What's the performance difference between 15 inch, 16 inch and 17 inch tires (all else equal)?

[clare]

Because consumer reports does not do a terribly scientific comparison.

[clare]

Weighing the fuel is NOT required if the tests are done at the same temperature - which would be a requirement for accuracy in fuel mileage measurementin any case due to the difference in air density.

[Ed Pawlowski]

I'm not doubting their test results but that sounds like a broad blanket statement. What particular car and engine? Will you get that result with a Tesla, F-150 and 1987 Chrysler Cordova?

Both AC compressors and auto aerodynamics have improved over the years. In any case, if it is 85 degrees, I'll have the AC on. My present car has the best AC of any I've ever had since my first car with ac, a '68 Olds Vista Cruiser I bought in 1971.

[clare]

The problem is getting an accurate reesult over varying conditions. Getting extremely accurate results in a fixed condition is simple. Same road, same speed, same weather conditions, repeatability is excellent and simple equipment can be very accurate. For instance, calibrated fuel supply, running start on measured distance at predeteermined speeds - make modification and retest.

[Ed Pawlowski]

I remember the Mercs. Wish I had the skills to build one. The 50's and

60's were the golden age of custom cars.

[rbowman]

That link didn't work for me but

appears to be the same article.

"There are some tech tricks you can use to help monitor your fuel economy as you drive. I'm fooling around with a couple of devices right now. I just installed a Scangauge on my motorcycle. This $180 device plugs into the On Board Diagnostic System (OBD II) port under the dash of virtually any post-1996 car or light truck. It operates as a scan tool (much more on that here), so it gives me trouble codes and streaming data, but it also works as an electronic gauge cluster and trip computer. I can track battery voltage, coolant temp and sundry while tooling down the interstate. But one feature is invaluable: the instantaneous fuel economy readout. It gets fuel quantity data from the injection timing--the longer the injectors are open, the more fuel they squirt. It's amazingly accurate, too."

The ScanGauge can be calibrated for the actual rolling circumference of the tires. It does instantaneous, which I don't find to useful, as well as daily or tank mileage reporting. On a fillup it predicts the gallons you will put in. That's the least accurate because I always use the automatic shutoff but even that is usually within a couple of tenths of a gallon.

Using the car odometer and a notebook with your gas purchases is strictly for the Luddites. It's like measuring acceleration with a stop watch and the seat of your pants when you can use an accelerometer.

[rbowman]

I worked in Ft. Wayne for a while. Indiana is big on car stuff since there isn't a whole lot of outdoor recreation opportunities. Fairmount was James Dean's boyhood home and is where he is buried so they have a James Dean Festival every year. The Mercs roll in from all over.

It wasn't a custom, but my soft spot is the '51 Ford from 'Thunder Road'. There is a nice one around town with the appropriate vanity plate.

[rbowman]

OBCII. You are plugged into the car's computer system, air mass sensors, injector timing, the whole works.

[rbowman]

The EPA has a much more complex problem. They are not measuring the fuel economy of a particular vehicle in the short term. They are attempting to compare apples and oranges

[rbowman]

But they look kewl... There is a similar phenomenon with motorcycles. The first thing many Harley owners do is replace the stock exhaust with aftermarket pipes. While they've gotten better at hiding it, the OEM exhaust has a cross over between the two pipes that people find unappealing. The new pipes may have less restriction and be louder. They may be shinier. In most cases they also result in a slight reduction in power. Most of the people aren't doing baseline dyno runs and retesting after modifications. They are buying bling and working on the intuition that more noise == less back pressure == more power.

[Mad Roger]

Yup. We agree.

Putting an M3 badge on an otherwise 3-series bimmer makes it look faster too! :)

And, whitewall tires look kewl to some, where nobody thinks that there's an inherent performance benefit (I hope!).

Interesting. The whole point of exhaust pipes on a motorcycle are, I would assume

• Get the hot gas out the bike without burning the rider (or the bike)

• Get it out fast without hindering the engine HP to push it out

• Keep the sound to a "sane" level (as determined by all within earshot)

• Look ok to the buyer of the bike (e.g., shiny things look good to fish)

My problem with bike "sounds" is that the potato-potato-potato sound isn't so much an issue with me as is the presumably child-like psychology of someone whose pipes are unreasonably loud.

Why? Do they need attention *that* badly and can't get it any way else?

Please note that I am not a psychologist so take my quick psychoanalysis of the loud-pipe crowd with a grain of salt and some humor.

Understood completely. It's sad, but loud pipes exist, so, there must be a *lot* of people working on those same (probably scientifically flawed) assumptions.

In a very real way, the loud pipes = more power argument stands on the same fundamentals that the bigger wheels = more performance argument.

Sure, you can get it - but you'd be hard pressed to beat the manufacturer in most cases, and even if you did - there are almost always engineering (aka scientific) cons for every pro.

