RC300h Fuel Consumption (v. IS300h)

Rabbers · March 13, 2017

From October 2016, when I took delivery of my RC300h, through February 2017, I drove 6390km (=approx. 4000 miles) of which 9% Motorway,

34% Town/City and 57% All Other Roads according to data I can nowadays summon up via a Tracking/Security device (ViaSat Blue Box) fitted

to the car at the behest of my insurance company in return for a reduced premium.

According to the in-car display I averaged 15.3km/l (=43mpg) over the distance. However, if I add the full tank supplied on delivery to my total

fuel purchases in the period and subtract the estimated content of my tank at end February, I calculate that the car actually consumed 440 litres

at an average of 14.5km/l (=41mpg). That the in-car reading understates fuel consumption by about 5% pretty much confirms what I and some

other owners reported on this site back in 2013, and the continuing inaccuracy could mean that Lexus considers the deviation to be within an

acceptable margin of error. Be this as it may, any discussion of fuel consumption for the 2016 RC and pre-2017 IS hybrids (and maybe other

Lexus models) based on in-car readings should allow for it.

Although Lexus' own declared combined-cycle figures of 20.0km/l (=56.5mpg) and 21.7km/l (=61mpg) for F-Sport versions of the RC and IS

hybrids respectively (the latter claimed to have improved to 22.7km/l (=64mpg) in MY2017) are in themselves wholly academic, the percentage

difference is, in my experience, roughly correct. This can conveniently be illustrated by a comparison between the displayed full-tank range

forecasts in the two models. Where I would regularly expect to see an anticipated range of 950km (=590 miles) - meaning a consumption of

55-56 litres (about 12 gals) before the next appearance of the fuel-reserve warning - in my IS, and was sometimes even able to exceed it, the

best I have so far seen and managed to achieve in the RC has been 865km (=535 miles), meaning that I am spending something like 9-10%

more on petrol. Clearly, this is not likely to be a deal-breaker for anyone who may be thinking of switching from an IS to an RC or is thinking of

buying one or the other, but nor is it an entirely insignificant amount.

The difference in consumption appears to be proportionate to the RC's higher weight of 1775kg (F-Sport) against the declared 1620kg of the

IS (I'm not sure of the trim but suspect my 2013 Premier was a bit heavier than this), almost all of the extra kg being attributable, I understand,

to a stiffened chassis borrowed, in part, from the GS. Since the cornering, stability and general handling of the RC are in my view better than

those of the IS despite the extra weight, my conclusion is that the cost of the extra fuel consumption is worth it.

March 13, 2017

For anyone that's interested.

since taking delivery of my RC300h F-Sport in July '16, at the last fill up I have covered 5764 miles and used 738 litres, by my calculations that's approx. 35.5mpg. I usually update my trip info each fill up and regularly see suggested whole tank consumption between 38-48mpg, which is nothing like reality.

The latest example, on my last fill up my trip computer suggested I had achieved 38.4 mpg, but in reality I calculated 31.9 actual.

Nb. Just to confirm the calc. I am using is 'miles/(litres/4.546)'

royoftherovers · March 13, 2017

59 minutes ago, James22 said:

For anyone that's interested.

since taking delivery of my RC300h F-Sport in July '16, at the last fill up I have covered 5764 miles and used 738 litres, by my calculations that's approx. 35.5mpg. I usually update my trip info each fill up and regularly see suggested whole tank consumption between 38-48mpg, which is nothing like reality.

The latest example, on my last fill up my trip computer suggested I had achieved 38.4 mpg, but in reality I calculated 31.9 actual.

Nb. Just to confirm the calc. I am using is 'miles/(litres/4.546)'

That is the correct way to do it. On Board Computer figures are misleading and at best advisory.

Regards

John

NemesisUK · March 13, 2017

Since taking delivery in Oct 16 and covering 2500 miles i have recorded 40mpg overall via the OBC and 37mpg via brim to brim calculation. Not bad agreement and am sure mpg will increase as the weather warms up.

royoftherovers · March 13, 2017

15 minutes ago, NemesisUK said:

Since taking delivery in Oct 16 and covering 2500 miles i have recorded 40mpg overall via the OBC and 37mpg via brim to brim calculation. Not bad agreement and am sure mpg will increase as the weather warms up.

VERY,VERY INTERESTING Peter. Your OBC reads 108% optimistically which is exactly the figure my Mercedes reads optimistically too !

I have not checked my Lexus OBC as I only check brim to brim results.

Regards

John

Regards

John

Zotto · March 13, 2017

The temperature has an important role , winter time is worse for mpg, as much fuel is used during engine first minutes of warming and keeping also interiors warm.

