A year or two on I have had a sports cat and straight exhaust fitted, did a similar 500 mile round trip on the motorway, one way fairly brisk, other way a bit pansy as I didn't want to stop for fuel and managed 36 mpg average. What with the cost of fuel this was on boggo Saino's fuel as didn't see the point in V-Power sitting on the motorway. Very similar to the 37 previously reported which was pre-exhaust and running on V-Power so I'd say that was within the experimental error. I rekon if you sat at 55 on the motorway you could eek 40mpg out of it, but where is the fun in that?
Just my thruppence into the ongoing chat between KungfuPanda and msgtv-
gives a torque vs speed brake specific fuel consumption map for the Ford Zetec, a fairly similar vintage 2.0 l engine like ours. This pretty much tells us all we need to know. The sweet spot is in the 245 g/kWh circle in the middle. For fuel economy we would want to stay in here at all times, ideally 2250 rpm at about 3/4 pedal position. When straying away from here, it is all about which loss factor becomes dominant.
Higher speed - friction increases linearly
Lower speed - heat losses increase at a faster rate than friction goes down, and at lower speeds, mixture enrichment is required for stable combustion
Higher load - again mixture enrichment is required as the engine begins to become airflow limited
Lower load - pumping losses increase due to throttling
As such, there are valid points made from both commentors. The strongest effect is running at high load, the contours of BSFC are very tight above the sweet spot as more and more fuel is required for a given increase in torque. Volumetric efficiency is low at this speed, meaning the engine simply doesn't have enough air to burn all of the fuel. This is why gasoline torque curves peak around 4000 rpm, where volumetric efficiency is highest - there is more air available so more fuel can be injected. The TS has a small trick up its sleeve here with the variable intake length and valve timing, so might be better optimised than the Zetec in this respect. Reducing load has less of an effect as there is plenty of air.
In the real world, all this means is that you want to be using whatever gear keeps you working the engine at least a little bit (to avoid silly pumping losses) while being in the 2000-3000 rpm range. Around town for example, using 5th gear at 1000 rpm and high torque (what I like to call Taxi driver mode) is not what you want, you are losing too much heat per cycle and have no power in reserve if you need it. Being in first gear is silly as you are revving the nuts off and friction is high. You really want 3rd gear at 30mph, where you will still be low on the torque axis, but BSFC is all pretty flat around there. Maybe you could gain a percent or so in 4th, but then you won't be anywhere near the power band for acceleration if needs be. On the motorway then of course you want high gear as engine revs are above the sweet spot even in 5th.
When it comes to acceleration, then I have to err on siding with KungfuPanda - but not totally. *Full* throttle is always going to hurt fuel economy as it tells the engine to deliver power above all else. But gently trickling up to speed is not ideal either. Think of it this way - we consider fuel economy in miles per gallon, when you are accelerating gently, you will be using less fuel, but you will be doing it for longer in a less efficient manner, covering less distance than the guy who is accelerating at say 75% throttle but changing up at 3000 rpm in an efficient manner. He has reached his top gear at 50 mph and is settling down to a nice efficient cruise while you are still at 25, putting fuel into accelerating. You will have put less fuel in so far, but he is closer to his destination
In the real world, as long as you are neither hammering it or trickling along, it is not going to make a hell of a lot of difference. The BSFC chart is fairly flat below the sweet spot, but it's nice to feel like you are both making progress *and* doing it in an efficient manner. I hope this is of some use to you guys.