Oil Viscosity Impact On Fuel Efficiency?

[CMC]

Hi,

I'm experiencing lower than anticipated fuel economy on my IS250. There are many posts on here quoting high 20s even 30 as an average on mixed driving. And then those quoting mid-high 30s on motorway runs. My tank average from mixed driving on my work commute has just struggled over 26. And this weekend I reset on a motorway - two junctions - all set at 70mph and it just scraped 30. Only just.

Now, lets take driving style out of the equation. I'he had that debate and am pretty comfortable its good - but I'm focussed on it too. I'm looking at other ways I can improve the figure.

I need to double check with them but I'm sure the dealership said 10/40 semi synthetic had been used on the last change. I realise that is not spec. But assuming it is true, would a change to 5w have a noticeable impact? if so how much.

As a slight aside. I'm also planning to reset ECU and change to a K&N panel filter. Any other suggestions?

Cheers.

Chris

[blackcts]

I think 10w40 can be used, but 5w30 is the recommendation visosity. You may notice a difference in fuel efficiency with 5w30, but remember the 2.5 is a big engine and the current cold weather will have an effect on mpg.

[is200 Newbie]

I may be wrong but different supplier fuels have different results on mileage - such as Shell vs Asda for example

Brakes binding / air con on / lights / exhaust / heater / tyre pressures etc are all contributing factors

[CMC]

I think 10w40 can be used, but 5w30 is the recommendation visosity. You may notice a difference in fuel efficiency with 5w30, but remember the 2.5 is a big engine and the current cold weather will have an effect on mpg.

Thanks. Is a 5mpg fluctuation likely? Its not been that cold.

I may be wrong but different supplier fuels have different results on mileage - such as Shell vs Asda for example

Brakes binding / air con on / lights / exhaust / heater / tyre pressures etc are all contributing factors

Yes, I was under that impresssion and i'm testing the theory.

I'll clarify this is a totally unscientific test!

Week 1 sainsburys 25.2

Week 2 shell regular 26.6

Week 3 shell V Power work in progress

I thing my brakes are Ok. i use aircon all the time, lights when its dark ;-) exhaust OK heater at 21. Tyres as book.

Chris

[is200 Newbie]

Aircon will have a big effect on fuel usage, also a small drain in performance.

I was using Sainsburys fuel as we have a local garage (just for the convenience) Miles around town not too good but motorway fine. I used the Shell V power on last fill up and miles around town and motorway were both good. I had the 360 miles out of what the computer expected when filled up.

Not too sure on what effect having the heater on will do

[CMC]

Aircon will have a big effect on fuel usage, also a small drain in performance.

How big do you think? 5mpg?

Its not the end of the world, I knew I wasn't getting an economy machine. But I had assumed that around 30mpg would be an achievable average if I was sensible, from the reports read on here.

If I can do something to get closer to that then great. if not so be it. I still love driving it.

Sounds like the consensus (or the silence) points to nothing major to look at though. So I'll plug on with the filter and reset and see what happens. And will spec a thinner oil when its next due a service.

I like having aircon on, it keeps the screen clear, so will probably stick with that. And then see what happens.

[dave1]

Are you going by the figures generated by the OBC or by manually calculating by brimfilling? The oil is not going to affect fuel consumption noticeably. The cold weather always reduces fuel economy.....more so on shorter journeys.

[CMC]

OBC at this stage as I'm comparing with feedback from others which mainly talks of the tank average.

I'm not hung up on the ultimate figure I achieve, the fuel consumption per se is not the issue. Rather its that my experience is lower than those quoted by others in similar driving circumstances hence my wondering if there is something I can/should do.

I will do a brimming exercise at some stage but suspect that will show lower figures.

[noby76]

few things you can try which might help.

• switch off your air con and only use when needed.
• use the recommended 5W-30 fully synthetic oil.
• try using V power shell as I found out burns slower than standard 95 RON giving more mpg.
• drive in ECT PWR mode all the time. not just me..but most guys on the US Lexus forums also noticed a slight mpg improvement when driving in this mode compared to normal mode due to the engine throttle and transmission being more lively resulting in the car picking up speed from a stop more quickly then you can ease off the throttle earlier and cruise.
• inflate tyres to about 36PSI this will decrease rolling resistance thereby increasing your mpg slightly.
• loose some weight in your car. I am not saying you have but most drive around with a lot of junk in their boot or back seats of their car which contributes to low mpg. the lighter the car, the better.

you also mentioned you do mixed driving. is this a 50 - 50 split i.e. do you do say 10 miles town/traffic driving and 10 miles open road/motorway driving? cus if your daily commute involves driving more miles in town/traffic driving compared to miles driven in open road 6th gear then you wont achieve the low 30's combined mpg stated for the IS250.

[Argento]

Some great advice and tips here from Noby.

I agree with all of them, especially the ECT tip.

