I've been using BP Ultimate for a few months now, and I had used Millers for a couple of months before that.
Neither product gives a noticeable improvement in MPG. That's not to say there's no improvement, just that it's drowned out by the natural variability in economy from one week to the next.
However, my engine is a bit quieter on Millers, and quieter still on Ultimate - especially when doing a cold start. I also notice an improvement in off-boost pull with both products, though it's more pronounced with Ultimate. This is particularly noticeable when pulling away from stationary before the turbo has spooled up, and it helps to smooth out the power delivery. The engine does seem generally a bit more willing on Ultimate, though it's a subtle imrovement rather than a "wow" factor.
Also noticeable is the reduction in smoke. On Ultimate I can hardly manage a puff of smoke for the tailgater behind at all.
My car is currently in need of refuelling, so I think I'm going to try a tank of untreated normal diesel for the sake of comparison. The last time I tried this, I couldn't stand it and ended up adding a shot of Millers. One thing I found was that the performance improvements happen immediately with diesel, unlike with a petrol engine that seems to need to "learn".
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