My mum is complaining about the amount of fuel the Legnum ST-R is using, it tends to get about 9l/100km with her driving it, whereas when I drive it I normally get around 8l/100km...unless I'm listening to that lovely exhaust note at high revs....
Anyway, it's already got lightweight Enkei RPF1 17x7 wheels on it, they weigh 6.6kg each, the stock ones are more like 9kg each, and 215/45/17 tyres are lighter than the stock 205/55/16 tyres. Furthermore the 215/45/17 tyres are slightly undersize by 1%, which isn't much and would only equate to 5-6km difference travelled per tank, but the smaller radius compared to correctly sized tyres would affect the acceleration of the car too, so I'm willing to bet that the difference is a little more than 1%. I normally keep the tyres at 38psi front and 36psi rear, so it isn't due to them being flat!
It also had it's fuel filter, spark plugs, engine, trans and diff oils replaced at 100,000km, Amsoil was used throughout, it's done 9000km since then so it isn't as if old oil is causing any problems. I'm going to do an oil change on it in when it has done another 1000km though, so I need to decide what is best, and what my other goals are.
The things I can think of to change it are:
Install a new O2 sensor, the current one is most likely 11.5 years old and has done 109000km, it is possibly reading slightly incorrectly
Remove the cat, it will reduce back pressure a bit which might decrease fuel consumption
Install my S-AFC I have sitting in a box, it might make a small difference. I've only got one AFR graph of an ST-R, and it sits bang on 12:1 the whole way through the graph, increasing it to 12.5:1 would perhaps decrease fuel consumption, but would necessitate using 95 octane..which brings me onto the next thing
Use 95 octane? I've always put 91 into it, and my mum would take a lot of convincing to put in 95 due to the increase in price. Especially because it would take two tanks of 95 to get all of the 91 out so I could reset the ecu to get it to learn on the new petrol
Put a stock exhaust system back on it. I'm guessing the original one had rusted out, the exhaust after the rear axle has been replaced, perhaps the stock oue would be better, or perhaps use a VR-4 exhaust, or even a aftermarket VR-4 exhaust? My mum quite likes engine noise, she has the Monaro after all
Lower the front, it does lots of motorway driving and the front is very high on it, there is about an 8-10cm gap between the top of the tyre and the wheel arch. I'm thinking that some Legnum lowering springs in the front would perhaps help the aerodynamics of the car and this might make a small difference with the amount of 100kmh driving it does
When the tyres are worn down perhaps replace them with a type that mentions fuel saving in the specs? It has GSD3's on it at the moment which are performance orientated. Hopefully this would also help with the road noise, because it is quite loud with those tyres on it
Install a longer final drive - this one is really at the bottom of my list, but perhaps when the clutch gives up it won't be a bad thing to do? The ST-R final drive is 4.055:1, while the 2wd V6 is 3.722:1. I don't know if they would be compatible or not, but it would drop the revs down from 2640rpm to 2420rpm at 100kmh which might help, but obviously at the expense of acceleration and fuel economy around town.
Do the MAF mod, perhaps meaning I won't have to install the AFC at all, can't see it making much of a difference, but worth trying because it is effectively free
Install MIVEC heads and set the change over cam point at 4500rpm which my mum never revs to anyway. My dream But ultimately way too expensive. Good fun though
Any thoughts or any comments at all? Any new ideas on what to do?