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Date:         Sat, 1 Apr 2006 07:54:18 -0800
Reply-To:     mark drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         mark drillock <drillock@EARTHLINK.NET>
Subject:      MPG measurement issues
In-Reply-To:  <001201c65596$475a1860$6400a8c0@masterpc>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Most of us have never tried to accurately assess the degree of error in our speedos at highway speeds. I do this for every vehicle I own and use for long trips. Every Vanagon I have owned was optimistic, some by more than 10 miles per hour at indicated 70. Speedo says 70, real speed 58, which number should I use when talking about MPG for that vehicle? I use the real numbers.

Odometer readings I check for accuracy also. With true stock size tires these were always pretty close. True stock size is 185R14. I seldom put new tires of that exact size on my Vanagons, sometimes smaller, sometimes much larger. This affects the odometer reading and speedo reading. Which numbers should I use when talking about MPG for that vehicle? I use the real numbers, not what the speedo reading say as they are wrong and I know it.

Many of us have smaller diameter tires than 185R14. This can give MPG figures a real boost, at least the phony ones based on the speedo readings. If you put smaller tires on, the odometer will record more miles than are actually traveled, giving the illusion of an increase in MPG. Also the speedo will read even higher than before, so if you keep at the same indicated speed as before, you are actually driving slower now and will get better MPG than before since you have less wind resistance to overcome at the lower true speed.

People often quote their highway MPG and say it was while cruising at 65-70, or some other narrow speed range. The problem is, readers don't know if that is ACTUAL speed, or merely the speed showing on the speedo while they drive. Readers also don't know if the poster corrected for odometer error, or just relied on what the mile counter said. That is why when someone posts MPG numbers that are very high, I suspect measurement error. I mostly mean the indicated measurements of the speedo, as well as mistakes in filling the tank.

Mark

>From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf >Of Geza Polony >Sent: Saturday, April 01, 2006 12:25 AM >To: vanagon@GERRY.VANAGON.COM >Subject: Re: MPG Question > >One thing to take into consideration: the wide variations in mpg we're >seeing here could very well be related to odometer variations in these >20+ >year old vehicles. Bentley Section 90.30 tells us that, brand new, >odometer >tolerance is from -1% to + 3.75%, in other words, a spread of almost 5%. >Couple that with over- or under-sized tires, and you could get >variations of >20% easy. > >I've noticed that going taking the exact same route from our home to our >vacation place yields different mileage readings on the odo. And I mean >exact same route! Maybe five miles difference over the course of 400 >miles. > >Claims of driving X MPH for such and such a MPG may be erroneous, too. >It >turns out the speedometers have a total tolerance, new, of almost 18%! >(Bentley 90.30) Incredibly, VW indicates that for a real 50 MPH speed, >speedometer readings can be up to 58.7 MPH! In other words, when you >think >you're going 75, you may only be doing 65 or so--and that's with >absolutely >stock tires. > > >What are we really measuring, then? This is all ballpark figuring, when >you >look at it that way. EPA must calculate MPG using measured distances, >rather >than odometer readings, along with carefully measured gas. > >I was originally asking a mechanical question. Is there something that >typically cuts gas mileage down even though the car runs fine? What >would >that be, if so? > > > >


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