Speed display

[Oldster]

My 720 reads the speed consistently (correction) LOW by about 5% compared to the speedometer on my 2000 Honda Accord. I have just learned of a class action against Honda because it overstates the speed and odometer reading (thus saving on warranty issues not to mention mileage rates on rentals).

I am curious to know of other's experience with the speed reading compared to the speedometer reading.

Thanks

 

[mvl]

My tomtom speed matches my car speedometer, and the car matches those "your speed is" radars on the road.

So tomtom seems accurate..

I had a 1998 civic that had the same speed issue, 5% too high (when compared to "your speed is" radars).

 

[Bama Rambler]

There has been tons of discussions about this very subject all over the net. Suffice it to say that the GPS is way more accurate than your speedometer. Even calibrated speedo's aren't in the same league with them. The general concensus is that, on a level road traveling at a steady speed with good signal lock, the GPS will be accurate to 1/10th MPH. That's good enough for me.

 

[Oldster]

There has been tons of discussions about this very subject all over the net. Suffice it to say that the GPS is way more accurate than your speedometer. Even calibrated speedo's aren't in the same league with them. The general concensus is that, on a level road traveling at a steady speed with good signal lock, the GPS will be accurate to 1/10th MPH. That's good enough for me.

Thanks, I couldn't believe that the GPS would be off and attributed it to there not being many straight level sections of road here in Nova Scotia.

 

[JimT]

My wife and I have Ford trucks and the speedometer reads about two miles per hour higher than my TomTom.

 

[canderson]

My wife and I have Ford trucks and the speedometer reads about two miles per hour higher than my TomTom.

Without getting into a long and detailed discussion about this problem, it's been around for quite a while, especially on trucks where tire size can be quite variable - not that cars are immune to it.

As an example of a minor shift for a car:

Switching from 185/60-14 tires to 195/55-14 tires and you'll be reading about 61mph when you're really going 60mph.

Switch from those same 185/60-14 tires to 195/60-14 tires and now your speedo is reading about 1mph LOW at 60mph, you're doing 61.

Move up from 185/60-14 to 205/60-14 tires and suddenly you're 2-1/2mph low at 60mph ... you're really doing 62-1/2mph.

Here's a link to a calculator that may prove interesting if you aren't running exactly the original spec tires for your car.

http://www.discountpartcenter.com/tirecalc.html

 

[sap283]

Without getting into a long and detailed discussion about this problem, it's been around for quite a while, especially on trucks where tire size can be quite variable - not that cars are immune to it.

As an example of a minor shift for a car:

Switching from 185/60-14 tires to 195/55-14 tires and you'll be reading about 61mph when you're really going 60mph.

Switch from those same 185/60-14 tires to 195/60-14 tires and now your speedo is reading about 1mph LOW at 60mph, you're doing 61.

Move up from 185/60-14 to 205/60-14 tires and suddenly you're 2-1/2mph low at 60mph ... you're really doing 62-1/2mph.

Here's a link to a calculator that may prove interesting if you aren't running exactly the original spec tires for your car.

http://www.discountpartcenter.com/tirecalc.html

I have Honda Civic 2009 LX-S, and speed on speedometer and TomTom always matches.

BTW I am not sure; speed has any relation to tire size., but yes if you are using non-standard tire sizes then there may be some variations as number of rotations will differ. Speedometer determines the speed based on tire rotation I think, while TomTom calculates distance between two co-ordinates (longitude & latitude).

In my view later should be more accurate.

 

[Ricochet]

BTW I am not sure; speed has any relation to tire size., but yes if you are using non-standard tire sizes then there may be some variations as number of rotations will differ. Speedometer determines the speed based on tire rotation I think, while TomTom calculates distance between two co-ordinates (longitude & latitude).

Of course tire size matters!
But you have the concept of your speedometer backwards.

From basic geometry: circumference = 2 π r
From calculus: v-perp = r x dφ/dt (or ω = dθ/dt)

Your speedometer essentially reads the driveshaft's rotations to derive your (apparent) velocity. There are several methods for doing this...older cars used a cable housing which connected to a gear in the transmission, some newer cars have a digital sensor in the transmission. And while the engine turns the driveshaft at the same rate, larger tires have to travel farther (ie. faster) to keep up. Hence, your speed is higher than the needle's reading. It doesn't take much size increase to cause it to be off by 2-3 mph.

The same thing can happen if you change the gear ratio (number of teeth) in your car's rear end.

