Obc Mpg Vs Pump Maths......

[PJ S]

Following on from the photo in this thread - http://www.lexusownersclub.co.uk/forum/index.php?showtopic=77913 - I'm having a hard time struggling to comprehend the maths.

Here's the story......

Picked up the car, range estimated at 43 miles or thereabouts.

Decided to fill the tank nearby, before heading onto the M40 and M6, where I'd refill with Tesco Momentum 99 for a first time.

So, 56.5L duly fitted in, I reset the trip and head off in search of the M40 at Thame/Oxford.

Arrive at Dumfries, ascertain directions to the Tesco station, and refilled with 23.5L. Didn't quite neck the tank like earlier, but from pump clicks, I reckon I was shy about 3L - needle on gauge wasn't all the way vertical.

Anyway, from the assumed consumption (circa 26.5-27L) and distance travelled (313 according to Google) - yes, I forgot to note exactly what the trip recorded, but recall the gauge being slightly above the half tank mark - the maths suggests I averaged 51-52 mpg!

The OBC was rather less ethusiastic, claiming it was low 40's.

Therefore, I know the maths is correct, and if the assumptions are accurate (which I believe they are), can anyone shed some light on the discrepancy?

Even if I was 5L low, that still equates to just under 50mpg.

If anyone else had said they worked out their mpg to be the same, I'd have laughed at them, and told them to put in a missing decimal point - but the more I work it out (there's only so many ways you can multiply and divide), the more I have a hard time getting to grips with the possibility that a 3L V6 can seriously do that sort of economy.

I know we all give OBCs a bit of leeway, erring on the side of more than it really was, but this is so far off the other way, it has had me questioning my sanity - a car that heavy and with an autobox, and that size of engine, just doesn't do those sort of figures.

I've never got anywhere near that in the Accord - and it was a 2.4L inline 4, with manual stir pot.

[noby76]

modern petrol cars have really improved in consumption but 52mpg is really optimistic for a 3.0 V6 petrol. I think you might have consumed more litres than you thought. the GS300 should officially acheive 39mpg on motorway driving so the maximum bonus mpg you can extract will be around 45mpg on motorway driving which in litres consumed will be.

313 / 45 = 6.9

6.9 * 4.54 = 31.5 how did i get this figure?

313miles which was total miles driven divided by maximum bonus mpg of 45mpg = 6.9 gallons. 4.54 litres makes 1 gallon so to get how many litres 6.9 gallons is we do 6.9 times 4.54 = 31.5litres

so i can confirm you used about 4.5 litres more than you thought and thats even with you acheiving a bonus of 45mpg rather than the 39mpg quoted so the OBC figure of 40mpg was more closer to the official motorway mpg figure of 39mpg.....

[PJ S]

Trust me noby, there was never another 8L of fuel fitting into that tank at Dumfries.

That would've been another £11, and the pump was clicking at £30 ish. I rounded up to £32 (135.9), which gave the 23.5L refill amount.

I'm happy to concede possibly 5L more may've been it necked - but that's still only 28.5L used, which is way over and above the manufacturer's suggested urban figure.

That's what's thrown me - had it been 2-3 mpg more, I'd have written it off as within tolerance, more or less.

That it's between 8 and 10 mpg more, is:

a ) a bloody miracle

b ) something wrong with the maths (the equation is correct!)

c ) Lexus chose to be pessimistic about what it can achieve

d ) Maradona was behind me the whole way!

Since I don't believe in Santa anymore, that's (a) out, and I know the maths stacks up, so (b ) is out too.

Can't see ( c) being the case, so that only leaves.........(d).

There we go, solution was under the rear bumper all along - Maradona was pushing me with his Godly hand.

Had I feckin' well known that, I'd have asked him for his autograph!

[noby76]

i do agree with the mordern lexus engines having dual vvti, direct injection, high compression ratio coupled with an effecient 6 speed auto box with you running a high ron fuel i would say it is possible but will still limit it to 48mpg and thats even me pushing it. I say that because i do get 33mpg in my IS3 on combined motorway and town crusing which is 7 mpg more than the 26mpg combined figure quoted by Lexus and I use Esso 97. people also forget big engined cars hardly work when crusing at 60-70mph i mean at 70mph my revs sit at 2500rpm which is about 1000rpm above a mordern deisel car doing the same speed and with your having 6 speed will be more of around 2000rpm at 70mph.

why bother with deisels huh....

[PJ S]

I neglected to mention, I sat at mostly 60-65 mph, with cruise on, and flicked the lever up or down to accommodate other road users.

A few times I had to push it to 80 to get ahead of the elephant racers and caravaners in the inside lane, to slot in, and not become one of those dawdling MLMs (middle lane muppets).

That and slowing down/speeding up at the various roadwork sites with 40/50 mph restrictions.

The only break in the flow was just coming up to Carlise - the TPMS threw a CHECK SYSTEM up, so drove to the next service station (a few miles iirc), whereupon tyre pressures were duly raised to what they should've been!

A quick adjustment of the personal fluid level as well, and off I set again — accelerating steadily back up to 65 ish once more.

Like I said, if the assumption is correct, then the maths indicates a superb figure for such a car, and quite why you'd want a diseasel is beyond me. Even with the hybrid's better town/city low speed economy, I'm really pleased I opted for "just" the 300.

As much as the maths doesn't lie, I still question the figures, even though I know they're reasonably accurate.

I'll refill to the brim again, and note the amount I can put in once the pump handle starts clicking a second time.

Wished I had really brimmed it in Dumfries, then there'd be no question of amount used.

Would be interesting to see how things may've differed if I'd known where the nearest Tesco was in Aylesbury/Oxford - 99 RON the whole way!

One other aspect that has me convinced the figure is not far off being true, is the amount of time the instantaneous mpg display was pegged at 90, and dancing back and forth between it and 60.

At the end of the day, even if it worked out at 40 mpg, I'd still be a happy chappy — this is just an added and most welcome bonus, and one where I wished I'd been able to buy a couple of years previously.

Funny enough, with your mention of (Esso) SUL, I rarely bothered with it in the Accord - never really seemed to make enough of difference over UL to warrant sticking with.

I normally stick to using Texaco, as ours has Techron additive, both Northern and Southern Ireland stations — the same thing (only less) as BG44K, which I'd never heard of until I called into a Honda dealer for directions, a god few years ago now.

Always felt the Honda ran a bit better with it, and Asda/Tesco were fine too, hence why I was keen to see how the Momentum faired.