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1995-2002 Range Rover P38A
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Discussion Starter #1
Just read this in an old thread:


The On-Board Computer is not very accurate and can only be regarded for guidance.

The calculation is carried out by using the fuel tank sender and the total mileage completed since last reset...so if you are on a side slope (even a slight one) the fuel gauge sender gives incorrect information for that part of the journey, or if there are long up/down hill sections, again the information used would be incorrect....I estimate the error could be around the 10-15% or even up to the 20% mark in some cases.

Is this correct? Using the fuel tank's sender would be horribly inaccurate and the MPG would also be affected by how full you keep the tank (since the gauge is non-linear). I'm sure my BMWs use the injector pulse to calculate MPG.

It might explain the wide variation in reported MPG between owners though.

Also is the part about "since last reset" correct? I had assumed the influence of old readings gradually decayed over time (an exponential filter is very easy to implement in software). I haven't reset mine in ages and it would explain why the readings don't change much.
 

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1995-2002 Range Rover P38A
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359 Posts
I hope it's incorrect, mine said it was using 83L / 100km last night, today it dropped to about 53L / 100km.
 

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2,202 Posts
Incorrect. The diesel probably does it the same way but on a petrol it knows the fuel pressure and it knows the injector pulse duration so it can calculate how much fuel is being used. The only time the fuel gauge sender has any relevance is on the range figure, it knows how much you are using and how much it thinks you have left so calculates that. So you can see one figure for range, park on a slope so the gauge sender sends an incorrect level and the range figure will change but the MPG figure won't. The calculation is done in the instrument cluster, not the BeCM.
 

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1995-2002 Range Rover Classic
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1,413 Posts
I have never relied on it, last time decided to play with it I was traveling on a down 5+ mile long down hill, basically I was coasting down hill at 80 mph. it told me was doing 90 miles to the gallon and I had 999 miles to go before empty on a full tank.
now a days it tells me my average speed on the highway is 23mph.
 

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LMAO! Yea I have seen numbers like that. It's only an average since the last reset or battery disconnect. Even my old Lincoln's trip computer is more accurate. The only way I trust a P38 info centre is if I reset it at each fill up. Only then is the AVG MPG believable.
 

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1995-2002 Range Rover P38A
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1,423 Posts
I have got the diesel and with the original factory engine ECU chip fitted, the average mpg figure was actually quite accurate compared with my calculated figure obtained by filling the tank brim to brim. Certainly within about +/- 10% I would say.
I have changed and experimented with a few aftermarket engine ECU chips, and most throw out the mpg figure. They work by pumping in more fuel by increasing the stroke in the FIP.
The car mpg calculation is still done on the original settings and suddenly the car thinks it is doing a lot more to the gallon.
I would therefore speculate that the mpg calculation is done in the BECM taking information from the odometer reading in the BECM and some region of the fuel map in the engine ECU.
There is one aftermarket chip I tried from Superchips that was accurate however. They obviously know what they are doing.
A couple of others I tried throw out the reading. They seem to be written by enthusiastic amateurs converted from the BMW engine chip.
I have got a J Fearne chip in at the moment and the car is showing 35-40 mpg ! I wish.
 

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1995-2002 Range Rover P38A
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121 Posts
Discussion Starter #7
According to the handbook the average mileage and speed are a straight average since the readings were last reset. Mileage from last reset is shown by the 'Trip 2' value - mine is currently showing over 800 miles, no wonder the readings don't change much!
 

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1995-2002 Range Rover P38A
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With reference to average mileage, you would need to do a huge amount of miles at a constant speed before the average and speedo read the same.
 
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