[Jamon]

Ive just recently bought a '96 Sierra Coily.
31" Tyres, 4" lift. 1.3L
Im getting like 240Kms to a $44 tank, Thats mostly highway driving at 90Kms, revs about 4000RPM in 5th gear. (Basically the fastest it will go)
Is that normal?

Apparently theres series 4 Rockhopper Gears in Transfer case, Does that change the ratio's for normal 2wd Highway driving? or just when in 4wd?

Are there any mod's i can do to get higher highway speeds/better fuel ecomony?

Cheers,

[Rusho]

Sounds about right, if the gears were not in there it would probably be worse

[lordfury007]

Assuming you're paying about $1.30/litre that works out to be about 34 litres of fuel which is about 14 litres per 100km.

I'm running 31" tyres with standard gears and i think i'm getting somewhere around 12 litres per 100km as long as i stayed below 100kph... its been a while since i accurately measured it though...

good point atari (next post down), i think i took that into account with mine, it was something like 100kph on the dial was nearly 120kph on my gps...

[atari4x4]

how far out is your speedo, you might be doing more km's than what's being indicated by the odo.

[royce]

Assuming your speedo is 15% out like most with 31's that about 12l/100km, so about right

[whincup]

yes, my speedo reads 88km/h when GPS reads 100km/h. id be checking yours with a gps, if you dont have one see if you can borrow one from a mate...

[Rusho]

My odo saids 280km's to a tank but it is more like 320-330

[Makie43]

Sat-Nav (GPS) is by design not very accurate for Speed. GPS measures Position and differentiating this to give Velocity is therefore noisy, ie a small error in position will give a big error in speed. Also, since GPS measures position essentially horizontally, it is therefore prone to errors vertically, ie. in altitude. The new Euro GPS equivalent is far better (self-calibrating and more trusted) but not on-line yet.
In an nutshell Sat-Nav is not really designed to measure speed well.

[ajsr]

bet you its still far closer than a sierra speedo after fitting 33's and reuction gears

[royce]

Makie43 wrote:

Sat-Nav (GPS) is by design not very accurate for Speed. GPS measures Position and differentiating this to give Velocity is therefore noisy, ie a small error in position will give a big error in speed. Also, since GPS measures position essentially horizontally, it is therefore prone to errors vertically, ie. in altitude. The new Euro GPS equivalent is far better (self-calibrating and more trusted) but not on-line yet.
In an nutshell Sat-Nav is not really designed to measure speed well.

looking forward to your explanation of why its not very accurate, include diagrams if you like

[lordfury007]

I sortof see what you both mean, but I decided to work it out for myself.

Todays GPS work out speed by first working out the position you are currently in, and the position you were in say, 2 seconds ago (depending on your GPS polling speed). Your speed is whatever speed you had to move (on average) over the last two seconds to get from point a to point b.

In a car, going at 100kph, you are moving at almost 28 meters per second, so a 2 second gap moves you 56 meters.

The accuracy of the GPS is variable, depending on atmospheric conditions, satellite clock errors, slight shifts in the satellites position, calculation errors and rounding by the GPS unit etc, but we can accept a normal accuracy of around 5 meters... This means the GPS can record you going anywhere between 46 meters per 2 seconds (83kph) and 66 meters per 2 seconds (119kph)

So Making a few calculations based on that

Code:

Ground Speed      GPS Accuracy   Poll      GPS Min     GPS Max   Accuracy
100kph                +-5M        2sec    83kph        119kph      +-18%
100kph                +-5M        1sec    65kph        137kph      +-36%
100kph                +-2.5M     2sec    96kph        105kph       +-5%
50kph                  +-5M        2sec   32kph         68kph        +-36%

Results obtained using following equation

GPS Min (in KPH) = (360((S - 2A) / P))/100
GPS Max is the same except its a plus sign

Which makes more sense in a longer equation (bugger bomdas, just go from left to right)

S - 2A / P x 60 x 60 / 100

S = Speed in meters/second
A = GPS Accuracy (multiplied by 2 as there are 2 polling points - beginning and end)
P = Polling speed


So this very quick bit of maths indicates that you get reasonable accuracy if you have high GPS accuracy (no Sh*t sherlock!), are driving faster and with a slower poll rate, but it possibly can be out by a significant amount if Accuracy drops (again, no Sh*t!), you slow down, or choose a faster poll rate.

If i really got excited i would have done some more results to graph it and prove if the results were exponential or linear, but i'm almost certain that it is exponential and i've done enough maths for one year.

In the end, from personal experience, GPS is more than accurate enough, and these results are "worst case scenario" results...

[Reubs]

Makie43 wrote:

Sat-Nav (GPS) is by design not very accurate for Speed. ... In an nutshell Sat-Nav is not really designed to measure speed well.

No, a GPS does not use position to determine speed, with speed in particular they are not subject to the same inacccuracies as they are in determining position. Google "Doppler Shift", and you might see this.
GPS Speed & Accuracy

[lordfury007]

Reubs wrote:

Makie43 wrote:

Sat-Nav (GPS) is by design not very accurate for Speed. ... In an nutshell Sat-Nav is not really designed to measure speed well.

No, a GPS does not use position to determine speed, with speed in particular they are not subject to the same inacccuracies as they are in determining position. Google "Doppler Shift", and you might see this.
GPS Speed & Accuracy

well that throws all my hard work out the window...

[got_bar_work]

my gps was spot on my speedo when i got it recalibrated for the 31's

[Zachius]

Also as of a couple of years ago GPS got a _lot_ more accurate due to the the US Military opening up part the system that they used to keep for themselves only.