Experiment: 7 aero mods + tire PSI = 10% better fuel consumption at 90 km/h
1) In case it's not obvious, fuel economy is a hobby of mine: both the effect of driving style, and also vehicle design. I experiment with this kind of stuff because I find it interesting, even amusing. I mod my own cars (eg. I installed taller transmission gearing in my Firefly, plus an "eco" camshaft, and more.)
2) But whoops, I sure made this Micra look UGLY! (Or did I?)
This experiment was more about function than form: I didn't spend time on aesthetics. But I believe that many of these modifications could be done in a way that is nearly invisible, or OEM-looking, or more aesthetically pleasing than my Red Green duct tape style.
Motivation: Versa Note envy ...
The idea for this comes from the Versa Note and the sedan, which get some efficiency features that are missing from the Micra. I wanted to see what would happen if I recreated those features and expanded on them.
There are 4 main ways to reduce fuel consumption: (1) improve drivetrain efficiency; (2) reduce mass; (3) reduce rolling drag; (4) reduce aerodynamic drag. Rolling drag can be reduced a little by increasing tire pressure. But aerodynamic drag is actually the biggest factor in most vehicles' fuel consumption.
Believe it or not, 50% of the Micra's fuel is burned to overcome wind resistance at speeds as low as 50-55 km/h. Going 100 km/h it's closer to 80% (which is why speeding on the highway kills fuel economy). So aero drag reduction is where I focused my efforts.
The mods ...
1. Tire pressure
Placard pressure is 33 front & rear, which is a compromise between ride comfort, fuel economy (rolling resistance) and handling. I find the taller sidewalls of the S (vs. the SR) make it super squishy comfy. I went to sidewall max (44) +1. I prefer the feel of the car at that level.
What about tire wear? I've increased pressure this much (or more) on every car I've owned, and have never seen unusual tire wear. Though some people will swear they've seen faster tread wear in the middle of the tire. It seems to depend on the vehicle, the tire & the driving style. If you go with higher pressure, you have to keep an eye on it.
There's no doubt that at higher pressure, the car coasts noticeably further.
2. Partial grille block
The Micra has a huge amount of open grille area for a small 109 hp car:
The diagonal lines show where the radiator is. Everywhere orange is wide open.
The problem with this? Excess air that crashes through the engine compartment and then exits underneath the car creates much more drag. Better to allow only as much air through the grille as is needed for cooling.
Considering this is the base Micra with manual trans and no A/C, its cooling requirements are bottom of the scale. There's absolutely no need for this much flow-through in normal conditions.
Solution: cut-out coroplast shapes, secured with wire ties...
In this configuration, the cooling fan didn't come on once during a 440 km round trip (cruising between 80 kph / 50 mph to 105 kph / 65 mph, including downtown Kingston city traffic with stop & crawl and many stop lights. The max coolant temp was 197 F; the cooling fan turns on at 207 F; ambient temp was around 75-80 F.
If you use AC a lot, or have an automatic, or drive in the mountains, or at much higher speeds, this opening wouldn't be big enough. You will want to monitor coolant temperature (I used a ScanGauge) or add some kind of indicator to let you know if your cooling fan is coming on when it shouldn't be (like an inline LED light).
Interestingly, the Versa Note, which has the exact same 109 hp engine/trans (manual, but not the automatic) has blocked off "faux grille" sections on the sides, plus it uses active shutters in the lower portion in the CVT version to minimize airflow into the engine compartment when it's not required.
Another thing the Versa Note gets that the Micra doesn't is a stock air dam ...
3. Air dam
In addition to reducing drag, it also probably reduces front end lift at higher speeds.
This was made of lawn edging. Clearance is about 10 cm / 4 inches, and it scraped occasionally, but only on very steep transitions.
It was fun coming up with a rigid attachment without drilling anything in Nissan's car! I re-purposed a bunch of existing holes underneath, plus liberal amounts of Gorilla tape.
Here's the Versa Note's front end for comparison. Also notice the blocked "faux grille" sections that the Micra didn't get?
4. "Kammback" style roof/side extension
The Micra SR gets what Nissan calls a "spoiler" on the hatchback. It's actually not technically a spoiler but a small drag-reducing roof extension:
Anything which tapers downward and inward at the rear of a vehicle will reduce the size of the turbulent, low pressure wake and reduce drag.
Bigger is better!
This is just cardboard with plastic over top, duct taped in place.
It doesn't have to be this ugly, though. I've seen people construct similar modifications from sheet aluminum, or professional looking fiberglass over foam core.
5. Smooth wheel covers
Wheel style has a big impact on drag. Generally, the bigger the gaps between spokes, the higher the drag.
These are coroplast discs taped over the stock wheel covers.
Again, it doesn't have to be this ugly. I'm a fan of the Bonneville salt flats style spun aluminum discs.I made this image for Aftica when he was debating what type of wheels he was going to go for.
What about brake cooling!?
In normal driving, a full wheel disc would be fine. Don't forget, the inside of the wheel is still open. On the other hand, if you're doing a Micra track day, or driving in the mountains, then you probably need more cooling.
(Coincidentally, lowering the car is another mod that helps reduce drag.)
6. Rear fender skirts
Even better than smooth wheel covers is preventing air from crashing through the open wheel well to begin with.
These are just cardboard with plastic over top. It's a simple flat piece -- the size & shape was dictated by where the tire protruded from the wheel arch.
Remember the 1st generation Honda Insight?
It had full rear fender skirts and a bunch of other "extreme" aero design elements. It remains the most fuel efficient, non-plug-in car ever sold here.
7. Rear tire spats/deflectors
They help guide air flow around the face of the tire tread. Both the Versa sedan (pictured, right) and Versa Note have them.
8. Passenger mirror delete
Expect to see conventional exterior mirrors disappear from expensive cars in the next few years, replaced with video cameras. Car makers are urging changes to regulations to permit them, to help with fuel economy ratings.
An exterior mirror can also be replaced with an internally mounted large convex mirror of various styles.
Testing the modifications & results -- next post.
View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...