 

[Dick Maxey]

Speed accuracy

My 720 reads the speed consistently (correction) LOW by about 5% compared to the speedometer on my 2000 Honda Accord. I have just learned of a class action against Honda because it overstates the speed and odometer reading (thus saving on warranty issues not to mention mileage rates on rentals).

I am curious to know of other's experience with the speed reading compared to the speedometer reading.

Thanks

If your location is being displayed properly and your satellite signal strength is good and your unit is refreshing itself every second, then I guarantee that your GPS calculated speed is a heck of a lot more accurate than your honda's speedometer. The problem with most auto speedometers is folks changing the sizes of tires on their cars which is directly proportional to the number of revolutions per minute (RPM) sent to the speedometer. BTW, my TomTom is perfect in 4 different vehicles. I suggest you try it in another vehicle and see how close it is in it.

 

[JimT]

My question now concerns the auto manufacturer. Do the calibrate the vehicles speedometer at the factory. Some vehicles come with stadard 16 or 17 inch tires. These same vehicles also may have 18, 21 or 22 innch wheels as an option. Are they just slapping on these bigger tires/wheels without making the appropriate adjustments? If your vehicle comes with the standard wheel size, the speedometer is probably correct.

 

[Barcard]

We've had one TomTom and 3 Garmins. All of them display 1 mph SLOWER than the 2 Fords, 1 Mercury (essentially a Ford) and 1 Honda speedometers we've compared them with. I won't say how I know, but the cop's radar agrees with the speedometer. This is important because where I live, the cops will tolerate 9 mph over and ticket at 10 over.

I turned off the speed display on the TomTom. Wish I could do the same on our Garmin.

 

[cnhmatthews]

My question now concerns the auto manufacturer. Do the calibrate the vehicles speedometer at the factory. Some vehicles come with stadard 16 or 17 inch tires. These same vehicles also may have 18, 21 or 22 innch wheels as an option. Are they just slapping on these bigger tires/wheels without making the appropriate adjustments? If your vehicle comes with the standard wheel size, the speedometer is probably correct.

When the OEM gives you a choice of wheel size the overall dimensions of the tires remain very close. If you go from a 16 inch wheel to a 17 inch wheel the tire's on the larger wheel are of a lower profile, keeping the rolling circumference the same. Keep in mind also the the speedo is calibrated to be close, not exact. You may find that at 30 mph you're dead on, but at 90 mph you may be way off. It's a guide to your speed, not absolute. That's why I'm a little surprised that the new Honda Civic has a digital speedo...seems like Honda might be opening themselves up to a little liability by giving such a specific reading.

 

[CorB]

+6 km/h off ( at 100 km/h ), 2001 VW

 

[Gilbert]

There's a bit I know and a lot I don't know on this subject...

Many years ago (60's and 70's) police spec'd cars had their speedometers calibrated because fines were different according to speed over the limit and the policeman had to have a validated speed for what he was going to put on the speeding ticket.

Car manufacturers became aware they could be sued for traffic fines if their speedometers were proved lower than reality in the sense that if it showed 70 and the car actually went 75. Many decided to err on the low side (show more than actual speed) because of this threat. Car electronics have evolved a lot thru the years but many manufacturers try to save pennies wherever they can and can result in getting lower precision and dependability than could be expected.

In many cars, the speed shown depends on some calculations done in some embedded computer. I know some cars (Intrepid & 300) can have the embedded computer calculation modified with the RDBII programmer such as for some tire sizes and gear ratios and available at some dealers. The same is used when a car's instrumentation is changed for a new one and they want to indicate the real mileage of the car or program the PAT keys.

 

[Bama Rambler]

In the states a speedometer on a new vehicle can read from -0% to +10% of actual speed. In other words, your speedo can show you going faster than you really are but can't show you going slower than you really are. Most mfg's won't get to close to the 0% due to the variances in parts affecting the final speed.

 

[denisj]

I have both a Garmin Etrex handheld GPs and a GO720. Both show the same speed which is exactly 5kph lower than car speedo. ie car speedo shows faster which is OK if you want to make sure you're not speeding.

 

[cacherjoe]

My TomTom 730 jives with my Garmin 60CSx, but my speedometer is 7kph off at 110. By the way the nice RCMP officer tells me that the speed on my GPSRs are accurate according to when he clocked me before kindly telling me to slow down and be on my way.....

When I had my Mazda before the Kia I have now, a tire store sold me a set of tires that were not the right size for the vehicle. That's when I really noticed a change. I ended up getting into the habit of using my GPS to gauge the speed rather than the speedometer in the vehicle.

Stick to the GPS it is much more accurate.

J