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Old 08-25-2017, 11:32 PM   #1 (permalink)
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From 38mpg to 45mpg on 07 Yaris hatchback by hypermiling and detailing exterior

I just got back from a 450 mile trip going from San Francisco to Los Angeles. I wanted to implement all my different hypermiling techniques so I decided to wash, clay, and wax my car before my trip. Detailing it took about 5-6 hours since I'm guilty of washing my car maybe once every other year or less. That's gonna change now that I realize how much drag a dirty car produces.

The best mpg tank to date was a trip to Vegas using the exact same driving style and that only got me about 38mpg. For the longest time I couldn't figure out why my car was getting such poor mpg on the freeway, and it didn't matter that I did a full tune up including replacing the AFR/O2 sensor, spark plugs, filters, fluids, pcv valve, 45psi in tires, etc.

These hypermiling techniques included drafting behind large cargo trucks, never exceeding 65 mph (staying around 55mph), speeding up before a steep incline, engine off coasting (used very rarely).

After detailing my car, I decided to try a front upper grill block and used packaging tape on the seams between the hood and the front bumper. My license plate was already relocated to the lower grill and blocks a portion of it. I drove at night and arrived in the early morning and did so for each trip so my car wouldn't over heat, nor did I have to deal with traffic.

If I were to try another experiment on my next trip, I'd remove the passenger mirror as well as the wiper blades since I clayed and waxed all my windows.

So for those that haven't washed your cars in awhile and experience poor mpg on the freeway, try clay barring and waxing it. I'd like to find out if I'm the only one who experienced a large increase in fuel efficiency from doing so. Not only do you save gas money, you'd be saving your paint from failing prematurely, which will help your car retain it's value when resold.

Has anyone else experienced something similar after claying and waxing their cars? Does anyone have any other mods they'd recommend such as a warm air intake?

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Old 08-26-2017, 01:43 AM   #2 (permalink)
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Congrats on the mileage improvement, great job. I doubt washing the car had much to do with the improvement though. From what I've read, any improvement from wax is too minimal to even be detectable. OTOH, the grill block and sealing the seams probably helped quite a bit. Your driving is probably improving as well, maybe more than you realize.

If you travel on the highway lots, I would suggest building a belly pan. I've never tried a WAI. I was going to but people here have said they are not very effective with a MAF sensor like on our cars. They work best with MAP sensors in older cars.

Almost all my driving is done 1-5 miles at a time.
Best short trip: 2.4 l/100 km, 3.9 km
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Old 08-26-2017, 05:14 PM   #3 (permalink)
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It's unlikely that clay-barring and waxing the car reduced its aerodynamic drag at all, as has been empirically demonstrated in wind tunnel tests. The explanation lies in boundary layer theory: the air molecules at the car's surface do not move in relation to the car. Aerodynamics textbooks represent this phenomenon with boundary layer velocity graphs like the one below:

The air velocity transitions between wall velocity and freestream velocity within the boundary layer, whose thickness depends on the fluid viscosity and body length among other things. To actually affect airflow, you need a fairly significant protrusion into the boundary layer; a little dirt on the car's surface isn't nearly enough to do this. You can safely chalk up your mileage increase to your grill block and good driving.


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