How accurate is my motorcycle speedometer?
November 29, 2007 8:30 AM   Subscribe

How accurate is my motorcycle speedometer?

And could it be less accurate at higher speeds? On surface streets, I feel that I am riding with the flow traffic and the speedometer reading seems somewhat correct. On the open road, though, I am always one of the slowest vehicles, almost always in the slow lane. Yet the speedometer reading is close if not equal to the reading I would have in my truck on the same roads.

How would I go about checking and adjusting the speedometer? The bike is a 2007 Yamaha Virago.
posted by ijoshua to Travel & Transportation (13 answers total)
Borrow a gps and see what it says. I have a gps mounted on my scooter (150cc) and the speedo is at least 5mph off.
posted by o0dano0o at 8:42 AM on November 29, 2007

The easy way to test is just by aiming at one of those "your speed is" radar signs. At least around here, they're pretty accurate. The other option is GPS; most receivers will tell you your speed to good accuracy.

Lots of speedos tend to read fast, and the error gets greater as speed increases. Mine reads 5mph fast at 75mph, and about 8-9mph fast at 125mph (6-7%). I gather that it's a CYA on the part of the automaker (better to read fast than to read slow and get blamed for speeding tickets).
posted by uncleozzy at 8:48 AM on November 29, 2007

To add to uncleozzy, it's probably going to be off in the same direction regardless of the speed. That is, it won't read slower at some speeds and faster at others. In my personal experience with both bikes and cars, they've all been optimistic. My '68 Plymouth read at least 10mph high at 55, more recent vehicles have been around 5mph or less depending on the speed.
posted by tommasz at 8:55 AM on November 29, 2007

GPS is the easy way; the traditional approach is to use a stopwatch and time yourself over a mile. (The safe way to do that on a motorcycle is to have your passenger operate the stopwatch while you concentrate on keeping your speed constant.) Sometimes the radar signs don't "see" a motorcycle really well, especially if there are other cars around, so take what the radar sign says with a grain of salt.
posted by Forktine at 8:59 AM on November 29, 2007

Speedos on motorcycles tend to average about 5-7% off (on the high side in my experience - meaning 75mph indicated will mean you are actually somewhat slower than that) at higher speeds (60+).

GPS is the way to go, there's no real way to adjust the stock speedo - though you could get one of those bicycle computers with the magnet on it... small as it is I'd be worried about it coming off at high speeds.

Once you know the error at certain speed ranges, you can safely adjust your indicated speed. Otherwise ride with the flow of traffic and you will proabbly be fine.
posted by clanger at 9:13 AM on November 29, 2007

60 m/h is 1m/m, find an Interstate with mile markers, keep a steady 60m/h and time yourself over 10 or 30 miles. Calculate... This is my favorite long-distance travel game... Bonus if you can do 70m/h and factor in +1/6 into your calculations....
posted by zengargoyle at 9:20 AM on November 29, 2007

GPS is the best (and least dangerous) idea.

All UK motorways have distance markers numbered every 100m (despite road distances being posted in miles) - keep a steady speed (ideally with an easy number on km/h e.g. 120) and then time yourself and count the marker posts. 120km/h = 2km/min = 20 marker posts/min = 1 post per 3s.

In the UK, at least, you'll normally find speedos over-reading - mostly because it's illegal for a speedo to under-read (all cars over 3 years old have an annual MOT Test and this is one of the failure points - and a failed MOT means the car cannot be used on the road).
posted by unsliced at 9:25 AM on November 29, 2007

n-thing the GPS route. Thats what i had to do with a vintage bike...speedo was over 15 mph too fast. If you don't know anyone with a GPS, just offer up 20 bucks on craigslist to borrow one for a couple hours (i did 20 bucks with a deposit of my ipod). You'll get a hit pretty quick.
posted by furnace.heart at 10:08 AM on November 29, 2007

Motorcycle speedos are frequently off by a percentage. My KLR650 reads consistently 10% high (and that's normal for that bike).
posted by flaterik at 1:21 PM on November 29, 2007

Speedo's are legally required to read at or over the speed you are travelling at. This is pretty much industry wide (cars, trucks and bikes etc), but I only found a EU directive at first look:

These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.

N'thing that modern speedometers are not adjustable (in the most part). Just learning your one for how accurate it is (it will be a percentage so will read 'out' at differing amounts at different speeds) is your best bet.
posted by Brockles at 3:51 PM on November 29, 2007

Ask a cop to run at an agreed speed. Compare. The one that did this for me flashed his blues at the limit increments of 30, 50, 70 for me. (How's that for service, huh!)
posted by DrtyBlvd at 5:31 AM on November 30, 2007

Make sure you've got your tires inflated properly as that can also throw your numbers off.

My motorcycle's speedo is generally off about 6% (as measured with my GPS unit).
posted by fenriq at 3:31 PM on November 30, 2007

My Suzuki has an electronic speedometer which reads 8% over. I have fitted a speedo healer that corrects it.

However, the odometer, which read correctly (thus proving the speedometer is designed to read over by the manufacturer) now reads 8% under. Which is a good or bad thing depending on how you look at it.
posted by hmca at 5:26 AM on December 3, 2007

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