With my GS 300h I do 14,5 km/l in winter time while with external temp in springtime it rises to 17 km/l or more. With summer season, needing AC, there is a slight decreasing to 16,5. My best with light hypermiling techniques was 25 km/l. (to obtain mpg multiply all for about 2.8)

Rabbers · March 15, 2017

I confess that I was surprised by the size of the reported gaps between actual and displayed consumption, not least because I was

left with the impression that they are considered normal or acceptable. To quote John: "...On Board Computer figures are misleading

and at best advisory...", and Peter: "[37 v. 40mpg is] not bad agreement". Maybe I'm being overly harsh but, normally, were I to be

demonstrably "misled" or "misadvised" by factors of 8% (see Peter and John), let alone >20% (see James), my reaction, on the

grounds of irritation alone, would be to sever my connection with the person or entity or machine responsible. Having said this, I

suppose I should consider myself lucky that the deviation of around 5% I saw in my IS and am seeing in the RC (see below for an

update) are apparently better than par for this particular course.

One more tankful of petrol since my OP has seen a slight improvement in cumulative consumption to an actual 14.7km/l (=41.5mpg)

= 497 litres for 7285km driven, and, as expected, this is continuing to improve with the warmer weather. I had been worrying that

the displayed consumption of 15.3 km/l had become stuck when it suddenly twitched upwards to 15.4 and then almost immediately

to 15.5km/l, where it currently stands, thus maintaining its reality gap of 5% or so. At this point, I would judge that unless my driving

style changes (which it won't) and except for occasional fluctuations in my driving mix for individual tankfuls (e.g. more motorway km

than hitherto), my consumption has permanently settled into a state of predictability.

NemesisUK · March 15, 2017

In reality the difference between 37 and 40 is insignificant, unless of course one were to recompensed on the OBC record and one did very high mileages. Otherwise it's all academic and coming from a car that routinely returned 15mpg I couldn't be happier, not that I do enough miles for fuel consumption to be of any concern.

Linas.P · March 15, 2017

@NemesisUK - well that is one perspective. 8% difference is still huge, regardless if 37 or 40MPG are otherwise great results. Would you say same for 100MPG vs. 108MPG.

Other thing, compared to older petrol models from Lexus this is high difference. I was comparing my brim to brim results on IS250 and they are pretty much spot on. They are slightly inconsistent throughout the tank e.g. if I refuel straight when the light comes up car usually takes 56L of petrol and and in car display usually shows ~ 28MPG, whereas in reality is more like 27.6MPG, however if I run well into reserve and refuel then ~61L (I usually driver 10-15miles after trip computer shows 0 range), then trip computer actually shows lower MPG than reality e.g. 27.4MPG whereas real one is like 27.6MPG...

One way or another the difference is very small and that I consider acceptable (0.5-1.5% difference), maybe rounded value. There is no reason why car would show 8% better fuel consumption except of misleading. I can appreciate "optimistic" range after refueling because it is just advisory prediction, but to have 8% deviation after the trip is unacceptable.

NemesisUK · March 15, 2017

13 minutes ago, Linas.P said:

Would you say same for 100MPG vs. 108MPG.

Yes, I would.But as I said fuel consumption is not really on my radar. I do keep records though, have done all my driving life. Any dramatic change could be an indication of some sort of problem.

Both my Mercedes had very accurate OBC reading, easily within 1% of my brim to brim calculations.

Bottom line is, OBC is an indicator of fuel metered over distance travelled. Brim to brim calculations are at the mercy of dispensing pump accuracy (which will always be pessimistic, you will always get more fuel than the pump displays (Weights and Measures,) and operator consistency. Too many variables to get hung up about the odd % differences

I fill it, drive it and enjoy it, especially on such a beautiful day as this

Rabbers · March 15, 2017

5 hours ago, NemesisUK said:

In reality the difference between 37 and 40 is insignificant, unless of course one were to recompensed on the OBC record and one did very high mileages. Otherwise it's all academic and coming from a car that routinely returned 15mpg I couldn't be happier, not that I do enough miles for fuel consumption to be of any concern.

Agreed. However, my admittedly academic concerns have less to do with the economic aspects of the RC's fuel consumption, with which I am perfectlycontent, than with so noticeable a degree of imprecision in its measurement and display as to constitute a blemish on Lexus' otherwise fair complexion.....

..... and

5 hours ago, Linas.P said:

....compared to older petrol models from Lexus this is high difference. I was comparing my brim to brim results on IS250 and they are pretty much spot on. They are slightly inconsistent throughout the tank....

