Average Speed Calculation

[Jonathan Mason]

I'm a bit puzzled. I was on the motorway today and realised I had not reset the trip meter after the last fill up so I duly did it with the speed set at by cruise control at a constant 75mph (according to the dash and 70mph according to the satnav unit)

This was a completely flat and empty piece of motorway so I am sure the speed of the car wasn't changing in anyway, but the car calculated the average speed as 64mph. Anyone know why this would be so far out from what I believe to be reality?

*edit changed reference from MPG to something more sensible. sorry.

[Grumpy Cabbie]

I'm a bit puzzled. I was on the motorway today and realised I had not reset the trip meter after the last fill up so I duly did it with the speed set at by cruise control at a constant 75mph (according to the dash and 70mph according to the satnav unit)

This was a completely flat and empty piece of motorway so I am sure the speed of the car wasn't changing in anyway, but the car calculated the average MPG as 64mph. Anyone know why this would be so far out from what I believe to be reality?

MPG? or MPH?

[Jamesbelfast]

I'm a bit puzzled. I was on the motorway today and realised I had not reset the trip meter after the last fill up so I duly did it with the speed set at by cruise control at a constant 75mph (according to the dash and 70mph according to the satnav unit)

This was a completely flat and empty piece of motorway so I am sure the speed of the car wasn't changing in anyway, but the car calculated the average MPG as 64mph. Anyone know why this would be so far out from what I believe to be reality?

Not a surprising result as the on board computer works by extrapolating data and not by measuring live fuel flow information. In the right circumsatnces you can also get it to read zero.:)

[Jonathan Mason]

Sorry. Corrected the MPG reference. Obviously not with it at the moment, but I am solely referring to the average speed calculation. I know the speedo reads about 10% more, but surely the on board computer uses this data for average speed calculation.

[DrCez]

Sorry. Corrected the MPG reference. Obviously not with it at the moment, but I am solely referring to the average speed calculation. I know the speedo reads about 10% more, but surely the on board computer uses this data for average speed calculation.

Hi, I tried something similar with my car and found it was slightly less than 10% different, e.g.:

- at a constant 54 mph (indicated) the average speed on the trip was 50 mph

- at a constant 76 mph (indicated) the average speed on the trip was 70 mph

I don't have sat nav or cruise control so it's not particularly accurate; the T3 wheel size is also slightly different than the T4 which might have a bearing on the results (but I would only expect a 1-2% effect).

So in summary I think for my car I see a 9-10% difference which is what you were expecting, but I don't know if this information is much help to you!

[phil4]

Hi, I tried something similar with my car and found it was slightly less than 10% different, e.g.:

- at a constant 54 mph (indicated) the average speed on the trip was 50 mph

- at a constant 76 mph (indicated) the average speed on the trip was 70 mph

I don't have sat nav or cruise control so it's not particularly accurate; the T3 wheel size is also slightly different than the T4 which might have a bearing on the results (but I would only expect a 1-2% effect).

So in summary I think for my car I see a 9-10% difference which is what you were expecting, but I don't know if this information is much help to you!

I tried the same in my T-Spirit this morning. Constant 75mph produced an average of 70mph, so underreading by a smidge under 10%.

Wonder therefore how this influences the MPG, and more worryingly the Odometer

[Jamesbelfast]

The speedo and odometer work from live data wheras the OBC works on historic data - etrapolation is the keyword. Also Satellite Navigation systems ain't 100% accurate.:)

[Jonathan Mason]

Sorry James, I don't agree.

Whether live or historic data the fact is the car was travelling with a speedometer reading of 75, the trip meter was reset and should now read the average speed during the new period. Allowing a couple of seconds to start calculating it then reads 64. The only logic there, using your extrapolation theory, is that on reset the car sets the computer speed at zero, and then instantly moves it up 75, this would quite drastically affect the average speed over the first couple of seconds.

Sadly, the average speed stayed the same over a longer period of unchanged 75 miles per hour driving.

This suggests an incorrect calculation and, as Phil4 suggests, possible incorrect odometer readings.

[phil4]

The speedo and odometer work from live data wheras the OBC works on historic data - etrapolation is the keyword. Also Satellite Navigation systems ain't 100% accurate.:)

I see what you mean, but I don't see it's relevance to these tests. In my case, I drove the car at a constant 75mph for several miles. Indeed, the average speed cannot be measured until some distance is covered. I think this is what you mean by extrapolation, speed over 1/10th of a mile, extrapolated to a whole mile. So in my case, the OBC may well have had an inaccurate impression for the first few 1/10ths of a mile, however once it is passed a mile, and onwards to the next the OBC figure is based on past mileage, and by now should have reached a sensible steady value.

As it happens because of the steady speed maintained, the 1/10th measurement and the final several mile figure were the same, the OBC had not changed at all. It's what I'd expect.

The speedo measurement may well be being taken from another place, I understand on non-hybrid cars, it's often the gearbox. What's puzzling is how such a wide variation can occur between the two. And as repeated, concerns me with regard to Odometer.

Also Satellite Navigation systems ain't 100% accurate.:)

Indeed, GPS is not. However I was not using the GPS and Nav for either speed or distance measures. This oddity also appears looking at these comments on models with no Sat Nav.

[johalareewi]

I have checked my prius gen3 odometer against my own satnav GPS over many journeys long and short and they pretty much agree with each other about the distance travelled.

However, the satnav and speedo disagree about the current speed. The speedo is about 7% higher than the satnav.

While satnav systems aren't 100% accurate, neither is the speedo. so who you gonna believe?

As for the average MPH, what happens to the average when the car is stationary?