Falling MPG any ideas?

DKLS

It may need an Italian service, my mother has a Yaris, and once a year I borrow it for a weekend and after being driven it runs fine.

Ultrasonic

Norman_Castle wrote: »

The real mpg. The trip computer will adjust for tyre wear.

We're talking small differences here but I'm doubt if the trip computer compensates. How could it do this?

In terms of an effect on real mpg I'm even more sceptical. As tyres wear their rolling resistance decreases so the more usual argument is that mpg improves with tyre wear. I hadn't thought of your point about slightly higher engine revs before which will have the opposite effect, but which effect is bigger I don't know. Both are MUCH smaller than the mpg differences discussed earlier in the thread though.

Mr_Mink

Ok, so the fuel I use is near enough always from BP. I occasionally use a different petrol station but it's still a BP one.

I think I'll try out the Italian tune up followed by alastairq's advice of making a conscious effort to drive economically, see if either of those do any good.

mcpitman

Tyre pressures - mentioned/checked
Thermostat - mentioned/checked
Brakes ceasing on - mentioned checked
Oil service - mentioned/checked

Air filter change - mentioned... checked?

Last thought, when was the last time the fuel filter was changed?

Has this model got an OBD port you can plug into and get any fault codes read?

Norman_Castle

Ultrasonic wrote: »

We're talking small differences here but I'm doubtfuk the trip computer compensates. How could it do this?

In terns of an effect on real mpg I'm even more sceptical. As tyres wear their rolling resistance decreases so the more usual argument is that mpg improves with tyre wear. I hadn't thought of your point about slightly higher engine revs before which will have the opposite effect, but which effect is bigger I don't know. Both are MUCH smaller than the mpg differences discussed earlier in the thread though.

The computer analyses the difference in wheel speed via the ABS sensors. If this is done while traveling in an arc, as when turning, the circumference can be calculated.

almillar

Agreed, but why empty the tank?

I did say not quite - and it's just to maximise accuracy - the bigger the range you do (miles) for as big a sample (gallons) as possible, will give you a more accurate result.

Ultrasonic

Norman_Castle wrote: »

The computer analyses the difference in wheel speed via the ABS sensors. If this is done while traveling in an arc, as when turning, the circumference can be calculated.

Think I'm with you now, I hadn't come across that before. Do many modern cars do this or just very high end ones?

Norman_Castle

Ultrasonic wrote: »

Think I'm with you now, I hadn't come across that before. Do many modern cars do this or just very high end ones?

Its only the high end ones, although mid range and cheaper cars all have it as well.

forgotmyname

How will it know the circumferance though?

The travelling in an arc will just mean one tyre is X% bigger than the other. It wont know that you have super knobly treaded remounds that are 20mm taller than the standard tyres will it?

It will know the difference between the actual wheels where you may have a new one and an old tyre.
I think its just an estimated value. 99% of tyres are this size when new and that size when worn so take an average which will only be about 5mm out either way.

Norman_Castle

How will it know the circumferance though?

It will use the circumference of the tyres when new as a median diameter. The reduction of this diameter over hundreds or thousands of miles is measured by the onboard computer which shows the amount of wear.. When new tyres are fitted the computer recognises the sudden increase in diameter and re-calibrates.