Submitted: Wednesday, Jan 14, 2009 at 10:03

ThreadID: 65103
Views:2921
Replies:1
FollowUps:17

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Hello

No this is not another worn out question, I was just going through the fuel consumption history I kept for my Patrol and thought this may be of interest to people. Over the course of 160,000km my 03 GU Patrol 3.0L diesel. I found that:

1. Going from a 265/70R16 to a 285/75R16 added about 0.3-0.4L/100km extra to the fuel consumption allowing for the difference in tyre sizes in the calculations.

2. Going from a Bridgestone AT pattern to a Goodyear MTR and then Mickey Thompson MTZ all in the 285 size made very little if any difference to the fuel consumption.

3. Bridgestone AT 693 in passenger construction, okay all round tyre, reasonable punture resistance but poor mileage out of them. Bridgestone AT 694's in LT 285, poor puncture resistance, 3 malley roots and one sliced sidewall, good mileage though, noisy towards the end of their life. Goodyear MTR's excellent tyre off the road, excellent mileage, okay on dry bitumen but not good in the wet. Mickey Thompson MTZ basically take Goodyear MTR's and add good on-road performance, AWESOME basically.

Finally if you are scratching your head wondering which tyres to buy then you have to check out this site;

Site Link

Hopefully the link works, cheers.

Snowy

No this is not another worn out question, I was just going through the fuel consumption history I kept for my Patrol and thought this may be of interest to people. Over the course of 160,000km my 03 GU Patrol 3.0L diesel. I found that:

1. Going from a 265/70R16 to a 285/75R16 added about 0.3-0.4L/100km extra to the fuel consumption allowing for the difference in tyre sizes in the calculations.

2. Going from a Bridgestone AT pattern to a Goodyear MTR and then Mickey Thompson MTZ all in the 285 size made very little if any difference to the fuel consumption.

3. Bridgestone AT 693 in passenger construction, okay all round tyre, reasonable punture resistance but poor mileage out of them. Bridgestone AT 694's in LT 285, poor puncture resistance, 3 malley roots and one sliced sidewall, good mileage though, noisy towards the end of their life. Goodyear MTR's excellent tyre off the road, excellent mileage, okay on dry bitumen but not good in the wet. Mickey Thompson MTZ basically take Goodyear MTR's and add good on-road performance, AWESOME basically.

Finally if you are scratching your head wondering which tyres to buy then you have to check out this site;

Site Link

Hopefully the link works, cheers.

Snowy

Wednesday, Jan 14, 2009 at 10:44

How did you allow for the trye size in the calculation? How did you arrive at difference in distance traveled?
AnswerID:
344254

Wednesday, Jan 14, 2009 at 10:54

By calculating the circumfrence of the tyres and then making their difference a percentage. This percentage then got checked by looking at the difference between the vehicle odometer and GPS odometer on a trip. This created what I called in my Excel spreadsheet a tyre factor, for 265/70R16 it was 0.97, while for a 285/75R16 it was 1.04. So when I had worked out the distance travelled on a tank from the vehicles odometer, I then divided it by this tyre factor to get the true distance travelled so that the eqution would be: Number of litres used / ((Distance travelled on odometer/ tyre factor) x 0.01)

Hope I didn't bore you but I like playing with numbers in Excel spreadsheets.

Regards

Snowy

FollowupID:
612156

Wednesday, Jan 14, 2009 at 11:05

GPS odometers are not very accurate. The difference, may well be due to this. I would expect your fuel consumption to drop with larger tyre diameter in most circumstances.
FollowupID:
612161

Wednesday, Jan 14, 2009 at 11:16

I must admit I am not fully conversant with the exact equations that GPS's use to calculate their position so I am not certain about their accuracy. However I am not sure about bigger tyres decreasing fuel consumption, I must admit that this is only my experience and not a set rule. But even if I had ignored the differences in circumfrence in the calculations this would have made the fuel consumption figures even worse as on bigger tyres the odometer shows that I travelled less distance than I actually had, less distance for the same or even more fuel when plugged in to the equation equals worse fuel consumption. I think the only way the fuel consumption could have improved or stayed the same is if at highway speed the drop in engine rpm caused by the bigger tyres put the engine in to it's power/torque sweet spot.Kind Regards

Snowy

FollowupID:
612162

Wednesday, Jan 14, 2009 at 11:19

"GPS odometers are not very accurate."Have you any reference on this? I'm not being 'snakey' or stirring, but would value any valid comparisons between GPS and normal vehicle odometers.

