Speedo is off a bit -- registers fast
11-13-2010, 10:14 PM
#1
Rennlist Member
Thread Starter
Speedo is off a bit -- registers fast
Comparing my speedo to the in-car GPS and other cars the speedo is a couple miles an hour faster then the GPS or other car. What could cause this?
Thanks
Ed
Thanks
Ed
11-13-2010, 10:24 PM
#2
Three Wheelin'
tire diameter, b/c the speedo is hooked up to an insane gear ratio to the wheels. so if your car is actually going SLOWER than she thinks, maybe your tire/wheel diameter is LESS than stock size. are your tires small?
now that i think of it, my speedo registers fast also.
now that i think of it, my speedo registers fast also.
Last edited by bonus12; 11-13-2010 at 10:40 PM.
11-14-2010, 10:08 PM
#3
Rennlist Member
Thread Starter
Thnaks for the thought Bonus12. I checked the tires -- 21560R15 - appears this is the standard for the '86 NA 944. What else can be an issue or is this system mostly Ed proff?
11-15-2010, 12:00 AM
#4
Three Wheelin'
I have the same issue, kind of. I went from original phonies to 18 inch turbo twist. So much bigger. The speedometer is off by a bit. Can I do anything about it?
11-15-2010, 01:11 AM
#5
Three Wheelin'
the factors, however, are tire size, speedo drive cable gear ratio, and the final drive ratio in the differential.
11-15-2010, 04:59 AM
#6
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In yurop every speedo is showing approx. 7kmh (4mph) more than the actual speed. It is supposed to be extra safety margin or something.
11-15-2010, 06:28 AM
#7
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we've spoken about the "speedometer defect" built into Porsches on several threads here and on other forums. it's real. if you can find the texas mile/nevada road-racing people, and they'll tell you the same.
i run oversized 25.65"/275-35-18 tires. many people run 25" tires. that's part of the problem, but not all, next, i turned back my speedo about 4 mph. now, when i'm going 84 mph, it says i'm going 80. at 140 mph, it reads 138, and at 160 mph it's reading 160.
i believe they built this defect into the cars so people would be going about 10~12 mph less than they believed during those rare times when people decided to "open it up." the cars have such good road habits that this would keep people at a safer distance from the limits of the cars, and thus save lives.
it was likely reasoned that when people were going over say, 115 mph, a widening speedometer defect would hardly be noticed..... and the idea of having less people dying in high-speed crashes cab only be reasoned to be a good thing at the corporate offices.... during road tests, true top-speeds would be measured using other methods with gentleman's agreement not to get into specifics about speedometers giving away exagerated speeds.
here's the math.
i run oversized 25.65"/275-35-18 tires. many people run 25" tires. that's part of the problem, but not all, next, i turned back my speedo about 4 mph. now, when i'm going 84 mph, it says i'm going 80. at 140 mph, it reads 138, and at 160 mph it's reading 160.
i believe they built this defect into the cars so people would be going about 10~12 mph less than they believed during those rare times when people decided to "open it up." the cars have such good road habits that this would keep people at a safer distance from the limits of the cars, and thus save lives.
it was likely reasoned that when people were going over say, 115 mph, a widening speedometer defect would hardly be noticed..... and the idea of having less people dying in high-speed crashes cab only be reasoned to be a good thing at the corporate offices.... during road tests, true top-speeds would be measured using other methods with gentleman's agreement not to get into specifics about speedometers giving away exagerated speeds.
here's the math.
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11-15-2010, 07:25 AM
#8
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This is done intentionally by car manufacturers so that they lower chances of blaming company for driving too fast and it is perfectly normal.
In my e39 bmw you can unlock secret functions and get the real speed value on the screen. Needle however always show 5~7kmh more than actual speed.
In my e39 bmw you can unlock secret functions and get the real speed value on the screen. Needle however always show 5~7kmh more than actual speed.
11-15-2010, 01:45 PM
#10
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make a mental note where the speedo needle rests while the car is stopped.
remove the glass and get your right hand index finger on the butt end of the speedometer needle and while using your left index finger to stop it from moving back too far, apply the most gentle amount of pressure until the needle finally catches and "jumps." now take a look at the position of the needle. it should be about 3~4 mph back... (don't touch the long end of the needle) !!
remove the glass and get your right hand index finger on the butt end of the speedometer needle and while using your left index finger to stop it from moving back too far, apply the most gentle amount of pressure until the needle finally catches and "jumps." now take a look at the position of the needle. it should be about 3~4 mph back... (don't touch the long end of the needle) !!
