[Mateo]

Just like the title says, how much do the rear wheel covers help with mileage? I am thinking of fabricating a set for my 89 Civic and am curious to know how many mpg gain I should expect to see.

Have any of you removed them to see if they drastically reduce your fuel economy?

 

[Rick]

It would require a real controlled experiment to determine the difference it makes, but I think the general concensus is about 5mpg. How much it would help a vehicle depends how much air actually drags across the wheels. I with I could find the link, but a while ago someone set out to increase the fuel economy of their Ford Ranger I think it was. He put wheel skirts and a sort of tear drop cover over the bed and managed to increase economy by something like 10 mpg. Anyone know what I'm talking about?

 

[Mateo]

It would require a real controlled experiment to determine the difference it makes, but I think the general concensus is about 5mpg. How much it would help a vehicle depends how much air actually drags across the wheels. I with I could find the link, but a while ago someone set out to increase the fuel economy of their Ford Ranger I think it was. He put wheel skirts and a sort of tear drop cover over the bed and managed to increase economy by something like 10 mpg. Anyone know what I'm talking about?

If it can honestly make a 5mpg difference, I'm going to do it. The problem, of course, is that the stock wheels stick out a bit. this would require me to make the wheel skirts pop out a bit .

 

[Rick]

The smoother you can make it the better. Maybe consider skinnier wheels and tires? Actually you could probably get a better gain by going with some Low Rolling Resistance (LRR) tires. Another area of you could focus on for aerodynamics would be the under side of the car. On the Insight there are covers under the engine, then there are fins to divert air flow around anything where it would otherwise drag. On electric conversions (where you don't have to worry about things like exhaust systems) some people add a belly pan to reduce aerodynamic drag so there may be some room for improvment here as well.

 

[Dougie]

I would be surprized if it's more than 1%.

If you read the published articles about the development of the Insight, they fought for every tiny improvement possible. What about the little air dams in front of the front wheels? The door seals? The underbody panels? The taper of the rear window?

Aerodynamic effects start taking effect at higher speeds, so I would concentrate on the rolling friction. For a regular car, my guess would be that the following are the most effective steps to improving mileage.

- Drive more slowly, in a higher gear. Lug the engine.
- Use the Insight LRR tires and pump them up. (Look at the specs for this particular tire size on http://www.tirerack.com/tires/Spec.jsp? ... tenza+RE92 to see how it's a completely different tire from the others sold under the same name: It's much lighter than the other RE92 tires. You can't just use any old RE92 tire; it's got to be the 165/65R14 size.)
- Get a zero degree toe alignment. Front and rear.
- Use lighter weight oil, and synthetic, especially in the gearbox.
- Reduce weight of car by taking out all the junk you're carrying in the truck!
- Back off the drum brake adjusters a bit more than specified, including the handbrake.

 

[Rick]

The Potenza RE92 in both the 165/65/14 and the 175/65/14 are LRR (175 being the original prius tire), but any other sizes aren't. Also the Goodyear Integrity has good economy characteristics, though not quite as good as the Potenza's, but your more likely to find a size in them you can use. I have an old set of Insight tires on my GEM neighborhood EV and despite having a much taller tire over stock they seem to have not made a difference on range even though it now goes faster.

 

[james]

My experience is that the effect isn't enough to notice. I got rear-ended back in January (and the guy's insurance still hasn't paid me a cent), so I took off the damaged rear bumper cover and wheel covers. The car was parked from March to mid-July 'cause I was in Europe, but since returning, I've filled it up 3 times, and gotten 78 mpg avg on the 3 tanks. Prior summer average was about 75-76.

 

[Mateo]

My experience is that the effect isn't enough to notice. I got rear-ended back in January (and the guy's insurance still hasn't paid me a cent), so I took off the damaged rear bumper cover and wheel covers. The car was parked from March to mid-July 'cause I was in Europe, but since returning, I've filled it up 3 times, and gotten 78 mpg avg on the 3 tanks. Prior summer average was about 75-76.

Ah, so they actually HURT mileage?

Seriously though, are there about 5 people who would be willing to do a few tests to determine if there is a great effect or not? I could set up instructions for a very controlled experiment, and it wouldn't be too difficult to do at all.

 

[james]

I wouldn't go so far as to claim that they hurt mpg. My driving patterns probably have been a bit different, weather has been a bit hotter this year than last, the rear hatch glass is currently replaced with lighter vinyl... Too many independent variables for a controlled experiment

 

[SVOboy]

I wonder how could they possibly hurt mileage? But is it possible to rip them off easily and do some sort of test without them to see what kind of percentage mpg stuff they do or how they affect the old coefficient of drag? I have no clue how to figure out the coefficient of drag, but yakno.

 

[Resist]

You probably can save more fuel by just keeping any car waxed or slippery for the air flow. Most cars are not kept in this slippery condition.

 

[Dougie]

Does anybody have an idea about which way the air flows through the openings between the front wheel wells and the engine compartment?

Theory 1: It flows out of the engine compartment, letting air from the radiator out into the wheel well.

Theory 2: The wheel well is a high pressure areaa, and the air is flowing into the engine compartment and then down under the car.

 

[b1shmu63]

Looking at the body contour around the front wheel well, it appears that they are designed to create a negative pressure and suck air out of the engine compartment and around the body.

Certainly use synthetic oil in the engine. Don't use synthetic oil in the transmission! It has been tried and in the case of the Insight produces difficult shifting. The effect is like having a non synchromesh transmission. Use only Honda MTF as it is specifically formulated for this task.

The wheel covers help or they would not have been worth the weight penalty. Quantifying the amount would be a job for a wind tunnel. The underbody pannels are even more important and are used on the Prius as well. Honda suggests that weight reduction, friction reduction and aerodynamic factors account for a third of the Insights doubling of gas mileage over a similar sized car. My guess would be that the total aerodynamics package acounts for 5 MPG.

The biggest factors for hypermilling are patience, practice, persistance, concentration and experience. Combine these with weather, speed, road condition, temperature, traffic, and especially the length of your trip. Sadly these cannot be bolted on or poured in. Fortunately they work on any vehicle.

 

[bpapa9013]

As many have already pointed out, aerodynamic features have most effect at high speed (some have argued the point of significant effect, I think they were battling back and forth over 45 or 55). But if you took the rear bumper and skirts off and drove an entire tank at 55 I bet you would see a MUCH bigger difference in mpg!

Especially if you are doing mostly city driving without them on now!

If you never drive your insight over 45, you would probably be best off removing all of the body panels and under body covers!