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Author Subject: aero mpg mods? has anyone done any?
jimmyhackers

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Location: birmingham

Registered: 14 Jun 2011

Posts: 1,213

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Post #1
a long time a go i removed a large portion of my 1.4 306s rear bumper to aid aero mpg motorway miles.

needless to say it worked but looked terrible,so ive been looking at less intrusive ways to gain some mpg.

probably start with an oem engine undertray.

ive been reading about tyre air deflectors, along with rear underside diffusers. anyone else tried these?

i know from fitting a front splitter (oem splitter was missing) to my xsi it made a huge difference to steering feel at higher speeds. but my gti6 already has its oem splitter.

ive seen people cover the rear underside gap with a sheet but thats a bit to much work for me.

is there anything else i havn't thought of?

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the world is changed by people in sheds
Posted 9th Jan 2025 at 00:39
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welshpug!

Capt Pedantic

Location: Bigend, Wales.

Registered: 27 Mar 2007

Posts: 25,847

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Post #2
flat rear floor/undertray would have been my next thought, my 1993 405 Mi16 actually had a plastic infill piece between the beam and the spare wheel

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Bring on the Trumpets.

Posted 9th Jan 2025 at 13:12
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silky

Seasoned Pro

Location: Chelmsford, Essex

Registered: 29 Dec 2002

Posts: 3,471

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Post #3
Fold the back seats down
Posted 14th Jan 2025 at 11:02
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jimmyhackers

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Location: birmingham

Registered: 14 Jun 2011

Posts: 1,213

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Post #4
thats weight distribution, not aero
i suppose if i can go through the corners faster i wont scrub off as much speed and lose mpg. :p

i got a cheapy rear underside diffuser but realised it will impinge getting the spare wheel out unless i cut half of it off. then i realised i could just attach a flat sheet of something to the spare wheel frame itself.

any suggestions on sheet material and thickness?

i also got a set of generic mud flaps to use "incorrectly" as tyre air deflectors, but their profile is too off to fit.

again if anyone knows what cars have a similar mudflap profile to the 306..greatly appreciated.

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the world is changed by people in sheds
Posted 16th Jan 2025 at 22:16
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jimmyhackers

Seasoned Pro

Location: birmingham

Registered: 14 Jun 2011

Posts: 1,213

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Post #5
fitted the engine undertray today. what a faff.

had to change my aux belt as its been eating itself :S its only 6-7 months old :S. cleaned/polished up the pulleys with a brass brush on a dremmel, im thinking maybe rust n grot speed up the belts decline as everything else was in order.
took all of 20 minutes to do.

undertray....2 hours :S

Generic thing from ebay was vaguely ok...had to do some drilling and mostly cable ties as the hardware provided was junk. if your going to get one, get it without the hardware and save your money for cable ties or better clips than i had to hand.

didn't fall off on the A-road at 70mph Wink
i noticed reduced road noise and the car seems a bit more stable at speed.
it also got to tempreture a lot quicker, about 5 minutes instead of 10+.
I will have to see what effect on mpg it has over this weeks commutes.

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the world is changed by people in sheds
Posted 8th Feb 2025 at 16:31
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jimmyhackers

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Location: birmingham

Registered: 14 Jun 2011

Posts: 1,213

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Post #6
Im noticing a small reduction in petrol used after the engine undertray is fitted..... before i was using a fiver each way on my commute im using about £4.50 now, my commute is 20 miles and about 30 minutes each way, mostly 70mph A-roads.

this is about a 10% reduction......which by itself seems too large for just an aero fix/mod. i imagine the car getting to temp quicker (including oil temp) is playing a bigger role in this than aero.

doing some wonky maths.... 9 quid gets me about 6.6 litres of fuel (1.45 gallons) 40 miles for 1.45 gallons is 27.6mpg... nearly normal.

i redid my boot floor with some sound deadening and have some left over,im contemplating fitting this to the internal side of my new engine undertray to further reduce road noise and maybe insulate the eninge/oil sump more.

Been doing some more google/youtube research. Essentially Aero MPG is about reducing turbulant air formation, and its best to work from the front to back...as trying to smooth air thats already been made turbulant is somewhat futile.

i now have the plastic angle to make some custom tyre air deflectors, which is next on the list. The fronts will be tiny a 2-3cm drop as ive already lowered the front suspension a little, but seemingly will be as important if not more than the rears which will be a mahoosive 10cm.

will report back with findings.

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the world is changed by people in sheds
Posted 17th Feb 2025 at 13:15
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jimmyhackers

Seasoned Pro

Location: birmingham

Registered: 14 Jun 2011

Posts: 1,213

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Post #7
well...my sticky passenger front brake being fixed seems to of helped a lot with my search for mpg.
not suprisingy.

i did end up putting foil backed sound deading on my undertrays topside, car heats up even quicker, road noise seems reduced further aswell as an reduction in engine noise.

my vss died on monday though so its somewhat difficult to gauge my mpg, new sensor turned up today so will get fitted tomorrow. sounds weird but im sure im only using between 6-7 quids worth of fuel now for the same daily 40miles.

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the world is changed by people in sheds
Posted 13th Mar 2025 at 20:00
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