What's Better?
 

I do about 12 miles of mostly highway driving to work everyday. I try to keep the speed down around 55 to 60 mph. A lot of times though I can get behind the big trucks on the way, but most of them like to run between 65 or 70. Should I pull in behind and draft at 65 or 70? or do I just maintain my 55 mph in the slow lane?

Slow down, drafting is not recommended.

What's wrong with drafting? It seems to me like it might be the best aeromod we can make.

Yeah, but it's not that safe, particularly in heavy traffic if someone is close behind you. You can always slow down much faster than a truck, but the person behind you can still jam your car under the wheels of a trailer.

On the upside, you can draft somewhat effectively surprisingly far behind a truck.

Agreed. Zero-drafting or what I call it "death" drafting is not recommended even though there are gains with it. The problems as mentioned are safety, lack of forward visibility and at those speeds some cars may not be able to negotiate the distance safely.

Better method would be to use distance drafting or at least give some distance (3-4 seconds) if you're going at highway speeds. Obviously it really depends on the situations but from what I've read and experienced it's just safer to give yourself some room and you'll still get some benefits of a lead blocker.

Also with HM, you're better off going slower or less than 60 (depending on your car) due to aerodynamics and the engine gearing. I know with my Scion she's very frugal at 50mph. :D

I know there has been a lot of negative press with HM and death drafting and when Wayne Gerdes (who coined the "hypermiling" term) says "NO DON'T DO IT - NOT SAFE", I gotta agree with the man. Plus there are other ways to squeeze the petrol that don't put you or other drivers at risk. Oh and welcome! :)

Here's a topic that addresses the issue of drafting:

also when drafting at close speeds you would be tailgaiting and wouldn't be using your gasoline as efficiently, you would be focused on staying close.

I never draft unless I'm lucky enough to find a semi going the same slow, constant speed. I stay 3+ seconds behind. It has only happened twice in my life.

well with me, I drive 55-60 until a semi passes me, then i follow them. If i just drive 60 and don't draft my truck downshifts going up hills. If i draft at 55-65 it doesn't downshift. And when drafting i will speed up using the same amount of throttle. I don't ride their back doors too close, usually a semi-trailer length. I feel it's safe. I have 3 seasons of circle track racing in my experience, and the front end only had a few bumps to it, and i drove hard in the division.
I think it's all about how comfortable you are behind them

Guys I understand the dangers of tailgating, but what I'm asking is what is more fuel efficient- 2 to 3 second Semi truck drafting at 65 or a steady 55 on my own?

I think it depends on a lot of factors. How good your car's aerodynamics are, whether there's a head or tail wind, the type of semi you're following. I think the only way to really tell is to do it in a car with a sensitive fuel consumption display, but that still wouldn't tell how your car will do.

Even with the Insight, "drafting" (by which I mean being behind a semi at what is standard separation on California freeways) sometimes gives me a 30-40% jump in mpg, but other times doesn't seem to have any effect at all.

A persons comfort level and safe distance can be 2 different areas.
You should factor in your overinflated tires and remember that your braking distance will be longer(slower to a stop) than properly inflated tires.

I think that you kind of touched a spot here, but I believe the answer to your question will only come with you trying both ways. I know that with my Metro I get better at 55 without drafting than 60 or more drafting, but that could just be because over 55 winds my RPMs up so much. :turtle:

Get a scangauge and see.

I would love to try the scangauge but my car is obd1. I have been considering a vacuum gauge, but I don't think that would help us in this situation anyway. Thanks anyways guys. Maybe I'll just try both over the next few months and see how it works out.

I don't believe that is a correct statement. Do you have a source?

The scangauge plain won't work out on your obd1 car. Build ye an MPGuino :)

All the tests I see so far only go up to 35psi. I believe overinflated tires(over max sidewall) would have a longer braking distance because of the smaller contact patch to grip the road.

Do you have a reliable source for your disbelief?
This is all I could find for now
http://www.rma.org/tire_safety/tire_...ips/summer.cfm
http://www.edmunds.com/advice/fuelec...0/article.html

Show me some test data (not editorial hogwash) to back up your asserion, i.e.:

http://www.geocities.com/barrystiret...tiongraphs.jpg

I've read it and Im confused that in autocross people don't set their tire pressure higher than a few psi since they would love maximum braking to improve their times.

It is measuring traction but "gradual braking" is not used to stop as fast as you can(neither is immidiate braking which would induce a skid). Looks like barry the tire expert isn't fully convinced either.
Barry's Tire Tech

"Dry Braking & Wet Braking:
Both of these tests were conducted using a traction trailer – pulled behind a truck. A brake is gradually applied to the test position on the trailer until the tire locks up and instrumentation on the axle measures the peak torque and then the stabilized slide value. For the wet portion of the test, water is sprayed on the ground in front of the tire at a prescribed rate. This generates a consistent water depth. This is a common way to conduct traction testing.

These results are a surprise to me. Given the results of the wear test, I would have expected the traction values to peak at some pressure and then drop. Perhaps it is the nature of the braking test and perhaps a different set of values would have been obtained if a test had been conducted on a circular skid pad using a vehicle – but they didn’t do that, so we don’t know."

I run a vacuum gauge and I can pull the same amount going 10mph faster when i'm drafting @ a safe distance. Its not worth it for me because of the rocks and such that get kicked into your windshield.

Thanks Vince that was the kind of data I was looking for.

Whats safer, being behind the semi, or having the semis blowing past you 10-15mph faster.