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Old 12-27-2017, 07:59 PM
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trickc trickc is offline
Bleedin' Gasoline
 
Join Date: Nov 29, 2000
Location: covington,ga
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Speedo question

So for Christmas Buttercup got new gauges in a custom dash overlay that I made. New tach,speedo,oil,fuel,temp,volt. They all work as expected but one thing that I found curious, I set my gps on the dash and went for a spin to compare. While the actual speed was accurate, pretty dead on actually, the trip and mileage counters were off. My factory speedo was off by 5-7 mph slow depending on how fast you were going and so was the mileage. I was pleasantly surprised how accurate the speed was on the new one but somehow the trip and mileage are still off and seem to be even more off than the factory unit. But how can this be if the speed part is right? Like I said I verified speed by my gps unit and can only assume the mileage part of gps is accurate also.
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Old 12-27-2017, 08:07 PM
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My SpeedHut GPS speedo is dead on.

How are you measuring the milage.

I went for a 20 mile drive and use the NM DOT mile markers.

Andrew
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Old 12-27-2017, 08:22 PM
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trickc trickc is offline
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My gps unit has a trip function that I set to 0 and rolled the trip meter to 0 on the speedo. For 1 mile on the speedo is 1.2 miles on the gps so 10 miles on the speedo is actually 12 per gps. Which as long as I know the formula, calculating fuel mileage is just a little more complex, I just find it wierd that the speed is right but somehow the mileage is not...
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73 j-4000 "buttercup"-SOA 36's 4 door cab


78 wagoneer choptop-SOA 36's, 6 point cage, blah blah blah
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Old 12-27-2017, 10:16 PM
joe joe is offline
 
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Keep in mind GPS readouts aren't true measurements made by counting wheel revolutions/distance,, they're mathematical (trig) calculations. They are made by measurements between two points in a straight line on the earths surfaced based on time between two points. On curves instead of thinking of a smooth curved line, think of a segmented line via a series of connected dots. The accuracy will depend on how often the unit refreshes position readings(dots) and how fast it recalculates those readings. Generally the better quality/cost of the unit the faster more accurate they are. A curvy up and down road through the mountains won't be as accurate as a straight shot hwy across Kansas.
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Old 12-28-2017, 10:32 AM
SJTD SJTD is offline
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Factory mileage is a geared counter. It should be accurate with the right size tires. Factory speedo is a spinning magnet induction type thingy that I've never really tried to understand. The two might even agree.

I too would expect the new aftermarket unit would have the speed and mileage match. Is it electronic? Cable driven? GPS?

If electronic it should be programmable for tires/gearing but the speed and miles should match even if programmed for the wrong tires or gear ratio.

If it's GPS they should match.

Who made it?
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Old 12-28-2017, 08:19 PM
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trickc trickc is offline
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Its an equus mechanical cable driven unit. Joe- I drove it to church and back, which is not kansas straight but a fairly straight 2 lane hwy distance of about 15 miles one way.

The gps is a garmin Ive been using it for about 2 years now and its been fairly accurate, and have used it in at least 3 other vehicles and reads speed and distance pretty close to what they've read +\- 1 to 2 mph.
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78 wagoneer choptop-SOA 36's, 6 point cage, blah blah blah
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