cheers

FollowupID:
612163

Wednesday, Jan 14, 2009 at 11:23

I can only speak from my experience. While GPS seems to give an accurate speed clcualtion I find they don't work so well as an odometer. Not sure why. I guess GPS odometers work on a distance traveled calculation derived from positions. Position accuracy is at best plus or minus 5 meters your vehicle odometer is better. Try it out next time you are keeping a log of distances on a road or track- I have many times and the GPS is hopeless compared to the Odo.
FollowupID:
612164

Wednesday, Jan 14, 2009 at 12:01

If I may be so bold, try GPS Vehicle Navigation in Australia by Robert PepperISBN 1 876296 22 4 Boiling Billy Publications

www.boilingbilly.com.au

Brian Purdue

FollowupID:
612175

Wednesday, Jan 14, 2009 at 12:02

I might get a couple of GPS units next trip and run them together in the truck.I have read that one of the Auto-clubs regard them as being very accurate for speed measurements and used GPS to compare speedometer accuracies between various manufacturers/models.

Given that: I would expect as time will be a constant, then distance will also be very close.

Any one else in a position to check distances on multiple GPS units? I'm not sure when I will be next on the road.

cheers

FollowupID:
612176

Wednesday, Jan 14, 2009 at 12:16

It comes down to the sampling rate of the GPS. The GPS records a position every so often according to the sampling rate. The GPS then "connects the dots" if the sampling rate is low, then the error will be significant. On the other hand your odometer is continuously sampling or at worst sampling at each revolution of the wheel.I am not sure how the GPS derives the speed calculation or display, except to say that as an indicator of speed between the last two samples, it would be very accurate, this however does not equate to an accurate measurement of distance via many sampling points.

FollowupID:
612180

Wednesday, Jan 14, 2009 at 12:20

In other words, the distance calculation suffers from cumulative error whereas the speed does not.
FollowupID:
612181

Wednesday, Jan 14, 2009 at 13:26

"distance calculation suffers from cumulative error"I'm not exactly sure how the distance is calculated, but I suspect it won't be from the first plot point to the last, otherwise it would short cut all the bends. So there must be some sort of summing between points going on there.

But is it only the +- 5m accuracy of a stationary GPS?

It is easy to prove a GPS is more accurate when moving to when it is stationary by looking at the scatter when stopped, then drive a straight road for a while and notice how the wide scatter of the stationary points is transformed into a very accurate straight line (very accurate width measurements) and if you drive at a constant speed, (by speedo or GPS) the lineal spacing will also be very consistent (very accurate length measurement).

How accurate is a GPS or Odometer?

Guess I will have to go for a drive past one of our surveyed 5km speedo checking sign sets and check the results.

cheers

FollowupID:
612197

Wednesday, Jan 14, 2009 at 13:39

The distance is calculated by summing all of the individual distances between each point sampled. If each point has an error, then the errors are also summed- hence cumulative error.eg.

GPS records points and b and determines distance between them is 10 meters, but the actual distance travelled by the car was 10.2 meters, so the error on that one distance is 0.2 meters, then gps samples point c and works out distance from b to c is 5 meters, but the actual distance traveled is 5.1 meters, and error of 0.1 meters.

So summing the two distances the GPS decides it has travelled 15 meters but in actual fact it has traveled 15.3 meters. The error is 0.3 meters which is the sum of the two individual errors of 0.2 and 0.1.

FollowupID:
612201

Wednesday, Jan 14, 2009 at 13:40

If doing a test, find a winding road of considerable distance, not a straight 5k runDavid

FollowupID:
612203

Wednesday, Jan 14, 2009 at 13:43

This one's like the Energiser bunny; "Keeps going and going, OI" - Jacko
FollowupID:
612204

Wednesday, Jan 14, 2009 at 13:49

We kinda hijacked the thread didn't we.What is annoying is I have to stop what I am doing in Firefox and change to Safari on my iMac to answer. It seems EO doesn't like my Firefox...ooops there I go again...off-topic

FollowupID:
612206

Sunday, Jan 18, 2009 at 16:30

No problems with Firefox on an iMac this end.Henpecked, that might account for some of the unrealistic fuel consumption figures quoted by some pootroll owners.

I distance-checked my old FJ62 recently and found the odometer to be under-reading significantly even though I'm on half worn standard issue tyres.

This might explain why I don't appear to be getting anywhere these days. ;)

FollowupID:
612928