Fuel Grade

[NAB]

I've got myself confussed with what petrol to put in my IS 250 SE MM - Automatic! I have had it for a couple of weeks and just put standard unleaded (95 RON) in it. However, have read a coupe of sites (and the manual) it looks like I should put premium unleaded (98 RON) in it. Is it necessary or worthwhile to use premium unleaded?

[Northern Boys Luv Gravy]

I've got myself confussed with what petrol to put in my IS 250 SE MM - Automatic! I have had it for a couple of weeks and just put standard unleaded (95 RON) in it. However, have read a coupe of sites (and the manual) it looks like I should put premium unleaded (98 RON) in it. Is it necessary or worthwhile to use premium unleaded?

It's not necessary to use 98 RON, and in a normally aspirated petrol engine you wouldn't notice any difference anyway. Audi recommends 98 RON for it's turbocharged 4 cylinder petrols, but even then the use of normal unleaded only takes a slight edge off performance.

Lexus recommends 95 RON and when you think about it, in the US they happily recommend the use of the ordinary unleaded you get there, which is the equivalent of 89 RON here. The engine adjusts automatically.

[Parthiban]

Yep 95 is fine, does it actually say 98 in the manual though? Find that a bit surprising actually as it's unusual for a car like this.

The US has a slightly different method of rating their fuel, so their 87 or 89 is the equivalent of our 95 - it confused me to begin with as well until someone explained it.

[SiWhite]

I run mine on both - sometimes at the same time. It doesn't seem to care.

With respect to a mixed tank though... I often put Shell v-power in the tank. There have been occasions where I've added half a tank of Esso standard unleaded. Why am I telling you this? Well because this mix gives me 41 / 42 MPG on a motorway at 'normal' speeds. At first I thought I was imagining things but I've tried a 50/50 mix several times now it always seems to give the same results. A full tank of either usually yields 35 - 38 MPG.

Go figure. Maybe it's a combination of the various additives that does the trick.

[Northern Boys Luv Gravy]

Yep 95 is fine, does it actually say 98 in the manual though? Find that a bit surprising actually as it's unusual for a car like this.

The US has a slightly different method of rating their fuel, so their 87 or 89 is the equivalent of our 95 - it confused me to begin with as well until someone explained it.

87 or 89 on the American rating is nearer 91 or 92 on our rating and we actually don't sell any fuel with that low a rating. Our lowest is, as you say 95 (Premium in the US).

[kenfoxy]

I run mine on both - sometimes at the same time. It doesn't seem to care.

With respect to a mixed tank though... I often put Shell v-power in the tank. There have been occasions where I've added half a tank of Esso standard unleaded. Why am I telling you this? Well because this mix gives me 41 / 42 MPG on a motorway at 'normal' speeds. At first I thought I was imagining things but I've tried a 50/50 mix several times now it always seems to give the same results. A full tank of either usually yields 35 - 38 MPG.

Go figure. Maybe it's a combination of the various additives that does the trick.

Sounds weird that. This points out 2 things. Firstly, there are differences in grades/brands of petrol. Secondly, If your car was CO2 tested on your mix it would end up in K not L and save £200 in VED per year.

[Gunther]

Lots of magazines have tested this to death, and have found that most of the differences are hard to tell for most cars. The only differences may be for detergents on the whole, and a slightly higher octane is largely not detectable.

If it works for you, use it, else save your cash.

[TestPilot]

I put 95 RON in my 250 when I had it... and plan to do so again when I get it back. I did a trial with Super in my turbo Volvos - there was a measurable - but small - increase in MPG, and a slight (but probably entirely imaginary) power increase - but not enough extra MPG to compensate for the higher fuel cost. Maybe if a car spends all day on the motorway it would be more worthwhile.