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Old 11-15-2012, 01:25 PM   #1
saturnhusker
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Default Speedo vs GPS

Ok I have a 2000 sl 1.9 base no ps. The tires that are OE are 185/65 14 well when I ran them the speedo was under what my gps was showing. I had to go like 63 on speedo to show actual speed on gps as 60. Ok running on interstate @ 75 rpms was at 2850. Now have 195/70 14 on there and have to go under the speedo going 60 gps said I was going 63. Rpms are down on all speeds. I aint changing the tires cause they are my winter tires I wasn't going to pass up deal on blizzaks cause they weren't the correct size tires. But speedo was off when I had oem tires on there do I need to replace speedo?
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Old 11-15-2012, 02:35 PM   #2
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Default Re: Speedo vs GPS

My code reader says the PCM/VSS is showing 70mph while the speedometer is showing 67mph. In this case, GPS is overkill for proving just how inaccurate the gauge's are from the factory
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Old 11-15-2012, 03:59 PM   #3
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Default Re: Speedo vs GPS

Google information. You'll find that car makers gave us a fudge factor; speedo reading will be higher than actual speed so this gives everyone a margin of error when using posted speed limits. No one replaces their speedo just because its 'off' a little. A difference of a few mph isn't going to warrant a speeding ticket.
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Old 11-15-2012, 05:03 PM   #4
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Default Re: Speedo vs GPS

Quote:
Originally Posted by fdryer View Post
You'll find that car makers gave us a fudge factor; speedo reading will be higher than actual speed so this gives everyone a margin of error when using posted speed limits. No one replaces their speedo just because its 'off' a little. A difference of a few mph isn't going to warrant a speeding ticket.
not to mention too, the officer's radar gun may also be slightly off as well...
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Old 11-15-2012, 05:51 PM   #5
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Default Re: Speedo vs GPS

Traditional radar guns are not likely to be off at all, that is if the officer takes the time to calibrate.....in most departments I am sure it is still mandatory to calibrate, to ensure that a speeding ticket is not thrown out by a judge.

Before shift use, they are calibrated with the supplied tuning fork.
Yep, the tuning fork's perfect pitch is what was used to calibrate.
Most units I have seen lately, still use the same method.

However, Vascar, X band and beyond, laser or infra-red, who knows what they use for calibration.
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Old 03-14-2013, 01:27 AM   #6
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Default Re: Speedo vs GPS

Well winter is nearly over and I just put on new 195/70 14's Kumho Solus KR21 cause I like the tread design. I was on my way into town and they have them radars on to show your speed so I stayed at what GPS said was 65 and what do you know radar gun was reading the same thing. I have done this experiment more than once and always read the same. I get around 37 mpg @ 75 mph on the interstate. I know it will go up cause the days are warming up and winter fuel is changed to summer. Plus that was with my Blizzaks on not a great highway tire. I will be back in to the mid 40's again if not higher.
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Old 03-14-2013, 07:51 AM   #7
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Default Re: Speedo vs GPS

Quote:
Originally Posted by saturnhusker View Post
Ok I have a 2000 sl 1.9 base no ps. The tires that are OE are 185/65 14 well when I ran them the speedo was under what my gps was showing. I had to go like 63 on speedo to show actual speed on gps as 60. Ok running on interstate @ 75 rpms was at 2850. Now have 195/70 14 on there and have to go under the speedo going 60 gps said I was going 63. Rpms are down on all speeds. I aint changing the tires cause they are my winter tires I wasn't going to pass up deal on blizzaks cause they weren't the correct size tires. But speedo was off when I had oem tires on there do I need to replace speedo?
It is not the speedo, the mpg/1000 RPM is programmed into the PCM and tire diameter is part of the calculation. Changing tire diameter changes the resulting speedo reading and there is no way to correct it. As also mentioned, the speedo usually reads slightly low by design with the stock tires (diameter) but not necessarily the stock tire size as tire size(diameter) as this diameter does vary slightly with brand and tread pattern/design.
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Old 03-14-2013, 08:08 AM   #8
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Default Re: Speedo vs GPS

How can you blame the speedo and not the GPS? The car and tire are firmly planted to the ground (if you don't drive like a maniac) with the wheels turning and calculating constantly. The GPS? It is using radio signals that are being transmitted every few seconds (depending on the quality of your unit) AND the GPS does not account for variations in elevation.

I guess the old adage, "The bigger the lie..."
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Old 03-14-2013, 10:32 AM   #9
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Default Re: Speedo vs GPS

Stop watch and mile markers works well and will yield similar results.
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Old 03-14-2013, 10:37 AM   #10
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Default Re: Speedo vs GPS

What about Radar Speed Trailer reading?

picture of Trailer:
westernstates.org/projects/coats/Updates/2011-09-01.html

My spedometer seems to match the trailer's number.
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Old 03-14-2013, 11:03 AM   #11
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Default Re: Speedo vs GPS

speedo and odometer change as tire wears also. new tire has a much different turn per mile than worn tire.
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Old 03-14-2013, 12:05 PM   #12
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Default Re: Speedo vs GPS

Tire revs per mile will vary with tire size, pressure and wear.
On most of my cars the speedometer has generally read about 2% to 3% higher than doing the milepost/stop watch on the interstates. That is with the OEM standard tire size mounted. The GPS-reported speed was closer than the speedometer.

Never got a speeding ticket as a result of speedometer inaccuracy.
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Old 03-14-2013, 01:07 PM   #13
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Default Re: Speedo vs GPS

Well when I had the oem tires on they registered slower the than the speedo and the result was higher rpms. Changed tires still off but rpm's lower with the bigger tire size. I have put on 22k since august I keep a milage log and with the change in tires my mpg went up and rpm's went down.
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Old 03-14-2013, 02:27 PM   #14
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Default Re: Speedo vs GPS

There are PCM calibration options and it is entirely possible that is incorrect. There could be a long list of reasons for that. For every set of tires you should calculate an odometer reading correction factor multiplier and the same for the speedometer as they are usually different.
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Old 03-19-2013, 12:24 AM   #15
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Default Re: Speedo vs GPS

Well it's winter and with the change of tires mpg gallon went up. Then we had a couple of days of temps in the 70's and I travel for my job well when I got there I still had 1/4 tank and had gone 375 miles a big jump from last summer. I have not done anything but change my oil every 3k.
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