M.p.h.

[SUEDEAPPLE]

can someone please tell me how accurate the speed figure is on tomtom satnavs

 

[dhn]

Very accurate! (assuming you have at least 4 satellites).

More accurate than the speedometer on your automobile.

 

[drledger]

From experience I can tell you that the difference can be up to 5 mph. That is not unique to the TT as my aviation gps units also have a variance. It has to do with topography and the difference between a truly straight line and the dips and minor turns of driving. Someone else may be able to give more detailed information also.

*Hi dhn..we must have posted fractions of seconds apart.

 

[GarCor]

As dhn has said
Very accurate! (Assuming you have at least 4 satellites).

The speedometer on your automobile can be off by as much as 8 MPH especially if you have larger wheels than was originally designed for your vehicle (Say standard equipment was 16" wheels and you opted to put 17" or 18" wheels) this can make a big difference.

This doesn’t matter what GPS unit you have your vehicle will not be as accurate as your GPS

(I have tried three different GPS units in my vehicle and they all show the same speed and that is about 5 MPH less than my speedometer)

I have 16" wheels and standard equipment was 15”

 

[mikealder]

Try doing the trig mathmatics to work out what sort of climb/ decent would make a significant difference, its around a one in three, and even at this sort of angle its a minor error, if you are driving on those sort of roads a GPS isn't of much use - Normal road use treat the GPS as 100% compared to the vehicle speedo.

As for wheels, I recently upgraded my V70 Ovlov from 15" to 17" wheels so I can use larger brake rotors, this has actually brought the speedo accuracy much closer to the true speed, although I have fitted much lower profile tyres to compensate for the increased wheel diameter. Wheel size doesn't make one jott of difference to the speedo accuracy its the wheel circumference that is of importance so the tyre side wall is reduced (along with ride quality when hit bumps).

I would never trust a basic comercially available GPS product to fly an aircraft, the height readings are never anything like accurate, you need to be within a few feet of vertical accuracy for landing an aircraft and the basic instruments fitted to any aircraft will outperform most GPS kit for height measurement - If its military grade then it might be better in terms of accuracy but they also carry far better equipment to work out altitude - Mike

 

[Arno]

I am surprised not to hear that usually when switching to larger wheels one also goes for a smaller cross section ratio to negate the difference in length per revolution traveled (which is roughly 8% per inch in diameter).
I like the idea to allow for larger disc brakes.
Most drivers switch to larger wheels to get the smaller cross section to get stiffer side wall for a higher cornering speed.

 

[drledger]

See Suedeapple I told you we would get some great input on your question.

When I fly I do not use my tt. I have a Trimble 2000 panel mount and a Lowrance 2000c portable (not certified). I considered an advanced portable that provides charts, instrument approaches and taxiway positioning but the screen was not visible in the cabin so it went back. There is a variance between my panel instruments and the gps units in the aircraft. Unlike the size of wheels etc in automobiles this difference is related to wind effect on the plane and the difference between modes of instrumentation along with several other factors, eg airspeed vs ground speed. The WAAS network improves positioning both vetical and horizontal.

BTW fwiw the two gps units provide identical information for speed, altitude, arrival timing etc. Just as one should expect. Even though the information from the gps is potentially more precise my panel instruments are considered primary in my plane. If I wished to invest about $13,000 I could have a Garmin 430W installed which could be certified and allow me to fly instruments using it as a source of information. I believe the exact terminology is "without reference to ground based navigational aides".

 

[GGW]

Most new car mfr's set their speedo's 7-15% fast. EG, they show a faster speed than you are actually traveling. This is to reduce problems of people trying to sue the mfr if they get a speeding ticket. Auto rental companies often set theirs 10-20% fast for this reason and to reduce wear & tear on the cars.

 

[tukatz]

When I did the S2000 engine conversion in my Morgan, I installed a new VDO speedometer that you calibrate by driving a measured mile in calibration mode. I have since checked it in highway speed check zones using a stop watch and it is extremely accurate. My VDO speedometer and my 920 agree on speed. Nuff said?

 

[Arno]

I installed a new VDO speedometer that you calibrate by driving a measured mile in calibration mode.

Why am I not surprised that a calibrated VDO is accurate?

 

[paspossible]

I was stopped for speeding by a cop with a radar and he told me 128km/h. I refrained to tell him that was exactly what my GPS was saying... My car speedo was telling me 130km/h

Soo... ANY GPS with at least 4 satellites will be very precise on speed. The difference cause by hills is minuscule since we normally don't change altitude by 500 feet per second. It's gradual enough the effect isn't worth talking about, even if it does exist. Trees, mountains besides you, canyons and building, all those do affect accuracy because you might loose some satellites.

A GPS defines speed as the time it took between 2 position reading so, the actual position isn't relevant as long as the positioning error is relatively constant. So when calculating the speed going uphill or downhill, the GPS don't need accurate measurement on the altitude, at least in a car. It just need accurate change of altitude so, if the GPS is really at 300 feet and going down to 250 feet, it doesn't matter if it think it's at 500 feet and going to 450 feet, as long as it know it moved 50 feet.

As far as curves, it's irrelevant since the horizontal position is what matters the most and horizontal precision is usually within 10-15 feet so, when driving 60mph, 10-15 feet is done very fast and curves are usually (?) longer than 10-15 feet. There is probably a difference on paper but that difference is likely to be even less than the one caused by hills and I doubt anybody driving cares to go into "0.0something mph" differences.