Hello everyone, Yesterday and the day before I was hitting unusual speed for a beginner like me. My max speed was 45.5 kph on flat usually with out help of winds ( I think so, because it happened on many occasions). The speed I reached was once 50 and the other was 52 kph respectively on my latest attempts, on both occasions when I go to gramin training center graph it doesn't show that speed on top of the speed graph yet it only shows this speed in "max speed" section.

Can I tust garmin for this max speed ? is it correct?

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Was your speed being tracked by the GPS or by the cadence/speed sensor? I would trust the sensor, but not the GPS.

I was just using GPS, sensor yet to leave the hold in customs with my clif electrolytes powder for since 26/8

How far off was the max speed reported by the unit v. Training Center? I sometime get the same thing using Sport Tracks, but the difference is small (0.1 mph). The application may actually recalcuate speed based on GPS information and the algorithms used may differ slightly. Just speculation on my part.

I've had my bike going 500+ mph, at least according to my Garmin. My CatEye said 12 mph at the time, and since this was on a crowded section of MUP going across a bridge, I don't think I was going about the speed of a Boeing jet. Another time, my Garmin said I rode my bike from Seattle to China and back in about ten minutes. I must have had a lot of air in my tires. ( In short, Garmins are unreliable, and if common sense ever tells you not to trust your Garmin, it's common sense you should believe. )

They both use the same math to calculate your speed, but SportTracks "smooths out" the data before applying the speed formula.

Have never had trouble with my 305.
Do you have the column set to show?
pacificaslim: Also if you have the cadence/speed sensor the speed is only used off it if you loose GPS signal like in a tunnel or heavily covered tree areas and such other wise the GPS is always used unless you manually turn it off and tell it to use the cadence/speed sensor.

I have the 305 and the speed sensor, and have specified tire size in the settings of the 305 (something that may help the speed reliability) but I still find the garmin to be off compared to the powertap or cat eye computer... max speed is usually 1mph+ slower than the others, and it gets funny in certain areas (I assume because its still using gps data) its spot on indoors, however, when I was on the trainer, and the average speed is dead on, so I dont care too much about the rest. just annoying.

I have a forerunner 305 and never had an issue at all. It is close to my bike computer which is dead accurate. I have done century's and almost do difference. THe last century my bike hit 100 and the garmin was 99.94. The top speed on the bike computer is a bit faster all the time the garmin requires you to keep the speed for a certain distance. Typically I hit top speeds on a ride around 31-35mph and the garmin will show about be 1/2 of a mph slower. Today on my 54 mile ride top speed on the bike was 33.1 and the garmin was 32.8. Do the roll out measurement for your bike computer and it will be dead accurate the garmin for pratical purposes it just as accurate.

Those are both examples of multi-path errors.

I'd trust the GPS over the cadence sensor. OP did you have smoothing enabled in Training Center?

Agreed. When I had my 305, when signal was lost, the cadence sensor would give me all kinds of wacky readings.

I've observed the same thing as the OP. In Garmin connect I would get a higher average speed which wouldn't show up on the ride data. I have a feeling that it's simply a stray data point. Usually when I plug the data into Ascent, it's gone.

After my third 305 died, I got a 500 and must say it's a significant improvement over the 305. It's faster, more accurate, and doesn't lose signal like the 305 did. I can turn it on indoors and ride under large canopies of trees and never lose signal in places the 305 would drop out frequently.

The difference was like.. top speed in graph is 47.5 kph and max speed 52.0 which is much difference considering my max effort -.0

Does it really matter?

That's a really useful post.

Ok. Why does top speed matter? Who cares if it's a + or - a few kph?

Personally, when I spend $250 on a GPS, I want it to perform the functions it claims it's supposed to. One of the big boons of GPS is supposed to be "sensitive" speed and distance measurement. If it doesn't do that, I've senselessly wasted $$ over a $50 Cateye.

We've all heard the lectures on how average speed is worthless... I don't believe that's what the OP is going after. He just wants to know if his GPS is accurate or not:

The problems I had with my 305 were not related to speed at all. They were auto-shut-off due to a poor design. Thus far, my 500 has been fine.

This is a very fair point. It sounds as though the OP is describing a transient peak - these can be caused by either a GPS/software glitch spiking your data, or (possibly the OP's case) when a transient speed reading is missed by the data sampling (especially if you have smart recording set). After using a 305 for getting on for 2yrs I've stopped worrying about anomalous spikes/peaks and focus more on the sustained efforts.

OP have you tried uploading to diffent software packages? I like to use Sporttracks on my PC, and IMO Strava is the best online GPS app. Maybe see if you get better numbers through one of thoses? (both free to try)

Umm, I would never trust the sensor, I have been using the FR60 (non GPS so i can only use a speed sensor) and I have yet to go on a ride with my top speed under 50, on most of my rides my max speed is around 98-109 mi/h but my average is correct, and the peaks of the speed sensor are on normal speeds. and by the way, I am fairly certain that I have never hit 100mi/h on a bike.

GPS is fairly accurate but only to a point. A few feet off one second and a few feet off the next second and it will think you are going a bit faster one second and a bit slower the next. Over a few seconds it averages out, it's not worth worrying about.

Thanks everyone for your inputs !

GPS can be very unreliable depending on many factors. You have satellite availability, reception, WAAS reception, sample rate, firmware bugs, hardware glitches, etc.

In short, NO you cannot trust your Garmin for much more than an overview. If you see outlying data, you have to disregard it. Newer firmware may improve the accuracy of your unit by automatically rejecting outlying data or by increasing the window that it uses to calculate velocity. With the 305, you can also increase speed data accuracy by using the wheel sensor, which can measure actual moving speed instead of calculating speed based on position.

That is what I wanted to hear, thanks for making it clear!

Just to emphasize Garmin's can be inaccurate in regards to top speed. I've looked at my Garmin at the end of sprints and seen a top speed that does not appear on the ride report when I upload.

Related to this my Garmin has occasionally shown 27% grade climbs! Please note where I live.

Why? Above explains it pretty well. W/o getting technical I always figured the top speed at the last second of a sprint wasn't held long enough to be "official."

FWIW I subscribe to the "doesn't matter"school.