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Speedo vs GPS = big mph delta


schwank

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So I've put in a bunch of days this summer with the Traqmate in my car. After analyzing the data collected from track sessions as well as just some driving around town while datalogging, I have found my speedo to be pretty far off the mark.

 

I am running spec RA-1 sizes... what sort of error rate are you guys seeing on the speedo numbers? When I am at redline in 4th gear, my speedo is up around 115mph while my GPS is recording 104. Even around town I am getting about the same 10% error rate... is that normal for these tires on phonies?

 

Also it's a bit odd but the tach input on the TM seems to be a bit off as well. It's right on at idle but once I'm at cruise on the highway it is about 200rpm low from what is indicated on the stock tach gauge.

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In your experience is the speedo on the car generally close at that speed?

 

BTW: If anyone here uses Traqmate, note they finally released v2.0 recently. the interface has improved quite a bit.

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In your experience is the speedo on the car generally close at that speed?

 

I have no idea. I generally don't look at the speedo. The tire diameter is smaller than stock however so it would not read accurate anyway.

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according to this tire size calculator the change from stock 215x60x15 size tires to the smaller diameter 225x50x15 tires, the speedometer reading would be 5.2% too fast.

 

doesn't fit with your 10% error observation but just a FYI.

 

btw- Thanks for the heads-up on the v2.0 Traqmate software release.

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Recently, my wife followed me while I drove my car, and via our radios, she confirmed that my steady 60 mph, was 52 in her SUV. Thats with Toyos, not Goodyears.

 

I'm going to give my 944 some exercise early this week, and I'll drive it thru the local construction zone's radar speed sign area, and report what it is at ....as fast as I can go at that moment. Since our Sedona HWY 179 construction project is never ending, I've tested the radar signs in our other cars many times, and found them to be surprisingly accurate.

 

Stay tuned.

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Yeah, the spec Toyo's are 5% smaller for the 944 and 6% smaller for the 924S, so that's part of it. With the Traqmate, I'd be curious if that's based on it's GPS reading or it's accelerometer reading, or some combination both... they both have their set of strengths and weaknesses. As for radar signs, don't forget about cosine error... if you're not going 100% straight at the radar antenna, it will say you're going slower than you really are. Some of those signs aren't calibrated very well either.

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The G2X uses GPS alone for speed and is accurate to 0.2mph, the GPS number is correct.

 

I did the DE at BIR during the club race weekend a couple of weeks ago and my speedo was showing around 138-140 going in to T1 in my 944S on 15" RA1 225/45's. I was pretty sure I wasn't going anywhere near that speed and later a guy in an 89 S2 that I was running very close laps with said his data logger had our max on the front straight around 128 mph, which is 7 to 8% slower than indicated. I also noticed that the data you posted on Trackpedia from Norm Hamden's 1:12.XX at PIR showed his max speed at 112 mph and I know that most of us are showing 120 or so on the speedo on the front straight before turning in to turn 1, which would be right on the 6% error rate that Joe estimated. I just bought James Reeser's Traqmate and hopefully I'll be generating some date on all of the Phoenix area tracks this fall. Bottom line is that I trust the data loggers a lot more than the speedos!

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I can concur with Joe about 6,000 rpm in 4th indicates about 100 mph in my 924S (mechanical speedo) and my 86 944 (electronic speedo).

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