Z4 E85 2.5i - Engine RPM v. Road Speed

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portculisz4

Member
Appreciate some feedback on engine performance v. road speed. Just getting used to my 2.5 Z4 and finding it very different to my LR Disco. in relation to engine speed when cruising at 70 mph. The diesel plonks along at around 1650 rpm at 70mph while the Z4 seems to turn over a little more briskly (around 2800rpm), I assume higher rpm's are normal on petrol engines s can someone confirm this is what I should expect from my 5-speed manual.
 
Perfectly normal.

Your diesel will probably redline at 4500-5000rpm

The petrol will probably redline at 6500-7000rpm

The diesel has different gearing to take advantage of the diesel's lower-down power band - as the petrol has different gearing to take advantage of it's power band higher up the range.

In a lot of cars the extra 6th gear is fitted to enable slightly lower revs at cruising speeds, to give better MPG.

For example, my Z4M sits at 3250rpm in 5th at 70mph - but then goes on to 170mph at 7900rpm (where both max power and rev limit sits). In 6th it sits at about 2750rpm at 70mph, but won't go over 160mph (due to soft limiter).

My 320D gets it's max power at 4000rpm, so almost 50% less revs, with it's max torque in a narrow range between 1750-2750rpm - so that's where the gearing puts it for efficient driving for the 8-speed ZF auto. So my 70mph cruise in 8th is at 1650rpm.
 
Mine sits at around that RPM so i normally sit at 60mph to give it less of a hard time lol! Shame really, needs another gear!
 
Many thanks for the input, pity you can't buy a 6th gear off the shelf as 'after thought' add-on, despite that she's remarkable quiet with top down at 60 to 70 mph, a darn sight quieter than my wife's old SLK 230
 
I had a 6spd Miata before 2.5i E85 like yours, and yes, I find that the lack of 6th gear is shown on the highway, especially at over 190 km/h. And I think with 6spd it would consume less fuel. But as you said, it is perfectly normal to sit on 70 mph at 2800 rpm.

But what I found very nice, is that for a roadster, it is not so loud at higher speed, so you can expect to go with it on 4.000 and 5.000 rpm easily, and 6 cylinder works calmly on those rpms.
 
That sounds about right. :thumbsup:

I had an E46 325ti Compact with the same M54 engine and in 5th that was revving just under 3,000 rpm at an indicated 70 mph, but didn't sound at all strained - after all the red-line was at 6,000! It also meant if you got slowed on a motorway it picked speed back up easily without needing a down-change. Later facelift ones did get a 6-speed box, but I couldn't find one when I was looking! Although strangely the facelifts had a 6,500 rpm red line. :?
 
When I collected mine not having a 5th gear annoyed me but managed to sit at 1500 RPM at 60MPH and gain 47mpg on a 100 mile jouney, all motorway so now I'm not to fussed! :P
 
Out of curiosity, is it possible to bolt a 6-speed box straight onto a pre-facelift 2.5?
 
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