Yes, I recall actual v. displayed consumption figures in my 250s would turn out almost perfectly matched by the end of a tankful after many fluctuations in between, especially around the halfway mark. I suspected, without ever confirming one way or the other, that the fluctuations were fewer during long drives with less stops and starts.

NemesisUK · March 15, 2017

But is it the OBC that's in error, or the manual brim to brim method?

5 hours ago, NemesisUK said:

OBC is an indicator of fuel metered over distance travelled. Brim to brim calculations are at the mercy of dispensing pump accuracy

Linas.P · March 16, 2017

On 3/15/2017 at 1:41 PM, NemesisUK said:

Yes, I would.But as I said fuel consumption is not really on my radar. I do keep records though, have done all my driving life. Any dramatic change could be an indication of some sort of problem.

Brim to brim calculations are at the mercy of dispensing pump accuracy (which will always be pessimistic, you will always get more fuel than the pump displays

I fill it, drive it and enjoy it, especially on such a beautiful day as this

Similarly, I am not really worried about actual consumtion - 37MPG is still good figure and I would be just as fine with something as low as 14MPG if driving something like RC-F. Fuel for me is not something I refill to go from A to B, but more like good beer or wine it is liquid to put smile on my face. Even more it is by far not the biggest expense (insurance is...)

However, here we discussing accuracy of trip computer, not the face values. It is clear that it is possible to show more accurate consumtion, but for some reason Lexus didn't do that in 300h set-up. I would guess that as always everyone does what they can get away with... 8% is not huge difference and most drivers won't notice, but it probably will help in believing that your car just that 3 extra MPG more fuel efficient which it isn't. Same can be said about speedometers - I hope you know that when it says 70MPH, in reality it is 65-66? I know that by law they should never show less, but 5MPH difference is slight overshot just to meet the law.

Finally, pumps are not charities.. yes they are required by law to never under fill, I believe maximum they can get away with is -0.05%. I remember watching 5th gear where they followed inspectors and conclusion was that every pump they checked was filling ~0.1-0.4% extra.... which is ~0.06-0.24L extra per tank... or even simpler the whole variable of brim to brim consumption per tank going to be that 0.1-0.4% due to extra fuel you get when filling. If trip computer shows around 0.5-1.5% higher consumtion, it means real consumtion is 0.4-1.1% smaller (even more accurate actually). So if Lexus could do that back 2005, there is no reason to believe they could not provide even more accurate readings today.

Rabbers · March 17, 2017

On 3/15/2017 at 8:09 PM, NemesisUK said:

But is it the OBC that's in error, or the manual brim to brim method?

I trust the manual brim-to-brim method more.

While some pumps can dispense and display wrong amounts of product some of the time (I imagine as often in the customer's

favour as not except in places where controls are fewer and fraud likelier), most of them do so accurately most of the time. Such

degrees of unreliability as may initially be seen or suspected in purchases-based consumption calculations will therefore tend to

become attenuated over time as the volume of purchases and number of re-fuellings grow. In other words, once the gap

between the OBC and brim-to-brim figures stabilises, the possibility of cumulative error in the latter, already small, will have

dwindled to insignificance. That brim-to-brim calculations seem, in the experience of owners of IS and RC hybrids, to be less

supportive of economy claims should not be taken to mean that Lexus is acting with dubious intent, but it is nevertheless strange

that a reputable car manufacturer should not err on the side of caution in calibrating its OBC - and be seen to be doing so -

especially when the consumption is by any token more than satisfactory.

On a connected subject, I have been glad to see that the total km displayed by the in-car counter (and by the satnav for individual

trips) and those recorded by the Tracking/Security device mentioned in my OP correspond exactly. I take this as a confirmation

that the RC300h's km counter is accurate (not that I ever doubted it) but, of course, a logician might argue that the possibility

exists that both devices are wrong ......

NemesisUK · March 17, 2017

So, if the distance measurement is spot on in the OBC, one must conclude the way the fuel is measured is erroneous? If it's similar to how MB do theirs it will be the duration of injection x flowrate?

royoftherovers · March 17, 2017

2 minutes ago, NemesisUK said:

So, if the distance measurement is spot on in the OBC, one must conclude the way the fuel is measured is erroneous? If it's similar to how MB do theirs it will be the duration of injection x flowrate?

I really think that we are trying to compare Apples with Pears.

OBC measures the current RATE at which fuel is being consumed having regard to past consumption as measured by the OBC and all of the data it holds.

Brim to Brim measures the fuel in total which has been lost prior to the last Brimful.

Simples aint it ?

No point in further discussion.

I will stick with Brim to Brim comparisons.

Regards

John