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Old 08-09-2016, 03:45 PM   #15 (permalink)
EcoModding Lurker
Join Date: Jul 2016
Location: Near Newcastle, UK
Posts: 19

330i Rag Top - '05 BMW 330i M Sport
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Originally Posted by MetroMPG View Post
Welcome to the forum!

Fun car. And the manual transmission gives you lots of options for fuel-saving.

I'm curious if you've compared top up vs. top down MPG at a the same speed, just for fun.
I did a top down test with the roof up and roof down today, at 55mph on cruise control, on my usual test route. Average of 3 runs each way, car was driven for about 30mins/15 miles before:

Roof up:
Direction A-B (Slight Downhill) - roof up @ 55mph: 48mpg (UK)
Direction B-A (Slight Uphill) - roof up @ 55mph: 39mpg (UK)
Average: 43.5mpg (UK)

Roof down:
Direction A-B (Slight Downhill) - roof down @ 55mph: 48mpg (UK)
Direction B-A (Slight Uphill) - roof down @ 55mph: 41mpg (UK)
Average: 44.5mpg (UK)

A real surprise, the roof down test seemed to yield a touch more mpg, but probably within the bounds of experimental error. The surprise is that mpg didn't drop off noticeably at all. I wonder if the zone from the windscreen to the flat rear trunk top surface is acting like an aerodynamic tail, with a high pressure zone inside the cabin area, deflecting air over the top and to the trunk, and is actually helping economy slightly. There is certainly not much wind inside the car with the roof down.

I can't say driving everywhere with the roof down would be a great hypermiling technique in the UK, the rain would cause a lot of expense with messing up the car's electrics....

Still an interesting result.
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