EPA ratings are relative. We don't expect to get those figures in actual driving experience, unless we're lucky or very careful.
But can anyone explain to me why a BMW with a manual transmission scores lower EPA ratings than the same model with an automatic? Manual transmission cars are lighter and more fuel efficient. Why don't the EPA testing methods reflect this?
As an aside, the 2006 550i carries a mandatory gas guzzler tax if the car if equipped with a MT but not with an autobox. Go figure!
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It has a shorter final drive, so the engine spins faster when you drive down the road and subsequently uses more gas. The upside is that it accelerates faster, like a 550i Auto is slow 
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| Reply » EPA ratings - Auto vs Manual |
Well, modern autoboxes are much more efficient than they used to be, but still less than a manual trans --- if all other variables are held equal.
The difference could just be differential gearing. Does the '06 550iMT have the same gear ratio as the '06 550iAT?
There are driving style differences too. Most autoboxes are tuned to shift for maximum fuel efficiency at the cost of maximum accelleration. If you shift your manual the same way, you ought to get slightly better mileage unless the differential ratio is to blame. Most people who have a manual don't do this. The EPA tests try to simulate "normal" driving. With an auto that's easy; just let the tranny shift for itself. With a manual they have to try to simulate what "normal" drivers do.
I've always owned manuals. I almost always have got more than the EPA estimated MPG. Not much more, but more. My Mazda RX-8 was an exception; it got fewer mpg. My BMWs seem to be very close to the EPA estimates.
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A shorter final drive ratio also means that you shift sooner, thus negating the logic.
Regardless, a manual transmission has less friction due to the lack of a torque converter, and is lighter weight.
Quote: | It has a shorter final drive, so the engine spins faster when you drive down the road and subsequently uses more gas. The upside is that it accelerates faster, like a 550i Auto is slow |
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| Reply » EPA ratings - Auto vs Manual |
Quote: | A shorter final drive ratio also means that you shift sooner, thus negating the logic. | A shorter final drive means that you shift sooner because the wheels turn slower per rpm versus a higher final drive where the difference is less pronounced.
Example
3rd gear stick 1 to 1 3.5(shorter) final drive wheels turn 1 for 3.5 turns of the engine
3rd gear auto 1 to 1 3.2(Taller) final drive Wheels turn 1 for 3.2 rpms. Which means that wheels turn 1000 rpm @ 3200 engine rpm auto versus 1000 @ 3500 engine rpm for Manual. Engine spinning faster uses more gas.
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| Reply » EPA ratings - Auto vs Manual |
Quote: | But can anyone explain to me why a BMW with a manual transmission scores lower EPA ratings than the same model with an automatic? Manual transmission cars are lighter and more fuel efficient. Why don't the EPA testing methods reflect this? | Acutally, it would be model dependent. For example, E46 330Ci mileage:
6-speed manual transmission - City/highway 20/30 mpg
5-speed automatic transmission - City/highway 19/27 mpg
Sequential Manual Gearbox (SMG) - City/highway 20/29 mpg
Of course, why would SMG get worse mileage on the highway when its the same transmission and final drive ratios?
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EPA ratings - Auto vs Manual