Two question for Colin Barber

[katabrontes]

Two regularly occurring themes on the Forum have puzzled me for a while.  They need an expert to consider them.

First is MPG.

My 2006 RX 350 averages about 24-25 mpg over a variety of uses.  However on a recent motorway run in France over about 300km it averaged 26mpg.  Others regularly claim much better figures for RX400 or 450.

Given that the hybrids have the disadvantage of carrying heavy batteries and, at motorway speeds rely entirely on the petrol engine to drive them how are these figures obtained.  Could it be regenerative braking which makes the difference - I am not sure how and I hardly touched the brakes on this motorway trip and used cruise control set at 130 kph throughout.  Could it be that certain accessories (eg Aircon compressor) might be driven electrically on the hybrids - I don't know about this?  Could it be the effect of the permanent AWD on the 350?  
 

After writing this I can't remember the second!  If I do I will post it separately.

 

 

 

 

[Sundance]

Hi Michael. Over the period of 16 months that I've owned my 400h I've noticed two stand-out factors that have a detrimental influence on mpg. One is use of E10 petrol (whilst I admit there is a cost argument involved, mpg is certainly higher when using E5) and two is constant use of A/C (turn off when not necessary to noticeably improve mpg). As for permanent AWD (which my 400h doesn't have) I believe 'yes', that will certainly have a negative effect on mpg.

All that said, I only manage 26/27 mpg on local short-trip driving but that can rise to 30/32 on motorway long distance.

Shall await with interest the response that you seek from Colin Barber 😉.

[LenT]

12 hours ago, Sundance said:

constant use of A/C (turn off when not necessary to noticeably improve mpg)

True…but deciding when it’s not necessary is going to vary from driver to driver.  It’s also a balance of benefits.

One aircon systems maker has these observations:

“Regular operation ensures its functionality and significantly increases its service life.  If the air con system is idle for too long, moving parts and seals can become porous, the risk of losing refrigerant increases and the oil in the compressor can degrade.

This can lead to costly repairs and shorter maintenance intervals. 

However, short time operations of less than ten minutes should be avoided.”

So, I should turn it off to pop up to our village shop and then remember to turn it on again for a drive to the next town.  I - or more likely MrsT - in the absence of a/c will then decide on the internal temperature while not bearing in mind that this can be individually selected between passenger and driver.

As I do a low mileage anyway, the potential mpg saving is outweighed by the maintained system efficiency and marital calm achieved by keeping the system switched on!

But that’s just me! 

[katabrontes]

I was surprised by your observation about ac Rowley.  I have only tested this on my Saab 900 turbo (sadly long defunct).  I ran two full tanks one with ac on and the other with it off and could barely detect any difference in mpg.  It was over 25 years ago and I can't remember the figures but it usually averaged about 24mpg.

I have always left ac on all the time ever since.  I am sure the benefits far outweigh any effect on mpg.

I agree about E10 and always buy E5 when I can get it.  In France E5 is readily available as either 95 or 98 octane.  In UK it's getting hard to find and only in 98.   Advice from the Lexus agent in Bordeaux was to stick with E5 as the saving on cost was more than offset by the poorer mpg.

[RONNIE W HODGEKINSON]

15 hours ago, LenT said:

True…but deciding when it’s not necessary is going to vary from driver to driver.  It’s also a balance of benefits.

One aircon systems maker has these observations:

“Regular operation ensures its functionality and significantly increases its service life.  If the air con system is idle for too long, moving parts and seals can become porous, the risk of losing refrigerant increases and the oil in the compressor can degrade.

This can lead to costly repairs and shorter maintenance intervals. 

However, short time operations of less than ten minutes should be avoided.”

So, I should turn it off to pop up to our village shop and then remember to turn it on again for a drive to the next town.  I - or more likely MrsT - in the absence of a/c will then decide on the internal temperature while not bearing in mind that this can be individually selected between passenger and driver.

As I do a low mileage anyway, the potential mpg saving is outweighed by the maintained system efficiency and marital calm achieved by keeping the system switched on!

But that’s just me! 

Totally Agree ken....

[Sundance]

20 hours ago, LenT said:

True…but deciding when it’s not necessary is going to vary from driver to driver.  It’s also a balance of benefits.

I too agree with you Len, like many decisions we make such an action is purely subjective.

