Civinfo - View Single Post - Computers don't lie

HumberMart · 7th November 2007, 15:43

I drove to Leeds and back this morning from Scunthorpe.

At the moment, there are 50 mph roadworks on the M18, with an 'Average Speed Check'.

So I thought I'd be smart and set the A trip on entry to the speed restriction zone, and monitor my average speed.

Recognising the speedo reads about 4.5 mph over the gps readings, I generally run through such road works at an indicated 55 mph.

Surprisingly, despite maintaining 55 mph, for the whole zone, the average speed was shown by the display as 50 mph!! (Ironically very close to the gps speed, but obviously coincidental).

Later in the journey I set the cruise to 75, for 10 miles, reset the trip, and sure enough, it showed an average of only 70 mph!

Now I changed the wheels recently to Askenlads 18" ones, and I did have to raise the jack slightly to get the new ones on after removing the standard 17's (both sets of tyres were almost new, so no tread differences). So I did wonder if the TR wheels have a very slightly longer rolling circumfrance, which would I guess, lower the speed readings. But this can't be true, 'cos it would also affect the speedo as well as the average speed, and the speedo is just as optimistic as it was with the old wheels.

Another consideration is the accuracy of the milo. I've checked this against the gps, and it does read less, but only by 2%. Although I guess a gps reading may under read itself, due to the way it 'joins the dots' to read distance, thus will to some extent shorten all distances.

Has anyone else spotted such a difference between speedo and the average speed computer

Interestingly, due to my current concern regarding how poor the economy still seems on my 2.2. I monitored the mpg separately on the trip to and from Leeds. Going to Leeds it did only 40.6 mpg, but 49.5 on the return leg!!!

I drove at the same speeds, but went over with an empty tank, and came back with a full one, so if anything it should have been less economic on the return. However the wind was quite strong, and it was more with the wind coming back. Could the wind make such a difference?

Due to the low average speed reading, I'm now wondering if this could be also causing low mpg readings, so I'm going to do another fill-up to fill-up check, and compare that with the computer on this new tank full.

If you're still with me at this point, you need to get out more, and I need to take my anorack off

7th November 2007, 15:43	#1 (permalink)
HumberMart Supporter Locking Wheelnut Join Date: 31st August 2007 Location: North Lincolnshire Posts: 112 Thanks: 0 Thanked 1 Time in 1 Post iTrader: (0)	Computers don't lie - or do they? I drove to Leeds and back this morning from Scunthorpe. At the moment, there are 50 mph roadworks on the M18, with an 'Average Speed Check'. So I thought I'd be smart and set the A trip on entry to the speed restriction zone, and monitor my average speed. Recognising the speedo reads about 4.5 mph over the gps readings, I generally run through such road works at an indicated 55 mph. Surprisingly, despite maintaining 55 mph, for the whole zone, the average speed was shown by the display as 50 mph!! (Ironically very close to the gps speed, but obviously coincidental). Later in the journey I set the cruise to 75, for 10 miles, reset the trip, and sure enough, it showed an average of only 70 mph! Now I changed the wheels recently to Askenlads 18" ones, and I did have to raise the jack slightly to get the new ones on after removing the standard 17's (both sets of tyres were almost new, so no tread differences). So I did wonder if the TR wheels have a very slightly longer rolling circumfrance, which would I guess, lower the speed readings. But this can't be true, 'cos it would also affect the speedo as well as the average speed, and the speedo is just as optimistic as it was with the old wheels. Another consideration is the accuracy of the milo. I've checked this against the gps, and it does read less, but only by 2%. Although I guess a gps reading may under read itself, due to the way it 'joins the dots' to read distance, thus will to some extent shorten all distances. Has anyone else spotted such a difference between speedo and the average speed computer Interestingly, due to my current concern regarding how poor the economy still seems on my 2.2. I monitored the mpg separately on the trip to and from Leeds. Going to Leeds it did only 40.6 mpg, but 49.5 on the return leg!!! I drove at the same speeds, but went over with an empty tank, and came back with a full one, so if anything it should have been less economic on the return. However the wind was quite strong, and it was more with the wind coming back. Could the wind make such a difference? Due to the low average speed reading, I'm now wondering if this could be also causing low mpg readings, so I'm going to do another fill-up to fill-up check, and compare that with the computer on this new tank full. If you're still with me at this point, you need to get out more, and I need to take my anorack off Last edited by HumberMart; 7th November 2007 at 15:49.