyance. Now I realize that factors like tread wear and tire inflation plays a big role in the accuracy of the speedometer, and I'm thinking one mph shouldn't be an insurmountable error to correct. To this end, I'm thinking just a couple PSI difference in tire pressure might be just the fix I need. The key will be to figure out what the exact correction is needed. Of course I don't want to put in too much air and cause poor tire wear just to get accurate speed readings.

 

One would think that this day in age where cars have built in GPS, cars would have some sort of self correcting mechanism to improve the accuracy of the speedometer so that we could stop entirely relying on a calculation method that was so dependent upon something as unreliable as the diameter of the tire.

[MaineFusion]

So I'm back from a 1,500mi road trip to Virginia, and made an observation. Before leaving, I had inflated my tires to 38psi and was carrying a much heavier load than my normal one person commute. There were three people in the car plus a full trunk of stuff. On the trip, my set cruise speed more closely matched my TomTom GPS speeds than normal. I also had much better fuel economy than I was expecting given the extra load and constant highway speeds. My last three Fuelly fuel-ups were for this road trip.

[Waldo]

Every car ever made in the history of cars has a speedo that reads higher than the actual. Manufacturers do this intentionally because the liability risk of having people going faster than they think is too great.

 

GPS correction would be an interesting idea though, but I would hardly consider the current state of automotive GPS to be "reliable", given all the things that can interfere with the signals.

[MaineFusion]

Every car ever made in the history of cars has a speedo that reads higher than the actual. Manufacturers do this intentionally because the liability risk of having people going faster than they think is too great.

 

GPS correction would be an interesting idea though, but I would hardly consider the current state of automotive GPS to be "reliable", given all the things that can interfere with the signals.

 

Interesting... I never considered that the speedo would intentionally be reading higher than the actual speed.

 

Even in cell phone based GPS systems, there is a great amount of ability to determine the accuracy of the GPS positioning. Since GPS antennas aren't so tightly restrained by size or weight considerations in vehicles as they are in cell phones vehicle based GPSs should be even more reliable than those on mobile devices. Working with long term averages, a pretty accurate recalibration of the speedo should be able to be able to be achieved. There may still be some imprecision in the speedo, but it could be made way more precise than it is by relying on the circumference of a pneumatic tire.

 

In general, I think having more precisely calculated speedos, and especially cruise controls, could help tighten up the speed range of typical drivers. Yes there are outlaying drivers who go way above or way below the speed limit, but I suspect the vast majority of drivers try to travel within preconceived safe speeds (e.g. 5mpg over the limit or at the speed limit).

 

A tighter speed range that had everyone driving more similar speeds could help improve the overall flow of traffic as there would be less passing and clumping up of traffic. Being one of those who drives at or slightly below the speed limit, what I have observed is that traffic tends to clump up in waves. When on highways, a wave of traffic will pass me and then there will be open road around me for a duration until the clump of traffic passes. Traveling in one of the open road section of the traffic wave is way safer than being in the tightly packed clump. I suspect that if speedos were more tightly calibrated, there would be less clumping of traffic.

[acdii]

The speedo in my 1992 Crown Vic was dead accurate. It was a calibrated police speedometer, a rare car to find back then. Under cover police interceptor but with a luxury package. I got it used and it was tinted with badges in the rear window under the tint for the IL Bar and FOP.

[Waldo]

I suspect that if speedos were more tightly calibrated, there would be less clumping of traffic.

 

You are giving people way too much credit. I'd estimate that less than 10% of drivers even use the cruise control on a regular basis. Of the remaining 90%, I'd expect half of them don't even check the speedo more than once every 5 minutes. To expect people to adjust their speeds continuously to a tighter tolerance than the current speedo accuracy just isn't going to happen. There are people out there who just like to follow someone else so that the other person can do the "thinking" for them.

Also think about the accuracy that would really be required. If you're out on the open highway and you are 100ft behind a car doing 70mph, if you are doing 70.1mph, you will catch up to that car in just 11 minutes. it will take you about 7 minutes to go past it, which will pile up everyone behind you who is going 75mph. Result is a huge clump of traffic.

[MaineFusion]

 

You are giving people way too much credit.

 

It's my commercial driver training that causes me to think that checking the gauges, displays and mirrors on regular basis is a normal practice. I forget just how bad most drivers are.

[acdii]

 

It's my commercial driver training that causes me to think that checking the gauges, displays and mirrors on regular basis is a normal practice. I forget just how bad most drivers are.

Same here. Holding a CDL now since 1988. I dont forget how bad drivers are, I see it on a daily basis.

