I have a Cooper S with the 6spd manual, and was wondering if anyone has ever done a taller 6th gear to lower the RPM at cruising speeds?

As it is around 70-75 the car sits around ~3K RPM in 6th. Given the torque of the engine, it seems like it would still do well around 2,500 RPM crusing, or at least a little less to improve mileage a bit.

Anyone ever done this? Thought about this? Looked into this?

Jason

 

I've also wondered about this. As much rural highway driving as I do I would really love a taller 6th gear. And like you said it would seem to be fine. I rarely downshift to accelerate it seems.

 

Honestly...don't think the motor could keep the car moving at 75 at such a low rpm...peak tq is 4500 rpm...and if you monitor mpg, thebetter mpg actually occurs in 5th gear at higher rpms till about 60...5 AND 6 are both overdrive....a scanguage will give you tons of load info....and actually SHOW why mpg is better in 5th....all about tq, and the bypass valve....you are not in boost at that low rpm.
One way to play with this to to get a slightly larger diameter tire than stock...will close up tye wheel gap...but you do loose acceleration due to taller gearing. Unfortunatly, changing gearing in a fwd car is kinda hard...not as simple as a rwd car anyway!!

 

Very good point Zippy, the more the engine is in it's power band, the less it has to work, for sure. I already run a slightly larger tire, but haven't tried running in 5th vs 6th. I'll give that a shot, thanks!

 

At 70-75 mph I would be in sixth. But I agree, taller gear doesn't necessarily mean better gas mileage. I find about 2300 rpms in 5th gear (40-45 mph?) to give me the best gas mileage. 2300 rpms in 6th (50-55 mph???) gives me worse gas mileage, because there's less power and I have to give it more gas to keep the car up to speed.

So I'm not sure a taller 6th gear is going to get you any better gas mileage.

 

Remember that drag increases as the square of speed. Or Drag = (speed)^2

A ten mile per hour increase in speed increases the drag by 50%. Check my math - (55^2 - 45^2)/45^2 = 0.494 or 49%

That is why you get worse mileage. If you want to compare mpg for different gears, you have to use the same speed.

Bob

 

That's a good point. I should have used same speed instead of same rpms. I'll check into that, but I'm pretty sure 5th gear still gets me better gas mileage in the 45-50 mph range than sixth does.

 

I've always thought it had more to do with throttle position than RPM. To maintain a certain speed of X may happen with the throttle not open as much in a lower gear, because the engine operates more efficiently when the rpm range is within the peak torque and horsepower range. That's my theory anyway. My other car (2.5l normally aspirated A/T -wifes car) gets better milage when I put it in sport mode, because it will hold the lower gear (higher rpm) for longer without the driver needing to floor it. The significance is greater when more load is required (like going up hill, or having more wind resistance). Otherwise, going down hill, higher gears are better because I don't need to open the throttle as far to maintain speed by preventing engine compression braking.

I think TIME is also an important consideration. With the mini I'm finding, that although I can gently accelerate to 60, without creating so much load that I hear the s/c whining, letting each gear run up to 4500rpm or so, being smooth etc.... It takes a while to get to speed that way. Also, cruising along at about 65mph, and approaching a slight grade (uphill), I obviously have to push down more on the throttle to maintain 65mph, and the OBC may indicate going down from say 29mpg to 18 or 25mpg. How much TIME the car is consuming fuel at the higher rate depends on how long the hill is, obviously. I found that giving the throttle a quick stab to run it up to 70 MPH drops milage to 14mpg, but only for a few seconds, then I can lift out and see the mpg go up to 50 or 75 MPG as I ascend the hill with speed gradually falling back to 65mph.

I know, I'm a little long winded, sorry. But think about this too -sometimes we think that milage falls in traffic, because it's stop and go... Well I try not to stop and go too much, creeping along at a steady pace as much as possible -but that aside, driving a distance of 60 miles at 60mph means you'll be burning fuel for 1 hour. Traveling the same distance at 30 mph means you'll be burning fuel for 2 hours.

Hmmmm

 

The supercharger is not load dependant, it turns all the time, boost is determined by the throttle position not load. The further the throttle is open, the more air flows, the more boost it makes.

Also, note that the automatics do turn at about 2500 at 70 mph, and they tend to get the same or even a tiny bit better (1 mpg) mileage.

The cost of fitting a taller diff ratio or just 6th gear would more than outstrip any savings in fuel mileage you might get.

 

I just put taller tires on my car (2005 mcs) and I DO get a bit better mileage. I'm running 215 45 18 (18" wheels). My speedometer is STILL a little bit generous with them on (it says 60mph and I'm actually going 58.5)