Lying Cars (Dash vs Reality)

[Debaser]

Jasandjules said:

Debaser said:

How do you work out what fuel economy you're actually getting?

Work out how many miles you have travelled (read the trip computer) and then divide that by the number of gallons of fuel you put in at the pump.....

What makes you think the trip computer mileage readout is accurate?

[tomjol]

bhstewie said:

Krikkit said:

You don't get the convenience of having a mobile spreadsheet that works out your mpg for you and makes pretty pictures?

When you say you work in IT... Are you an Oracle dbadmin who sits in a cupboard all day?

Google Docs is a mobile spreadsheet

I too work in IT, and take very little pleasure in continuing work-like activities at home.

When I wanted to start tracking fuel, I had a choice: make my own spreadsheet with pretty graphs etc, or use something like Fuelly. I chose the lazy, and less work-like, option.

[bitchstewie]

I chose the lazier option and just copied someone else's.

This is fast turning into an IT thread so rather than derail it, all I'll say is I just don't get this constant need for "apps" to do simple mundane st like bash in my MPG once a fortnight

[tomjol]

Understood, but there's a difference between "needing" and "using"

[gizlaroc]

DickP said:

My company car claims on the dashboard that it is averaging around 55mpg......I presumed a slight variance may happen of a couple mpg. Reality is the car averages betweeen 46 and 47 mpg. That's a difference of over 10%!

17%


I don't want this to turn into a petrol vs diesel debate, but every single diesel I have owned has been between 10-20% out on the OBC, bar one, where as every petrol has been pretty close to real world.

Really not sure why this is?

I wonder if it is something to do with warm up times?

I was reading how the BMW computer calculates mpg and they showed the curve on the warm up period, from cold they estimated it was using 40% more fuel than when up to temperature and as the temp climbed the amount of extra fuel used was reduced. Problem with this was when it was at 75ºc it said it was only using 10% less than when up to temperature, which was 89ºc. However, if you then look at how the car over fuels when cold 75ºc was overfuelling by over 25%.
With many doing sub 10 mile journeys this would through the MPG calculations out by quite some margin.

Petrols obviously get up to temperature far far quicker, my 2.0T is within a mile, which would make calculating mpg far easier.


My 2.0tdi A3 used to show 52mpg while only getting 40/41.
My 1.9tdi A6 from a few years ago used to show 46mpg and calculated it was struggling to hit 35mpg.

My 3.2fsi Petrol A6 used to show 28mpg and was actually 29mpg calculated.
My 2.0T showed 33mpg on the last tank, but got 500 miles, calculated at the pump it was actually 38mpg.

Our X3 20d xdrive was showing 48mpg average, reality was 39mpg.

My E350cdi however was pretty lose, showed 33mpg average and was 32/33 on most calculations, but then I used that as my workhorse and it did more 100 mile journeys than sub 10 mile journeys.


I may be wrong, but I reckon it is to do with how they calculate the fuel used before up to temperature that throws the obc so much.

[eldar]

tomjol said:

Understood, but there's a difference between "needing" and "using"

Is there is an app to help you decide?

[tomjol]

eldar said:

tomjol said:

Understood, but there's a difference between "needing" and "using"

Is there is an app to help you decide?

Probably! I don't need or use one of those

[blank]

Mine is always within 0.5mpg in either direction. Other cars have often been up to 10% optimistic.

Wonder with the VW recall, if 'real life' mpg drops, they will include a trip computer re-cal to make it look the same as before. Would probably fool 90% of drivers. Doubt they'd risk it though!

[bitchstewie]

Mine is a VW and it reads +/- a couple of MPG on/around 55mpg so very accurate IMO.

[Bonefish Blues]

Computer generally says 60ish, calculations generally say 50-51.

Volvos of that generation seem to be pathological liars - Mrs BFB's is exactly the same.

Edited by Bonefish Blues on Sunday 4th October 16:23

[Jasandjules]

Debaser said:

What makes you think the trip computer mileage readout is accurate?

Well it is about the only way to measure distance. It won't be wildly out.

[Garybee]

Current astra is within a couple of tenths of an mpg. Previous car (BMW 328) consistently claimed 7-8 better than it was getting.

[Debaser]

Jasandjules said:

Debaser said:

What makes you think the trip computer mileage readout is accurate?

Well it is about the only way to measure distance. It won't be wildly out.

I find it interesting that in a thread about trip computers lying about mpg, people are happy to accept the odo as accurate.

Your accurate mpg calculation may be less accurate than the 'lying' trip computer.

[justanother5tar]

My E46 330i says average MPG of 32.

I think thats a tad (read; very) optimistic at best. Still, keeps the mrs happy as i told her it'd be doing around 20MPG when I bought it.

[GroundEffect]

It calculates it using the same speed as displayed on the cluster. What did you think would happen when they are by law forced to over read?

[DKS]

gizlaroc said:

I may be wrong, but I reckon it is to do with how they calculate the fuel used before up to temperature that throws the obc so much.

I think I get what you're saying, but in that case why does it need to calculate how much extra fuel the engine needs when cold? It knows exactly how long the injectors are open for.

[BrownBottle]

Debaser said:

Jasandjules said:

Debaser said:

What makes you think the trip computer mileage readout is accurate?

Well it is about the only way to measure distance. It won't be wildly out.

I find it interesting that in a thread about trip computers lying about mpg, people are happy to accept the odo as accurate.

Your accurate mpg calculation may be less accurate than the 'lying' trip computer.

So if the general consensus is that speedos read about 10% over, does that mean the overall mileage displayed in most cars is 10% more than they have actually done?

[blank]

BrownBottle said:

So if the general consensus is that speedos read about 10% over, does that mean the overall mileage displayed in most cars is 10% more than they have actually done?

No, the car knows how fast and far it is going. The speed reading from the CAN is usually accurate (compared to calibrated GPS), the speedo is deliberately set to over read a little.

[paulrockliffe]

DKS said:

gizlaroc said:

I may be wrong, but I reckon it is to do with how they calculate the fuel used before up to temperature that throws the obc so much.

I think I get what you're saying, but in that case why does it need to calculate how much extra fuel the engine needs when cold? It knows exactly how long the injectors are open for.

It knows a hell of a lot more than that, all the engine maps are calculated based on an exact air-fuel ratio as a starting point. The computers control every aspect of the ignition cycle, it know exactly how much fuel is being burned, certainly several orders of magnitude greater than the accuracy of the MPGs that are being reported.

[caelite]

CaptainMorgan said:

My old Fabia vRS was 100% perfect, tested it over numerous different drives and conditions and every it was within 1mpg or so. Havent bothered checking the 320d as it's crap anyway (35mpg) and the Panda I've not had to put any fuel in yet lol.

This, My old VRS got essentially bang on what the manufacturer stated through town (actually a bit more i average 45mpg urban), Extra Urban (62mpg) is a bit optimistic but can easily be achieved cruising at 60-65 (If you sit at ~80+ the bricklike aerodynamic characteristics of the fabia come into play. Sitting at 70ish i used to get ~55mpg, ~50mpg at ~80.

The dash readout was close enough to bang on what fuelly used to tell me I was getting.

New car though... MG ZS, I dont know yet I just put money in it and it makes a nice noise. Doesnt have any dash trip computer .

Edited by caelite on Sunday 4th October 21:57