Your Car’s Underbody Plastics Keep Wind Drag Down and Fuel Economy Up | Autance

Riding around without fender liners or undertrays? Here’s why you really should fix those!

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Your Car’s Underbody Plastics Keep Wind Drag Down and Fuel Economy Up | Autance © Your Car’s Underbody Plastics Keep Wind Drag Down and Fuel Economy Up | Autance

Ever scrape a curb, get too ambitious with low-low ride heights, or just leave the underside of your car partially disassembled because you’re too lazy to remount some plastics? Yes, I’m talking to those of you riding around without fender liners or undertrays. Here’s why you really should fix those!

Hi there, socially responsible responsible wrench-turner. Check out the Green Gearhead tag to get even more useful info on being as environmentally conscious as possible while keeping your car on the road.

In the case of my 2010 VW GTI, I’ve given the car a hard time. I bought it for $6,000 or so, could afford to maintain the mechanicals comfortably, but I let cosmetics fall by the wayside. I’ve put 33,000 miles on the car in 16 months, and with those miles comes road hazards, hard driving, and good ol’ mistakes. Suffice it to say, I lost both of my fender liners, front undertray, and broke an expensive bumper spoiler over the course of my adventures.

The unfortunate aftermath of an animal strike. – Image: Chris Rosales

It never bugged me enough to actually fix it until I had some extra cash, and no more mods to do to the GTI. I also was deep in the process of de-shitboxing the car. What I didn’t realize was that there were big fuel economy penalties for not replacing replacing some dumb plastic!

The plastic panels under your car and lining the wheel wells serve as important aerodynamic devices, and protection for important parts of the car. The fender liners help control the high pressure air buildup in the wheel wells. The undertray protects the oil pan and other accessories from dirt and debris, as well as smoothing all-important underbody air flow. Without them, you will lose mpgs, top speed on track, and expose sensitive parts to the open road.

How do cars lose under trays and fender liners? Generally, fender liners exit stage right when a large puddle is hit a bit too hard, or when the car is lowered and rubs through the push clips. Under trays generally get ripped off on parking bollards, deep bumps, or road debris. That, or they get yanked during an oil change and never replaced. A lot of people don’t notice when it’s a clean removal and the liner doesn’t hang up, but most of the time one brave clip will hold the piece to the car and require a quick tear-off before you can continue driving.

If you bought your car used and it’s not immediately obvious whether or not your underbody plastics are intact, Google around for other pictures of cars like yours or plastic parts and see if you’re missing anything! If you are, or something’s been damaged and you haven’t gotten around to dealing with it, here’s your inspiration to make it your next little repair project.

The only way to do this right is to buy genuine plastics from your dealer or supplier. Don’t even bother with the eBay stuff or any knockoff products; they will fit incredibly poorly, be warped, and sometimes won’t even fully line up with the original push clip holes, requiring some custom fitting. To boot, they’re often made with cheaper plastic that’s noticeably thinner. 

Adorable try-hard golf ball pattern from my new R32 belly pan, which actually does help airflow! – Image: Chris Rosales

Mileage may vary with cost, but genuine plastics won’t be cheap. Honda, Toyota, and the like won’t break the bank but it’ll still cost $100+ for an undertray and liners. For my GTI, lower fender liners were $97.46 apiece. I upgraded to a full-length belly pan from a Mk5 Golf R32 for $112.35. The most expensive piece for me wasn’t closely related to those common replacement pieces, but something close to it. I had an animal strike a few months back with a large hare and broke my lower bumper spoiler, which was the main thing holding me back from replacing the other stuff. That set me back $278.28. Ouch. Don’t be discouraged though, Volkswagen plastics in particular are notoriously expensive.

Yeah, this dumb piece cost me damn near $300. – Image: Chris Rosales

For most cars, you’re looking at $150-$200 max for some basic plastics. It’s not cheap, but the returns are worthwhile and they’re easy to install. Not to mention, a lot of modern cars depend on fender liners and undertrays to keep other plastics and bumper skins in position, so sometimes it’s necessary to spend the money, unless you’re cool with having no bumper at all.

The good news is that installation is very easy! Most cars use cheap push clips available at any auto parts store. European cars tend to use actual screws specific to the manufacturer, like VW and BMW. The screws need to be purchased with the skid trays for maximum originality, though any old short screw will probably work. Safely jack the car up and support it, remove the necessary wheels, identify which plastics go where, line the holes up, and fasten them to the car with your push clips or screws! Undertray is similarly easy.

Fully under-trayed! – Image: Chris Rosales

With that, your car is now fully protected and designed to work aerodynamically as intended. Indeed, I enjoyed the idea of not having turbulent underbody airflow and a fully protected engine compartment. More importantly, I gained a decent bit of fuel economy. I used to get about 360-380 miles to a full highway-driven 13-gallon tank, and now I get 400-420 miles! That’s 10 percent in calculated MPG on average. Nothing to scoff at! I suspect that my actual MPG is much higher because of my non-factory staggered tire setup making my speedometer read about 5 mph slow, according to the GPS speed on my radar detector.

A 10-percent mpg increase over a long commute, over months, and over years is huge! My car is was particularly sans-undertray with uncontrolled airflow everywhere, so you might not gain quite as much if your aerodynamics aren’t as borked as mine were. Either way, the takeaway is that aerodynamics make a huge difference in highway economy and the effect that missing undertrays and liners have should not be scoffed at! 

Sure, $200 isn’t cheap. Over time, however, the increase in mpg will pay for itself, and continue to save you money. Go and replace those plastics!

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