Does your trip computer tell you fibs?

lordrobs

lordrobs

lordrobs

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I guess this question is more for the nerds out there that have a trip computer but also have a spreadsheet with your fuel economy on, just like me :D

a lot of people quote the MPG that their trip computer reads out as gospel, I've been keeping tabs on mine since the beginning of Feb and with my car atleast this isn't the case at all. So I'm wondering how much yours is out by. Might be interesting to see the difference in accuracy between the different manufacturers.

I'll kick things off- Octavia vRS 1.8T

On average the trip comp. over reads by 2.5 MPG, worst over a single tank it over read by 4.3 MPG.

Interestingly the 4.3MPG over read was on a tank that was about 90% free flowing motorway so the indicated motorway economy of my car is way off :(

So how good or bad is your trip computer?
 
thepedster said:
my car doesn't even have a low fuel light :(
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Haha, mine does but it doesnt seem to want to work, am thinking the sensor has gone as i also cant get to the screen on my computer that tells me how many miles i have left.

With regards to accuracy i'm not that sure how accurate my mpg is. On long motorway trips staying between 70-80mph i can get 45mpg, any higher and it goes to the 38-40mpg mark. JUst tootling to and from work (which is 4.5miles each way) i can only average about 30.

For a 1.6 Megane i think they might be a bit low, but oh well.
 
schumi84 said:
Haha, mine does but it doesnt seem to want to work, am thinking the sensor has gone as i also cant get to the screen on my computer that tells me how many miles i have left.

With regards to accuracy i'm not that sure how accurate my mpg is. On long motorway trips staying between 70-80mph i can get 45mpg, any higher and it goes to the 38-40mpg mark. JUst tootling to and from work (which is 4.5miles each way) i can only average about 30.

For a 1.6 Megane i think they might be a bit low, but oh well.
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1.6 Phase 1 megane? They don't have multifunction trip computers, so the button on the end of the wiper stalk is useless ;)
 
lordrobs said:
a lot of people quote the MPG that their trip computer reads out as gospel, I've been keeping tabs on mine since the beginning of Feb and with my car atleast this isn't the case at all. So I'm wondering how much yours is out by. Might be interesting to see the difference in accuracy between the different manufacturers.
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Your trip computer is not telling you fibs, it's calculating a different average mpg to the one you get from the brimming method. The trip computer gives you your average mpg over time, so if you travel at 20 mpg for half an hour, and then 40 mpg for half an hour it will tell you you averaged 30 mpg.

Your brimming method on the other hand averages over the total fuel used, so if you drive at 80 mph, and 20 mpg for half an hour and then 40mph at 40mpg for half an hour, you will have used 2.5 gallons of fuel and travelled 60 miles, giving an average mpg of 24 mpg.
 
My first couple of tanks and mileage agreed with the trip computer pretty much exactly, so I've stopped taking much note of precise amounts now - 00W Mondeo.
 
Trip computer shows an average of 36 mpg, which seems very unlikely as the car is driven hard almost all of the time. I'd rather not know the actual figure though to be honest!
 
Mr Jack said:
Your trip computer is not telling you fibs, it's calculating a different average mpg to the one you get from the brimming method. The trip computer gives you your average mpg over time, so if you travel at 20 mpg for half an hour, and then 40 mpg for half an hour it will tell you you averaged 30 mpg.

Your brimming method on the other hand averages over the total fuel used, so if you drive at 80 mph, and 20 mpg for half an hour and then 40mph at 40mpg for half an hour, you will have used 2.5 gallons of fuel and travelled 60 miles, giving an average mpg of 24 mpg.
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Both methods should, in an ideal world, give you exactly the same reading provided you reset the computer once you've brimmed the tank so I'm not sure what you are talking about.

If you drive at 20mpg for 30 minutes then 40mpg for 30 minutes both brimming and the Trip Computer should in theory give 30mpg.
 
howardcanning said:
1.6 Phase 1 megane? They don't have multifunction trip computers, so the button on the end of the wiper stalk is useless ;)
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I am thinking mine is a Phase 2 (is a V reg). The button on the end of the stalk cycles me through all the usual options, and as i say it just has a flashing line on the 'miles left to refill'.
 
[TW]Fox;11402463 said:
Both methods should, in an ideal world, give you exactly the same reading provided you reset the computer once you've brimmed the tank so I'm not sure what you are talking about.

If you drive at 20mpg for 30 minutes then 40mpg for 30 minutes both brimming and the Trip Computer should in theory give 30mpg.
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I'm glad someone else saw it that way. I read his post 4 times trying to work out the point at which they diversified.

There should be no difference.

[edit]To answer the question in the OP mine is fairly accurate.
 
[TW]Fox;11402463 said:
Both methods should, in an ideal world, give you exactly the same reading provided you reset the computer once you've brimmed the tank so I'm not sure what you are talking about.
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That's not so. Look at the numbers I gave in my post.

If you drive at 20mpg for 30 minutes then 40mpg for 30 minutes both brimming and the Trip Computer should in theory give 30mpg.
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This is just wrong. The result from the brimming method depends on the speed you're doing during that time.
 
The "brimming" method isn't that accurate either. As far as I'm aware, the fuel pump cut off works by detecting back pressure however the point of cutting off can vary quite a bit depending on many factors such as the caliration of the pump, the flow rate, the ambient air pressure and a few others which I cannot recall.

Whilst I don't have a complete understanding of it, I was told this by a chap I know and trust and who is a fuel wholesaler with his own chain of petrol stations.

My trip computer is as accurate as I care. I'm not going to lose any sleep over 2 MPG over a tank because as much as it matters it makes no difference.
 
Mr Jack said:
Your trip computer is not telling you fibs, it's calculating a different average mpg to the one you get from the brimming method. The trip computer gives you your average mpg over time, so if you travel at 20 mpg for half an hour, and then 40 mpg for half an hour it will tell you you averaged 30 mpg.

Your brimming method on the other hand averages over the total fuel used, so if you drive at 80 mph, and 20 mpg for half an hour and then 40mph at 40mpg for half an hour, you will have used 2.5 gallons of fuel and travelled 60 miles, giving an average mpg of 24 mpg.
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Is this for the OP's particular car?

We had this discussion in work after I noticed my Veccie had this 'odd' behaviour when showing 999.9MPG on overrun, so we did some 'tests' on different peoples cars, and one of the things was time and distance averages, we looked at our journeys into work, and I was commuting 50 miles a day with a good test of this, 10% of the journey distance was around 20MPG average, 90% was at 50MPG average.. but the 20MPG section took almost the same time as the 50MPG section (Motorway/A road Vs town..)

If the computer only used time for the average, then I should have averaged 35MPG on the fuel computer, The actual brim'd fuel consumption was 46.8MPG, My veccie showed 43MPG, my Zafira 46.6MPG, both do give average speed and distance travelled as part of the 'info' and both seem to the be closer to the mileage average then time average..


I do agree with your maths though, and my old Ford Orion Ghia I had (an 88 model) had the most inconsistent fuel computer ever, but with no speed input at all, it must have just been purely based on a time average..
 
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Demon,

I'll freely admit I don't know about the OPs car in particular - it's how the one on my old Mondeo did it, and I'd just assumed that it was the same on other cars as the differences seem to be consistent. If you've done more testing, I'll happily accept your conclusions :)
 
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