308 II Tyre pressures and fuel economy

[Fred17]

Hi All,
I checked the tyre pressures on my new car and they were lower than the placard, so I took them to 37F/33R. The ride was awful! It seems the former owner (who drove track cars, and had just bought a Clio) knew a thing or two, so I dropped it back to the original 35F/31R. This is the first time I've ever come across a car with high placarded pressures.

The lower pressures certainly don't affect fuel economy. My first "proper" fill, where I'd worked out how to reset the trip computer, gave me 6.0 l/100km. The car reading was 6.3. The second fill gave me 5.7l/100km with a car reading of 6.2. I was astounded, because while there was little town driving in both tanks, the second included long sections of 110km/hr zones which used to kill economy in my previous Fiesta. It's also a bit odd to have odo/fill figures better the trip computer. My Koleos is regularly 0.5l/100km optimistic.

Those economy figures are on E10, too. The previous owner said he'd only ever used 95 or 98. The "Trip 1" in the trip computer was stuck on 9999kms and 8.8l/100km, which is pretty ordinary economy but it was a city car previously, a shopping trolley. It took me some time to work out how to reset it (you have to be stopped, for one thing).

Not only do I have a quick, nice handling and comfortable car, but it gives me superb fuel economy. It's many years since I bettered 50mpg

 

[Whippet]

Hi. Ethanol has lower calorific energy (heating value) by volume than petrol So E10 will have slightly less calorific energy than petrol, so to perform the same work, slightly more volume of E10 will be consumed. Therefore your fuel economy will be slightly worse with E10.

Unless the engine design and engine management system can take advantage of the higher octane rating, there is nothing to be gained from using 98 compared to 95. Except more profits to the fuel companies.

Others here will have different thoughts on this. My approach has been to add 5% to 10% to the pressures recommended by the tyre placard on the vehicle. Better tyre life, and no diminution of fuel economy. Personally I would find it very difficult to observe a difference through the seat of my pants of 2PSI inflation pressure variation.

 

[Fred17]

Hi. Ethanol has lower calorific energy (heating value) by volume than petrol So E10 will have slightly less calorific energy than petrol, so to perform the same work, slightly more volume of E10 will be consumed. Therefore your fuel economy will be slightly worse with E10.

Unless the engine design and engine management system can take advantage of the higher octane rating, there is nothing to be gained from using 98 compared to 95. Except more profits to the fuel companies.

Others here will have different thoughts on this. My approach has been to add 5% to 10% to the pressures recommended by the tyre placard on the vehicle. Better tyre life, and no diminution of fuel economy. Personally I would find it very difficult to observe a difference through the seat of my pants of 2PSI inflation pressure variation.

Yes, I am aware of the lower calorific value of fuels containing ethanol. That was one of the reasons I was so surprised at the stunning economy, especially as I hadn't driven for economy and a significant section of the trip was through windy and hilly country with several overtaking manoeuvres.

I could definitely feel the difference between 2psi front and 3psi rear. The point I was confused about the placarded pressures and conversions, and think the 35F/31R is about right. I recently had new tyres fitted to the Koleos and had a noticeable improvement in economy (albeit over a short distance) with higher tyre pressures. I had previously kept tyres at placarded presssures.

 

[Brodie Shields]

"It took me some time to work out how to reset it (you have to be stopped, for one thing"

Just press the RH stalk, then cycle trough to the trip you want, then long press to reset it to zero. And you can do that on he move.

 

[Fred17]

Hi Brodie,
It took me a week to get the long term trip reset, the one that was sitting on 9999km. When I long pressed on the move, it just jumped to another screen. It was only resettable for me with the car stopped. Now I'm baffled by the stereo, I pressed something wrong and instead of my USB stick playing from the start, it's jumped to albums, and plays a small group of songs over and over (the USB isn't set up in albums, so I don't understand why the system would decide there ARE albums lol).