For example, why not just get rid of the pipes altogether? Then you'd have the least resistance (although no tuning) of the air outflow from the engine.

I guess the argument is that would be too loud (but some pipes I hear riding past my house, which is on a busy street, seem *that* loud!).

[Mad Roger]

Thanks for checking out the articles because they were ancillary to our conversation on tire-diameter effects on performance, but it's an interesting question so there's now a new thread opened to iron it out.

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

My contention is, at the start anyway, that it's going to be really bad for the people who love decimal places - but - let's see how it turns out scientifically because I don't want to make unfounded claims without stating their foundation realistically.

[Mad Roger]

We could wax prolifically on Consumer Reports, where it would delve into the fact that they're a "business" (sort of) and that their customers want to hear contrary stuff (remember the Bose speakers "wandering" around the room lawsuits that proved what I'm asserting?).

However ...

This is Usenet for heaven's sake.

So, if you forced me at gunpoint to choose to believe something that a Consumer Reports engineer published as fact, versus something someone on Usenet posted as fact ... whom do you think I'd choose to save my life?

[Mad Roger]

Since mpg measurement at home is a tangent offshoot from mpg performance of larger-diameter wheels and tires, I opened a separate thread with the science guys over here

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

[Mad Roger]

That could be the case that the EPA cares about something else (and they can be driven by politics) - but - they must be subject to the scrutiny of scientists at the car companies - so - we can assume they're held to a higher standard than the mom and pop is who claims mpg figures with decimal places.

Since MPG measurement accuracy & repeatability is a slightly different subject from tire performance, we can take this subject over to this thread:

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

[Mad Roger]

I agree. But I suspect most people don't understand what you just wrote.

My rule-of-thumb take-it-with-a-grain-of-salt "assumption" is that anyone quoting decimal places using "home methods" is probably not understanding what you just said.

There's now a separate thread, with science guys on it, over here:

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

[Mad Roger]

Being the progeny of two PhD chemists, I'm well aware of density, but are you sure every mom and pop who "claims" their spectacularly "accurate" 22.9 mile-per-gallon numbers understands what you and I both know?

Do they realize that you have to keep the *entire* test at the same atmospheric conditions, and where the typical mom and pop doesn't do that for an entire tank of gas (which might take a week to use up during which time the atmospheric conditions change greatly).

Do they even know what "test conditions" they're testing and do they run "controls"?

Point is, my assertion (as yet unproven) is that the mpg figures with a decimal point in them that people "say", are an indication of their ignorance of what they measured.

As for what accuracy "can" people measure with the typical tripmeter divided by pumpmeter test, I opened a separate thread on that topic because it's a valid question (where I added the science & car guys).

What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

[Mad Roger]

I found *plenty* of articles which debunk the A/C myth.

I think the A/C myth is born out of the same things most myths are born out of, e.g., the high-octane myth, which is that marketing departments are very good at planting the seeds of urban myths, because they understand, I suspect, that most people trust their intuition more so than they trust facts to the contrary.

Also most people, I suspect, hold on to long-held beliefs, and can't shake them, no matter what.

Luckily, for you, the reason you can get good information out of me, is that I hold neither of those two traits all too strongly. While everyone holds those two traits, I can easily change my mind if you show me facts to the contrary, and, I don't trust my own intuition as much as I trust reliable facts to the contrary.

Having been raised by two PhD chemists, I mainly trust that which I can reliably reference.

But, particularly, I trust an article that *knows* the myth, and debunks it, rather than an article that doesn't indicate any knowledge of the myth, and just propagates it.

I don't own a pickup so I just skimmed this part, but, as an example, CR, released today an article that shows that leaving the tailgate open on a pickup truck, apparently, worsens gas mileage.

[Mad Roger]

That article was published *today* in the print magazine (it says), so, we can presume they used a recent model year vehicle - but maybe more is in the print article. (Anyone have it handy?)

The first line of the online version I quoted says they tested lots of cars though... "We?ve tested this at various temperatures with multiple vehicles,? says Jake Fisher, CR?s director of auto testing "

BTW, there were quite a few articles on the net which said the same thing.

So if you want to beat up the CR article, I can show you plenty more which say the same thing that you'll also have to beat up. :)

BTW, most people trust their own intuition more than they trust facts to the contrary, so you might be surprised when you read the part about what they found out when they tested cars that SAID to use high octane fuel, but CR tested them with the low-octane (regular) fuel instead. :)

[trader_4]

I think like with wheels you'll find that most people aren't doing it for the power. They like louder pipes because they sound cool. There is also the saying that loud pipes save lives. I don't know if that's true or not, but it seems logical that drawing attention to the fact that a MC is there, is a benefit, because most of the car and bike accidents occur when the car driver for whatever reasons, behaved like the bike never registered in their mind.

That's probably because they are straight pipes. A MC normally has not only pipes, but mufflers.