Tom Scott video - SRT subtitles

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- This is test one.

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The Catesby Tunnel in
Northamptonshire, England

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used to be for trains.

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It was built by the Victorians in 1897,

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but fell into disrepair after
the line that went through it

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was closed in the '60s.

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It was abandoned for many years,

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but now it's a vehicle testing facility.

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You know a wind tunnel?

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This is a more realistic version of that

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where the air isn't moving,
but the car definitely is.

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- So Catesby Tunnel is
2.7km long.

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It's got a cross section of 42m²,
perfectly straight.

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It's quite an amazing feat considering

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the Victorians built it
without a laser level.

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Obviously the surface
of the earth is curved,

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but Catesby Tunnel is flat.

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So if you put a laser line along it,

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we're within 3mm of
being off that laser line.

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But when we first inquired about
repurposing Catesby Tunnel,

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it was totally derelict.

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They'd ripped up the track,
they'd taken away the ballast,

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they'd destroyed the drainage.

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So at the far end,

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the tunnel was actually
knee-deep in water.

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There were no facilities
or utilities here.

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There were bats living within the tunnel

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that we had to tread carefully around.

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So at the far end of the tunnel,
they've actually got their own "bat hotel".

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When we test outside,

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we've got the wind, we've got the rain,
we've got other vehicles,

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we've got lumps and bumps in the road.

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We want to remove those and
make this controlled a condition

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as possible so that the test

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is only seeing the changes
that we're looking for.

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- We're doing a "coast-down test" here.

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Julian got the car up to
130km/h,

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very quickly(!), and then he took
his foot off the accelerator

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and just let it coast in neutral.

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This tunnel's long enough
that there'll be a couple

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of minutes before he
has to touch the brakes.

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From that, you can work out
the drag and friction forces,

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which is really useful data
for car manufacturers.

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- Because Catesby Tunnel is a
bit different to a normal wind tunnel,

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we have to collect data on board.

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We can't use GPS,

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because we can't see the
satellites in the sky.

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The approach that we've come
to is an image-based system.

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- There's 1200 tags now in the tunnels

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and they are 5m apart,
and they've been surveyed

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to within a few millimeters,
each position.

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And then we measure the time it takes

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to go between each tag,

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and that's picked up by
this high speed camera

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that's working a hundred times a second.

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And that gives us a very high accuracy

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for speed of about a hundredth
of a kilometer an hour.

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The tunnel is a fantastic place
for this kind of measurement,

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because there is no extraneous sunlight

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and sunlight is very
difficult to cope with

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when you are working with infrared.

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- But I said that this was test one,

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and it was, I wasn't lying.

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But like any video about cars,

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you didn't see it from just one angle.

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There were camera shots
from all around the tunnel

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to get different angles
and keep it interesting.

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There were shots from
cameras that were attached

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to the car with suction cups

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for dynamic shots from
almost impossible angles.

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But this is a proper glossy car video.

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We don't want to see those
suction cup cameras from outside,

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and beside, we've only got two of them,

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and we don't want to
see the camera operator

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in the back either 'cause he's not there.

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There's got to be a little
bit of unreality to this

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that no one will notice once
it's all edited together.

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So when I said this is test one,
that is actually true.

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This right now is test one,

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but the shots from outside,
we haven't filmed them yet.

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We're going to have to go back and forth

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in this tunnel 15, 20 times

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before we've got every angle we need

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to make this look interesting.

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So I had a thought.

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Do those cameras actually
affect the results?

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If you've got a camera on
the outside of your car,

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how much more fuel are you actually using?

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How's that for timing?

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- Good timing.

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Spot on!

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- Typically, we go out and back for a test

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and average over both of those sections.

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We have a turntable at each end.

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The turntable takes about
20 seconds to turn around.

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We don't have access at
the far end of the tunnel,

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so we have to turn around inside.

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It saves doing an Austin Powers
37-point turn at the far end.

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- So exactly the same test the other way.

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- Yes.

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- Okay.

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Go for it.

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- In order to get the best accuracy,

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the driver has to be very consistent

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and he has to do the same thing
time after time.

