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Accuracy Of Trip Computer (Tank Average)


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Hi Guys

Anyone noticed that the Tank Average reading on a Lexus IS250 is out (2010 F Sport) ?

With the driving I do (mainly short trips) I get about 9-10L/100km on the trip computer, however when I compare it to my Car Log app (which is bascially km's / Litres of fuel) it's typically about 1L more (ie 10-11L / 100km.

This is pretty consistent per tank irrespective of whether I do city or country driving that it's about 1L out

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Yep..i do that. 6 times now (every tank since I bought the car). And it's consistently out by roughly 1Litre per 100Km, with the correct figure (the manual calulation) showing less econonmy than the trip computer

I believe I have some carbon build issues (I have another post about this) as the car has stalled twice now in the morning, so I'm wondering if this impacts the trip meter calculations.

I've used some fuel cleaner and also redlined it for about 5 seconds a few times which hopefully will address this and see if this changes anything.

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Two thoughts:

1. How is the app calculating the distance? By GPS? It could be a variance between the app's estimation of the distance travelled, and your odometer. This, in turn, could be caused if your tyre diameter is different to stock. Having said that, you would need to have installed smaller wheels - or have your tyres significantly under-inflated - in order for your IS 250 to understate the litres of fuel consumed per 100 km.

2. You say that your IS 250 reports between 9 and 10 litres per 100 km. Let's say 9.5, on average. 9.5 L/100 km = 10.53 km/L,which is between 10 and 11 kilometres per litre. Are you sure your app is reporting litres per 100 km, or kilometres per litre? They are two different methods of measuring fuel consumption.

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Hi Lance

1. i calculate using the old method. ie input the no of k's driven and the no. of litres i fill to full. Then work out how many litres per 100km. Been doing this for years, and doing this on the lexus since i picked it up in december.

Your point about the wheels is interesting. the rear should be 245's, but i noticed the are 225's (same as front). Do think this could explain it ? The car passed both RWC and a detailed inspection I paid for so was surprised this wasn't picked up

it is litres per 100km.

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Your point about the wheels is interesting. the rear should be 245's, but i noticed the are 225's (same as front). Do think this could explain it ? The car passed both RWC and a detailed inspection I paid for so was surprised this wasn't picked up

Yes, for 2010 IS 250 F Sport, there should be 225/40R18 front wheels and 255/40R18 driving wheels. According to an online calculator for tyre sizes (http://tire-size-conversion.com/tyre-size-calculator/), if you are using the same tyres for the rear as you are for the front, then there are 804.3 revolutions per unit length (mile) when the IS 250 is only expecting 775.2. That means the distance travelled is overstated by the odometer, for a given true distance (and a given volume of fuel) by 3.8%. Thus, a reading of 9.5 L/100 km would be closer to 9.9 L/100 km in reality.

That's not quite the difference of 1L/100 km you're trying to reconcile, but it does partially explain things.

Another factor is if the level of error between the speedometer and the true speed. My IS reports 114 km/h when I'm actually doing 110, so there's an error of 3.6% there; and, according to Wikipedia, odometer errors are usually proportional to speedometer errors. So a reported fuel consumption of 9.5 L/100 km is actually 9.5 L x (1 + 3.8%) x (1 + 3.6%) = 10.2 L/100 km in reality.

Add to the fact that distance readings can be off by up 3% when tyres are worn. Assuming your tyres are partially worn, let's say a difference of 1.5% from new tyres; so now, your true fuel consumption is 10.4 L/100 km when the trip computer says 9.5 L/100 km.

Edited by Lexus Nerd
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haha..now I get why you call yourself Lexus Nerd :-)

Thanks for your very detailed explanation...which makes sense.

Given that no one else has reported the same issue, it sounds like it explains at least part of the problem.

I will continue to monitor and see if after running the fuel cleaner if things even out a little or the gap remains the same.

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I will continue to monitor and see if after running the fuel cleaner if things even out a little or the gap remains the same.

A dirty engine would explain excessive fuel consumption, but it would explain a discrepancy between a trip computer reading and your own logs. There are some additional factors I didn't mention before:

"The fuel consumption figure produced by an on-board trip computer is calculated from the distance travelled and the theoretical amount of fuel consumed by the engine (often based on the number and duration of fuel injection pulses). In theory, these consumption figures should be reasonably accurate, though in practice there are sometimes differences between these and the figures derived from on-road testing. This may be the result of variations in service station pump readings, odometer readings and filling errors ... or in the process used to calculate the trip computer figure. In view of this, these figures should be regarded as a reasonably close guide rather than a firm consumption figure." (Source: RACQ)

In addition to my previous points:

- Service station pump readings and "filling errors" such as "the consistency of fills" and the "ambient temperature and temperature of the fuel during filling" could be factors, but we can discount those issues in your case, because your discrepancies are consistent after six fills and you probably aren't always filling up on hot days.

- If your engine is dirty, this would affect how much you throttle the engine in order to obtain the desired acceleration. This would, in turn, increase the number of fuel injection pulses. That would be reflected in your trip computer's fuel consumption reading. So a fuel injector clean could improve your fuel economy, but it won't affect the percentage of discrepancy.

Sorry for being wordy again, but I wouldn't want you to spend $ on an engine clean, in the absence of any other factors that would warrant it.

Edited by Lexus Nerd
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Thanks again Lance

That was the premise of my original question really..ie to see if it's just a general inaccuracy of the trip computer that would explain the difference, or could it be something else. Seems like it could be a mixture of both

I read that the 75km service includes a fuel injector clean so also wondering if that will make a difference. (just done over 60K)

I'll see how I go in the next few tanks now that i've tried a few things and report back on if there is any difference.

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  • 4 weeks later...

The trip computer would be off as it fundamentally calculates off the innacurate speedo. Pretty much all factory cars are a bit conservative with speedo calibration, likely to help people getting caught out with police/speed cameras. connect an aftermarket gps & you'll see this immediately, or when on the long hwy's with distance markers.

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