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« SumoSac Giveaway, Round 2 | Main | How Much do You Spend on Kids' Sports? (And Ways to Save) »

August 26, 2008


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Regarding point 1 - I agree. But in my neck of the woods if you're doing less than 65 you'll get run over! ;-)

Regarding points 2&3 - While not bad suggestions, they both presume that your car is badly out of tune or improperly inflated. Both would affect your daily driving anyway and would have most likely been addressed long ago. Also, modern electronic controlled cars can go very long periods of time before needing service. In fact, a "tune up" as it we knew it just 20 years ago doesn't really exist any more. The on board computers adjust the engine to compensate for aging of ignition parts.

I've got a compressor and it is quite handy for keeping my tires properly inflated. I don't think I would buy one just for that purpose, though- most service stations have air compressor pumps that you can use for free to fill up your tires. The main reason I have my compressor is for using it with air tool- the tire inflating is just a nice side perk :)

Great points.

I wonder how low tires need to be to affect fuel economy, there seems to be some debate on that point.

I find it easiest to use a bicycle pump to keep my tires inflated properly, there is a bit of exercise envolved but not too much at only around 15 pumps per tire.

I have a compressor for my nail gun, but I find the bike pump to be more convienent and the "exercise" is welcomed.

I definitely agree that the more you hit the brakes the worse your mileage. But I'm not entirely convinced that rapid acceleration is as bad as they same it is. Obviously jackrabbit starts followed by hard braking is bad, but basic physics (F = MA) suggests that the work needed to reach a give velocity is exactly the same regardless of the acceleration (obviously this leaves certain frictional effects out of the equation). So it really makes little sense to "granny it" when merging onto a highway or accelerating from a light when you will reach your top velocity long before the next stop.

Jon, while I appreciate the idea of using physics, the problem is that I can't pay per Newton at the gas pump. The efficiency of your engine in converting the chemical energy stored in the gasoline into the kinetic energy that accelerates your car varies with how hard you're pressing the gas.

So while you're right, you're using the same amount of Joules (engine output) either way, the difference is in the amount of Joules you're putting _into_ the engine in the form of gasoline.

Look for a rechargable battery powered compressor at Wal-Mart. I bought one several years ago for around $40, and I use it regularly to check and fill my tires.

Overcoming inertia uses the most fuel. F=MA -- the A is for acceleration. Mass is fixed. So it takes more F(orce) to accelerate at a greater rate. Energy = Force x distance. So, to go a given distance, using more F(orce) you use more Energy, which as we all know equals more fuel.

Your commenter got his basic physics wrong. This is why lighter cars get better gas mileage than heavier cars. It's also why a Corvette can get 30mpg at 55mph but 15mpg at 25 (city driving, lots of overcoming inertia).

Second, inflating your tires properly has a second, more tangible benefit -- greater longevity. Underinflated tires wear out on the outside, causing you to replace them sooner (they also have less traction on turns if their outer tread is worn). Tires are fairly expensive. I had to replace mine at around 20,000. If you drive 20,000 per year, you may be talking $800 per year on tires, or more. If you do the math, that adds about 4 cents per mile. Gas at $4 per gallon costs you 8 cents per mile if you get 25mpg. Under-inflated tires can add 50 percent to your cost. Plus, you may get worse fuel economy too (some say 5 percent).

Third, there are things you can do to "tune" your car to get better mileage. Exhaust, revised computer chips, and whatnot, though they're cost-effectiveness is dubious (it may cost more to buy them than they save). However, a properly tuned car will give you better performance and you won't feel like you have to "floor it" just to get the car moving.

I bought a rechargeable battery powered compressor at Wal-Mart a few years ago for about $40. It works great for checking and adding air to my tires.

Bill Davis, You left time out of the equation. When accelerating hard the force is applied for say 10 seconds vs. 30 seconds for grandma to get to 50 MPH. There's nothing wrong with the basic physics other than the failure to account for friction which is mentioned in my comment. Jeb is right that the frictional effects (engine efficiency) are what account for the difference between equations and reality. An engine has an efficiency curve that correlates output power and fuel usage. It varies greatly from vehicle to vehicle. However, most vehicles get their maximum fuel economy at whatever speed is high enough for the car to run in the overdrive gear. Getting the car up to that speed quickly is actually beneficial. There are alot of factors at work here and it varies greatly vehicle to vehicle, but the argument for granny style acceleration is not as cut and dry as most of the fuel saver articles would have you believe.

What is true is that to the average stupid American, telling them to accelerate slowly will prevent them from doing alot of revving and braking between red lights 1 block apart. But accelerating slowly onto the _open road_ may not be the saver you think it is.

Jon, I respectfully disagree. Time is built into the equation. Acceleration is change in velocity divided by TIME. So, time is factored in.

Oh for Heaven's sakes! Arguing over high school level physics is ridiculous!

The parameters of your car that hind or help mileage and you can do much about are pretty narrow. In the long run it may cost/save you several hundred or a few thousands these tweaks but the die is cast when you chose your car on the lot. Period.

You may as well recommend washing your car every week to cut back on air drag. ;-)

Bill, Rather than perpetuate the argument, I'll refer you elsewhere. The hypermilers have put the math on paper about pulse and glide (hard acceleration followed by engine off) and it's positive effects on fuel economy. Read more at While hard acceleration followed by glide in overdrive gear is obviously not as good as true P&G, the math they have done shows that granny starts do not pay off when you have a glide area ahead of you.

My last word on this: Here's my empirical evidence -- I used to drive a 1983 Mustang. When I drove it "granny style" (which was almost never), I got 29mpg. When I drove it hard (all city driving, maximum acceleration, stoplight to stoplight), I got 15.

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