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You have to drive in a very straight line.

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So any deviation, that
adds to the distance

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and the drag that the car's experiencing.

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You can start off at
pretty much any speed,

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as long as it's above the
start of your coast-down speed.

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You want to let the car settle a bit,

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because the moment you
put it into neutral,

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the car will start to rock backwards
and forwards a little bit,

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and all these little things all add up.

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- Is that a garage door opener?

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- It is.
- Just for the turntable?

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- Yeah.
- That's nice.

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- Smart.

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- I feel like this needs the music
from "Thunderbirds."

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- So the whole idea of Catesby Tunnel

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is to create constant
atmospheric conditions.

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We shut the doors at both ends

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and we seal the tunnel

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so that there's no wind coming through it.

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Because we're surrounded by earth,

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it actually means that the
temperature within the tunnel

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is within one or two
degrees all year round.

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We had the Aston Martin
Valkyrie in the tunnel,

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we've had cyclists in
here during drafting,

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which is fairly unique,

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something that you can't
do in other places.

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One of the biggest issues is

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what happens if there's
an accident in the tunnel

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and how do we get to them?

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So throughout every test,
we have a fire safety team on site.

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They're there to respond
if there is an accident.

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- There is a very slight
gradient to this tunnel

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and while I'm sure some
of the marketing team

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would be very happy if
it was perfectly flat,

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turns out there is a
little bit of an advantage

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to that gradient,

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because drag increases
with the square of speed,

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but friction increases linearly.

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So if you have one test
going very slightly downhill

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and one test going very slightly uphill,

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in theory you can
separate those two factors

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out from each other with a lot of maths

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that I do not understand.

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- The Catesby Tunnel surface
is an asphalt surface,

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typical of an A-road that
you would see outside,

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but just finished to as high
tolerance as we can get.

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- I do like how you stick

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to one side of the tunnel,
like it's a two-way road.

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- It just seems wrong barreling that fast

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down the center of a road.

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- Yeah.

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- And also, for consistency,

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for the aerodynamic wake.

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- Oh of course, it'll
be different depending

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on where you are in the tunnel.

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So I've been kept in the
dark for about 20 minutes now

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while you wrangle data.

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What do we have?

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- Well, this is an example of
one of the coast-down tests.

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The dark blue line is the speed.

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So that's us accelerating
rapidly up to 130,

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then pop it into neutral and
then coast down to about 40.

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- And then brake so you
don't hit the barrels.

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- And then brake so I
don't hit the barrels.

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- So we had four or five runs
with a clean car.

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- Yeah.

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- We had five, six runs with the GoPros

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in varying configurations.

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So we've got some sort of
drag from some sort of camera.

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- Yes, exactly.
- Alright.

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- And you can see, from 130
to 120 kilometers an hour,

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it took 7.04 that time and 7.03 that time.

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So a hundredth

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of a second difference.
- That's so close.

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- You wouldn't get that outside.

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- Yeah.
- It just wouldn't happen.

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And then with the GoPros on,

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from 130 to 120, took 6.98 and 6.97.

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Again, hundredth of a second difference.

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- Yep.
- Incredible.

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But there is a very noticeable difference

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between having a clean car

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and when you've got the
GoPros on the outside.

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It's measuring just a small difference
of about 0.8% between the two.

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- And does that map to fuel usage?

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- Yes.
- And this is not the most scientific test.

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We did a couple runs, the
GoPros were all over the place,

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but there is enough drag from them

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that you can measure it in there?

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- Yes.
- And there would be some effect on fuel.

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- Yes. Now of course, we were using
the ideal car for this test,

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because the weight doesn't
change from test to test.

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- Right, it's electric.
- Whereas,

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in a fossil fuel car, of course,
you've got to make sure

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that you don't let that tank go too empty.

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And there's lots of things
you have to take into account.

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- Yeah. So there's roughly 0.8%
more drag on the car

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from sticking a couple
of GoPros somewhere.

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- Yes, absolutely.

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- That's the closest I've got
to science in a long time.

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Thank you very much.

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- No problem. Thank you.