Poor aerodynamics affect MPG

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Scooby_Doo

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I had an 85 mile trip to the old Severn Bridge today to collect my new kayak , set off a bit earlier than I needed to meet up with the delivery driver so decided to see what the best mpg the car could achieve.. All motorway miles, didn't exceed the speed limit , there were long stretches of 50 mph limits and the M48 was virtually empty. I got to the Severn View services and the trip was showing 50.5 mpg for the 85 miles.
Loaded the kayak on the roof rack and set off home, even slower , 60 - 65 max. When I got home the trip was now showing 46 mpg for the 170 mile round trip. By my reckoning the return journey with the kayak was only 42 mpg , I was somewhat surprised how much the difference in mpg had been affected.
 
We found the same with roof boxes years ago (and bikes)

Windows open and roof down on a vert, ' hurt' as well.

All those hours in the wind tunnel do apparently count for something it seems
 
Makes a huge difference, especially at higher speeds. Around town, acceleration (weight) and rolling drag forces dominate. On the open road, aero drag dominates.

Aerodynamic drag forces square with speed. So at 60mph, the aerodynamic load is four times that at 30mph. Just leaving the roof rack/bars on the car can result in a 5-10% fuel economy penalty. Putting a roof box, bikes or canoes on the roof can easily add a much bigger penalty, especially if the return trip was at a higher speed (versus long stretches at 50mph on the way out) and/or into a headwind.
 
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I seem to remember (some time ago) Land Rover published different emission specs for vehicles supplied fitted with OEM roof lights and roof racks due to the extra fuel used due to the additional drag.

We all know driving with the windows open uses more fuel so driving around with a roof box of for years (like a friend of mine has been doing for about 10 years) or a kayak/bikes on the roof ill eat up more fuel .
 
Windows open and roof down on a vert, ' hurt' as well.
Absolutely. At motorway speeds, roof down on an SLK can cost around 15% in fuel consumption.

On a saloon car travelling at motorway speeds, running aircon is much more fuel efficient than opening the sunroof and / or windows for cooling - something that many people (mainly the French ;) ) don't seem to understand.
 
Absolutely. At motorway speeds, roof down on an SLK can cost around 15% in fuel consumption.

On a saloon car travelling at motorway speeds, running aircon is much more fuel efficient than opening the sunroof and / or windows for cooling - something that many people (mainly the French ;) ) don't seem to understand.
When driving a convertible, I like to think of it that there’s an opportunity to improve fuel consumption by closing the roof, rather than it worsening by opening the roof 😁
 
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