I have issues. More or less and obsession is forming with regards to mpg and my 2015 CVT Fit. I do my best to avoid starring at the info panel readout to so my mpg but its hard to resist. And with the tiny tank the gas light comes on every Friday to or from work. If I do too many errands outside of my commute I worry about being forced to fill up more than once a week. So eaking out as much mpg when I have done errands during the week is my latest issue.

Now before I continue when I mention drafting I am not talking about drafting 10 feet behind a truck. I am talking about being 50+ feet (probably closer to 100) behind a big vehicle. Mythbusters showed a %~20 mpg improvement at 100 feet. Sure their car probably had horrible mpg compared to the Fit so I figure %~20 improvement on that car probably means 2-3 mpg improvement on the fit at that distance. Anyways...

So this morning my car was getting close to showing the low gas light when I realized I was 50-100 feet behind a semi. He happened to be going just a little bit less than the 70mph I had my cruise control on. So I thought...drafting might allow me to wait til fill up til tomorrow. So I stopped the CC and settles in behind the truck.

This went on for a while when another truck, without a trailer, got in front of me. So I backed off some. Eventually the full semi in front left and I was sitting behind a truck without its trailer. So I wondered:

Does the fact that a semi truck did not have a trailer behind it affect the drafting when compared to a truck with trailer?

What if the lead vehicle is just a SUV or mini-van? Would drafting be worthwhile at all (again at 50-100 feet) with smaller lead vehicles?

What about sitting behind a subcompact?


Just curious. I normally do 40-43 mpg on my trips to and from work. Today I averaged 47.0. But to be fair traffic was kind to me today.

 

Or don't draft so you don't make all the rest of us who work for good fuel economy look like tools.

 

100 feet at 70 mph is only a second following distance; 50 feet is half a second. Half a second is much to close to be following for safety, and even one second is very marginal. (People do follow that closely, I know, particularly in urban areas. People also far to often get caught and have massive multi-car pileups because of inadequate following distance.)

You'll save more fuel by slowing down to, say, 50 mph than you will by following trucks.

As to your question, the vehicle size and shape does make a difference, but I doubt it would be at all easy to quantify what is good and what is bad. It has a lot to do with the aerodynamics of the lead vehicle and how the air circulates and has turbulence behind it. These things are rather tricky to predict with any degree of precision by casual observation.

 

Every little bit helps, but...

Following unsafely is bad, and anything closer than the area's normal interval is unsafe.
Focusing on the truck you're drafting takes attention away from other traffic and is unsafe.
Going out of your way to catch up with something to draft isn't worth it.
Drafting something that doesn't keep a constant speed isn't worth it (see: going out of your way).
Drafting something that isn't going your desired speed isn't worth it (see: going out of your way).
Smaller things give less of a benefit, larger things block your view.

The other aero problem is that the rear end of a car, especially something boxy like the Fit, is where the real drag is: the squared off truck, bus or SUV is such a great thing to draft because its back end creates a huge hole in the air. The Fit's rear end is just a smaller version of that and drafting behind someone doesn't help that. So regular traffic itself makes the best draft, with cars in front of and behind you. Simply keeping up with regular traffic may be the best way to help your car's aero.

That said, it's a happy day when an 18 wheeler winds up in front of me for a while.

 

Rush hour traffic...where do people live where 100 feet behind a car is not what happens? Heck...that's a huge distance in my experience. In fact people easily merge in front of me because i leave far more space than others do. Should people be spread out more? Sure but its not realistic.

 

This^ Semis are the best to draft, and yes size and shape of the vehicle you're drafting makes a difference.

If you can't outbrake a semi in a Fit you have problems, even at a short following distance at speed. Besides, semi drivers drive defensively and are generally easy to predict. It's not like you're following some asshat in a Expedition going 90... and if you are, don't do that.

Semis on the highway are great because they usually keep a constant speed, don't stop and don't brake unless they have to. I wouldn't draft anything else personally.

 

Be the problem, not the solution.

 

if your not 20 ft off the back of the semi you are just in dirty air,not worth the issues


also if the semi comes across something in the road,,,,say a brick he will just go over it leaving you little time to do any thing but hit it!

 

I live in Vermont. It's completely realistic here to stay at least a couple seconds behind the next vehicle while driving.

I do realize that things are somewhat different in quite a few other parts of the country. It still doesn't make it at all wise to have a half-second or even one second following distance on a public road.

 

Drafting isn't very useful at distances of 50 feet between lead car to following car unless you're going 200 mph. Drafting at highway speeds needs only a couple feet separation to gain much and of course that dangerous at highway speeds. an suv will offer improvement but not as much as a semi.
the aero dynamics of the lead vehicle matters immensely. the bigger and the flatter the back end are much better than a small and aero slick shape. We've not experienced even 10% better mpg behind at semi at 100 ft and wew've had the chance to follow one over a whole gas tank refill. that was towing a race car on a trailer and yes at times we got closer because we were talking with the semi driver on cb.

 

Very useful as compared to what, and do you have instrumentation to really make that decision?

With cruise on in another car, I've watched my instant mpg slowly climb as I caught up with a van. I lost 3 mpg immediately when I pulled out to pass. In a rig that I had to fight to get 19 mpg in, 3 is a lot.

In the Fit if I'm lucky enough to meet a big truck going my way, I can keep up with it and get normal mileage while going 10 mph faster than usual- at normal traffic following distances.

The OP didn't latch onto a truck, but observed a significant jump in mileage when traffic conditions changed- the vehicle in front of him was a big truck. I've seen that to be the case myself, that at normal following distances (which can vary widely depending on the road and traffic) there is a benefit to being behind someone- and the bigger, the better. Being in a line of cars also helps, but a collection of random cars is less likely to drive smoothly than one or two trucks.