On < switch off your air con and only use when needed. >

If you do want A/C on all the time.

Try switching the A/C off a mile or so from home.

The evaporator will remain cool during that distance (and thus still drying and cooling the air) even though the compressor is turned off.

Argento (V-Power +Nitro all the time!)

[It wasn't me]

Lower viscosity oils will defiantly improve fuel consumption to a degree and so will inflating your tyres, the only downside to inflating your tyres is they will bow a bit like this ) but the other way round, causing your tyres to wear out prematurely in the centre, but yes, less rolling resistance.

Don't forget manufactures do a lot of testing before deciding on the optimum tyre pressures, based on ride comfort and efficiency.

Your tyre pressures will also increase as the temperature rises as you drive.

Your choice of tyres will make a difference as will staying away from cheap supermarket fuel

[It wasn't me]

Just looked the Q8 oils application database it says you can use

0w-20

2 types of 5w-30

And even a10w-40

The latest and best spec being the 0w-20, so if your dealer doesn't supply it, it might be worth buying it yourself and giving it to them to put in on your next service ,but only use reputable makes.

[It wasn't me]

A K n N filter will not necessarily make much of a difference if anything at all,they tend to be more free flowing less restrictive allowing moe air through on high performance cars when req'd,and will filer even smaller particles.

On cleaning and re - oiling you mustn't over oil,as it has been known for the excess oil to contaminate the air mass sensor sometimes, causing catastrophic engine damage,Subarus spring to mind.

[Creo]

I got my IS250 early December and was initially a bit dissapointed by the first couple of fill-ups. Read some advice to check the air filter, and possibly put some fuel additive at fill-up.

The air filter was very dirty! Replaced it, and added AR6200 fuel supplement for the next 3 fill-ups.

Result was my MPG went from under 30 to since mid-December to now I have averaged 33.4mpg in a mix of driving, including a lot of short hops to the local shops with the car cold.

I have the aircon on about 50% of the time, tyres are inflated to Lexus specs, I buy fuel from wherever I am passing when I need it, no idea what oil is in it, but I hadnt heard of the ECT power mode thing before so will give that a try.

The fuel additive may have helped a little to clean or whatever it does, it was £8 for the trial size but didn't last long and I havent used it for a month and still seeing same mpg.

So, my own experience of mpg improvement is check your air filter!

[CMC]

Thanks.

The filter has arrived, not fitted yet.

But, Creo your point is interesting. I also ordered some additives as discussed in posts on here. At fill up on the weekend I added the one recommended for de-carbonising (BG44K). Again as stated before, totally unscientific, but... The journey following fill up - around 75 miles on steady A roads and Motorways was high 30s. Then on my commute today I've averaged 33mpg, where last week it was slightly over mid 20s.

I've no idea if carbon can have that effect on economy, but early signs are good.

I also got some A6200 as my brief test of V-Power showed not much improvement over standard Shell. I've not used it yet but plan to at the next fill up.

Then I'll change the filter and do the ECU reset. I want to wait until the de-carboning fluid has done its work and things have settled down.

So early days but definitely positive signs.

I'll come back with further feedback.

Chris

[North west beast]

I've only ever got 30 to 31 max out of my two 250 s mate

[Argento]

It wasn't me, <it might be worth buying it yourself and giving it to them to put in on your next service ,but only use reputable makes. >

I'm up for that.

Have you ever done that, at a Lexus dealer?

If so, what was their reaction?

As far as I know, supplying your own oil is standard practice in Germany, I'm particular.

Argento

[Argento]

Creo, < ... now I have averaged 33.4mpg in a mix of driving, ... >

That looks about right.

I get very similar to that with my IS 250 F-Sport.

Easy to get 37+ on a long M-way run - even with a/c on.

I'll still try some of that BG44K and AR6200 stuff soon though.

BTW There's a big discussion on Club Lexus at the moment on carbonisation problems with IS 250s.

Argento

[Mouser]

Biggest thing IME is driving style. I get 30-35 on short to medium runs, and the other half gets 20-25. On long runs I'll be seeing ~45.

Once up to speed, the car doesn't slow down anything like as fast as most do, so you can let it coast down gentle inclines and as you approach drops in speed limit / junctions / roundabouts. The other car is an Audi A3 diesel, and in comparison that feels like the brakes are dragging it slows down so quickly off throttle. Remember that hard acceleration burns slightly more fuel than gentle acceleration (for the same change in momentum), but all the energy you lose when you brake is wasted, so it's best to work on that angle with mixed driving.

I'm running 5W20 full synthetic, you can get 0W20 if you prefer. When the car is being used primarily for shorter journeys thinner oil will lead to less wear as you achieve sufficient flow to lubricate much earlier. Tyres are at, or slightly above spec as the car's often loaded with stuff (babies - yay!), and aircon / heater / seat heating / radio are all used a lot.