 

Just tonight, a cop tried to roll a stop sign, but I had the right of way. Would have been interesting had he pulled me over, since I came to a full stop before he reached the line on the road. :)

[MaineFusion]

Same here. Holding a CDL now since 1988. I dont forget how bad drivers are, I see it on a daily basis.

 

I got so used to bad drivers as a tour bus driver that I could continue narrating my tour while preforming evasive proceedures. Of course the tour was etched into my brain to the point I didn't even need to think about what I was saying.

[dkegel]

Every car ever made in the history of cars has a speedo that reads higher than the actual. Manufacturers do this intentionally because the liability risk of having people going faster than they think is too great.

It's always dangerous to use absolutes like "every". The speedo in my 2013 Focus actually reads 1 mph slower than our Garmin GPS. I think my daughter's Honda Civic also reads a bit low. I agree this is not the norm, but I would not assume that all cars read high.

 

Dave

[hybridbear]

You are giving people way too much credit. I'd estimate that less than 10% of drivers even use the cruise control on a regular basis. Of the remaining 90%, I'd expect half of them don't even check the speedo more than once every 5 minutes. To expect people to adjust their speeds continuously to a tighter tolerance than the current speedo accuracy just isn't going to happen. There are people out there who just like to follow someone else so that the other person can do the "thinking" for them.

I agree! This is exactly the problem. For example, in Minneapolis we have large sign boards along many of our freeways that will sometimes display a speed lower than the speed limit. The speed on the sign does not become the new speed limit like in some countries (i.e. the UK and others) but is a suggestion. If traffic slowed down to the speed posted on the signs then there would be less traffic ahead and they would mitigate a traffic jam. Most motorists will continue to fly along at 55+ MPH when the sign says 40 MPH and then soon find themselves stopped in gridlock. I have found that it really does work by slowing down and enduring the glares and fingers for going slower on a stretch of freeway that is still free-flowing, but then I get vindication when I catch up to the people who went flying by me because they got stuck in traffic. And while they had to often come to a dead stop, I often barely have to slow down because by the time I get there the traffic is flowing again. Often I even pass the people who went flying by me earlier because from my vantage point I'm able to more quickly identify the most free-flowing lane rather than having to hit the brakes hard to stop for gridlock.

 

The signs are computer controlled and can change about once a minute based on the traffic flow ahead. They're a really neat tool. We've even had stories on the news telling drivers to slow down to the speed on the signs because they'll actually get where they're going faster, but most people "drive faster, not smarter".

[corncobs]

They have a lot of these smart traffic control system around the bigger cities on the Autobahn.

Problem is that no one really pays attention even if those electronic signs reduce the speed limit and then you end up in a gridlock.

[murphy]

The route 495 bypass around Wilmington, DE has electronic speed limit signs. They aren't recommendations, that is the speed limit.

 

A very long time ago Ohio had speed limit signs that displayed a lower speed at night when illuminated by headliyghts than what was displayed during the day. The first time I encountered them I was wondering how they managed to change all of the signs so quickly.

[acdii]

Another indication people aren't very bright is when they go flying along in heavy fog. I guess they think that massive pileups in fog only happen in movies. It gets foggy my me quite often and it amazes me that morons still drive over the speed limit when you cant see more than a quarter mile if there are lights on, if there arent any tail lights on, you wont see them in time until you hit them. I get passed by these idiots all the time since I back down at least 10 MPH when its like this.

 

There was one time this one idiot was following me too close, I swerved to avoid a dead animal, he wound up hitting it and ran off the road. Then you get those following so close they are fixated on your bumper. I like to mess with them and serpentine across the lane, then LOLL as they run onto the shoulder.

 

Drive smarter, not faster

Edited November 19, 2013 by acdii

[MaineFusion]

I catch up to the people who went flying by me because they got stuck in traffic. And while they had to often come to a dead stop, I often barely have to slow down because by the time I get there the traffic is flowing again. Often I even pass the people who went flying by me earlier because from my vantage point I'm able to more quickly identify the most free-flowing lane rather than having to hit the brakes hard to stop for gridlock.

 

The signs are computer controlled and can change about once a minute based on the traffic flow ahead. They're a really neat tool. We've even had stories on the news telling drivers to slow down to the speed on the signs because they'll actually get where they're going faster, but most people "drive faster, not smarter".

 

On my trip to VA I tried out the traffic alerts my TomTom Android app provides for subscription. Same basic concept as your adjustable speed signs. It even provided forewarning as to which lanes were closed. Like your signs, I was able to know to slow down early and change to a better lane. I also liked that it showed not only where the jamb was starting, but where it ended.

 

There is no sense to rush into a traffic jamb.