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  #1  
Old 10-27-2004, 11:09 AM
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Using some sort of device to get actual MPH readings?

I thought I heard something on the message boards about people using a GPS or something else to get their actual MPH readings, I would be interested in one because I have oversized tires on my 300D, and according to that Miata calculator, 60 on the speedometer is actually about 62.3 on level ground, but with others driving beside me with normal sized tires for their car, they read 70, while I read 60, and I just wanna know where I can get a device to get me the actual speed I'm going until I get new tires, which will not be for a while.
-Joe

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  #2  
Old 10-27-2004, 11:16 AM
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Borrow a GPS or buy one, like you said, that would work.
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  #3  
Old 10-27-2004, 11:58 AM
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H-town,

Use the mileage markers along side any Interstart highway and a stop watch. Drive different constant speeds about 60 MPH until one takes exactly one minute to cover 1 mile (10 minutes for 10 miles would be more accurate). The speed that takes 1 minute for one mile or multiples thereof, will be the speed on your speedometer that is actuall 60 MPH. This method works best if you have a good cruise contol.

Another way is to look for a radar speed detector along a road that displays your speed.

P E H
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  #4  
Old 10-27-2004, 12:00 PM
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Quote:
Originally Posted by boneheaddoctor
Borrow a GPS or buy one, like you said, that would work.
Where can I purchase one of these GPS systems? Do I need a certain brand or something?
-Joe
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  #5  
Old 10-27-2004, 12:12 PM
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Quote:
Originally Posted by H-townbenzoboy
Where can I purchase one of these GPS systems? Do I need a certain brand or something?
-Joe
They are quite common these days. I think you can get them at places like Best Buy or Circuit city just to name a few.

I can't recomend any one over another. Never bought one as I could never justify the cost vs. utility. To many other toys to spend money on.

Maybe someone else has a recomendation of a specific modle/ brand.

You could ask salesman to show you features and things like rate of travel etc. That will be more accurate than stopwatch and mile markers. Those seem to have dissapeared from the highways around here.

But if money is tight and you know someone with a stopwatch and have a convienient stretch you can maintain a steady speed and get time over a mile stretch, you could do the math and get an fairly accurate true speed average.
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Old 10-27-2004, 12:14 PM
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You can buy one at sporting goods stores for $150 or less for a small hand held unit. I have a Magellan 315 and it works very nicely.
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  #7  
Old 10-27-2004, 12:30 PM
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Gps

You can get a basic Garmin E-Trex for right at $100, that will do anything you'd ever want to, unless you are planning to scale some uncharted mountain.

Steve,
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  #8  
Old 10-27-2004, 12:38 PM
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Some of the way-cool gps units interface with a laptop so you can run navigational software and get voice directions while driving. Pretty cool, as long as the software is really correct. My buddy and I found out the hard way that his software didn't know the Wrigleyville area of Chicago very well. I will just say I was glad it was light out and not dark...
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Old 10-27-2004, 12:53 PM
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Quote:
Originally Posted by Habanero
Some of the way-cool gps units interface with a laptop so you can run navigational software and get voice directions while driving. Pretty cool, as long as the software is really correct. My buddy and I found out the hard way that his software didn't know the Wrigleyville area of Chicago very well. I will just say I was glad it was light out and not dark...
I know www.Mapquest.com gets it wrong about 1 time in 20 I use it.
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche
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Old 10-27-2004, 02:00 PM
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I have seen basic Garmin's on sale at West Marine or Sports Authority for $89. You might even be able to get one with WAAS now I know that has been filtering down into the cheap hand held ones. WAAS is pretty cool it will give you your location to the inch no more within a few feet. Also the gov has had the interference turned down for the last few years so even none WAAS gps's are very accurite.
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  #11  
Old 10-27-2004, 03:10 PM
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No sense renting/buying a spendy GPS just to see what your speedo is off. Do what PEH said and use the mile markers over a least 5-10+ mile stretch or find out what size your stock tires are in dia. Call a tire shop or search the internet for dia of the originals and of the new tires. Basically the % increase in tire dia is going to make your speedo/odo read slow by the same %.
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  #12  
Old 10-27-2004, 08:03 PM
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Quote:
Originally Posted by jseries
No sense renting/buying a spendy GPS just to see what your speedo is off. Do what PEH said and use the mile markers over a least 5-10+ mile stretch or find out what size your stock tires are in dia. Call a tire shop or search the internet for dia of the originals and of the new tires. Basically the % increase in tire dia is going to make your speedo/odo read slow by the same %.
Been there, done that. I would use PEH's method, but my CC does not work, and mile markers are kind of scarce in Houston. The Miata online tire calculator is of no help, since when I read 60 on the speedo, the 300SD driving exactly alongside me read 70, and the calculator claims I'm really going 62.1mph when I read 60. I would rather have a GPS, $106 isn't so bad considering what a set of properly sized good new tires go for.
-Joe
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  #13  
Old 10-27-2004, 08:16 PM
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The cops always set up those radar trailers in my area. There supposed to slow you down by showing you your speed in large numbers. I have checked my speedo a few times using them and have found it to be right on. It is also fun to screw with them, imagin the guy who is going over the speeds 45,39,109,39,41 ect Not that I would ever do that.
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  #14  
Old 10-27-2004, 08:33 PM
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I have a question for anyone who has actually used one of these GPS units for this application, measuring vehicle speed of travel: How fast does it update? How fast does it respond to changes in speed? Is it only good for average speed on the highway? If I am traveling at 30 and then speed up to 40 how fast will it register that? I assume that there is some lag and averaging going on and it would not really work very well in terms of getting an accurate speed reading all the time. But I don't know.

Thanks,

Mike
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  #15  
Old 10-27-2004, 09:02 PM
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Their is a slight lag time but they are pretty fast.

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