[Greisingel]

While driving at 65 mph average in France over 1000 miles on those wonderful long straight autoroutes (typically 2-lane) I crept up to 36 MPG (much of the driving I left to the cruise control). I had wondered if the quality of French fuel was in some way superior to the Brit E10 I typically use or that the CC was more accurate than my lead foot.

I am back down to 24 MPG for my typical station run and short journeys.

 

[ColinBarber]

On 4/6/2024 at 9:19 PM, katabrontes said:

My 2006 RX 350 averages about 24-25 mpg over a variety of uses.  However on a recent motorway run in France over about 300km it averaged 26mpg.  Others regularly claim much better figures for RX400 or 450.

Given that the hybrids have the disadvantage of carrying heavy batteries and, at motorway speeds rely entirely on the petrol engine to drive them how are these figures obtained.  Could it be regenerative braking which makes the difference - I am not sure how and I hardly touched the brakes on this motorway trip and used cruise control set at 130 kph throughout.  Could it be that certain accessories (eg Aircon compressor) might be driven electrically on the hybrids - I don't know about this?  Could it be the effect of the permanent AWD on the 350?  
 

I just looked back at my Fuelly stats when I had a 400h and it would achieve 32-33 mpg on longer runs - I'd expect similar with an RX350, but once you get over 70-75 mph then you economy will start to drop off quite considerably due to the size and aerodynamic shape of the RX.

It's certainly the case that the hybrid benefits are mainly below 50 mph, especially for older Toyota generation 2 hybrid system that the RX400h had - there isn't much assistance that can be provided at higher speed, and very little regeneration braking going on. Many of the differences between hybrid and normally aspirated balance themselves out - e.g. 400h is heavier but the 350 has higher drivetrain losses. Aircon, whether it is electrically driven or mechanically via a clutch use about the same amount of energy. Therefore high speed economy is roughly the same.

This is how Lexus US positioned the two:

Q: How should I choose between an RX 350 and an RX 400h hybrid?

A: If most of your driving is highway cruising or you leave your vehicle parked for several weeks at a time, the RX 350 may best meet your needs. If most of your driving is stop-and-go city and you want improved acceleration and fuel economy, the RX 400h may best meet your needs.

[katabrontes]

Thank you Colin for these helpful observations.

Mostly I avoid very short runs.  In UK most are about 15 miles or so to do shopping.  Occasional longer runs.  Speeds are usually about 30-50 mph.

In France runs are about the same for shopping and speeds a bit faster as the roads are clear.  About 4 round trips of about 350 miles mostly motorway when speed is more or less constant 85mph.

I haven't ever bothered much with fuel consumption.  I think in terms of full or empty!  Petrol is a relatively small part of the cost of running a car.  If you buy and change new ones it's trivial.  I tend to buy a good example at about 5 years old and keep it until it starts to cost more to service than it's worth, or even longer if I like it.  The inevitable odd scratches neither matter not bother me.  The first one on your shiny new car must be very upsetting.

I dislike new cars.  They smell horrid, everything is stiff and differs from its predecessor.  My experience has been that they often go wrong soon after you buy them and depreciation is massive.

Although I still enjoy driving I regard a car as a useful tool and instrument of freedom of choice.  I like it to be comfortable, quiet, big enough to provide for my needs and easy to drive. A few bells and whistles are nice if they enhance safety or the driving experience.

 

[Ken R]

18 hours ago, katabrontes said:

I dislike new cars.  They smell horrid, 

Can't agree sorry! If you could bottle the smell of a new car you would make a fortune!😁

[Sundance]

5 hours ago, Ken R said:

Can't agree sorry! If you could bottle the smell of a new car you would make a fortune!😁

You'd need to ... to compensate for the depreciation the minute you drive it off the forecourt! 😉

[katabrontes]

Just as well Ken because you will need it to pay for it!

[Malc1]

17 hours ago, Ken R said:

Can't agree sorry! If you could bottle the smell of a new car you would make a fortune!😁

I’m sure some clever business somewhere has done that ……. wonder who 🤔

Malc 

[Ken R]

1 hour ago, Malc1 said:

I’m sure some clever business somewhere has done that ……. wonder who 🤔

Malc 

Magic Tree tried................and failed as far as I'm concerned ☹️