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General Category >> Rubber Side Down! >> Speedo accuracy
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Message started by babyhog on 06/30/10 at 05:42:01

Title: Speedo accuracy
Post by babyhog on 06/30/10 at 05:42:01

Hubby has always said my speedometer must run a little low. He has followed me on both of his bikes, and his truck too. Then I had a guy from work follow me to work the other day and he said I was running a little over 60, when I thought I was going about 57-ish. Is this normal for this bike? I still have the stock tires, need to replace rear this summer, was planning to go with a 140/90 instead of 140/80. Is that going to make a difference?

Title: Re: Speedo accuracy
Post by justin_o_guy2 on 06/30/10 at 05:57:08

Generally, I think the speedo is a bit optimistic. Have you changed the rear tire to a taller one?

Title: Re: Speedo accuracy
Post by Charon on 06/30/10 at 06:11:05

Checking the speedometer against another vehicle is always iffy, because you have no way of knowing how accurate the speedo is in the other vehicle. And it is quite common for makers to have speedometers read a little high. That is reputed to be their method of avoiding lawsuits from people who get speeding tickets.

The easiest way to check your speedometer is to use a GPS. Another good way is a stopwatch and the mileposts along the Interstate (or anywhere else). At 60 mph, it takes 60 seconds to travel a mile. Within the range of 55 to 65, it is "close enough" to assume 65 mph takes 55 seconds, and so on. The difficulty with the stopwatch method is in maintaining your speed exactly for a mile. A third good way is to find one of the "Your Speed is - " units somewhere along the road. Problem there is making sure you are the vehicle it sees. Motorcycles are relatively small targets, and the radar has an easier time seeing the semi behind you.

Changing tire size will affect the speedometer reading. A bigger tire will make the speedometer read lower. If you have ever watched a drag racer do a burnout, you may have noticed the rear tires expand as they speed up. This happens on all tires as speed increases. This, along with innate inaccuracy of the speedometer itself, can mean speedometer error changes with speed. The Savage/S40 drives its speedometer off the transmission, so rear tire size is the one that affects the speedo. Some bikes, and most cars nowadays, drive the speedometer via the engine control computer, so makers can cause the speedometer and odometer to read anything they please.

Title: Re: Speedo accuracy
Post by Tanker2Biker on 06/30/10 at 06:17:33

I have checked mine against my GPS, and it seems to be within a mile or two, taking into account there is a little lag in the GPS changes.

Title: Re: Speedo accuracy
Post by Serowbot on 06/30/10 at 06:24:18

Stock speedo usually runs a little optimistic... a 140/90 will put it close to accurate...

Title: Re: Speedo accuracy
Post by 12Bravo on 06/30/10 at 06:25:39

Like Charon said, most motorcycle speedometers read fast from the factory. The speedo on my Burgman read about 9% fast. I went with a 140/90 on my S40 and the speedo is closer to being correct now. I found that I can't go by those road side radar signs either. They won't pick up my S40 or Burgman half the time and the other half read either fast or slow. Use a GPS or use the old fashion stop watch trick.

Title: Re: Speedo accuracy
Post by Routy on 06/30/10 at 06:37:57

I'm always comparing it to the bicycle speedo w/ trip meter I have mounted on a turn signal arm. I programed it to the exact (measured) circumference of the front wheel, and it shows the bike speedo to be very close,....maybe 1 mph slow @ 60mph.

Title: Re: Speedo accuracy
Post by Charon on 06/30/10 at 06:43:23

I thought about editing my earlier post, but decided it would be "fairer" to post again. My S40 seems to be within a mile or two at all speeds up to about 65 mph, checked with a GPS (and a stopwatch at 60 mph). The odometer is surprisingly accurate, being off no more than a tenth of a mile in ten miles. This is with the stock tire, and is the reason I replaced the rear with another stock tire.

My 250 Ninja is a little optimistic. It seems to read higher speed than actual, by 4 or 5 mph at 60. Its odometer is also optimistic, by perhaps 6%. Its speedo is driven from the front wheel, so whenever I change the front tire I will see if a larger tire will fit. The Ninja folks seem to think that a larger tire will call for "adjustments" in the fender mountings. Unfortunately, due to some recently-developed neck problems aggravated by the seating position on the Ninja, it may have to go.

Title: Re: Speedo accuracy
Post by justin_o_guy2 on 06/30/10 at 06:46:02

Find a good chiro. YOu need a good neck, bike or no bike. Took me many, many years to get to the "I am no longer in pain daily" spot after landing on the shoulder/side of head at amost 60 mph, but w/o chiropractors, I woulda never made it.

Title: Re: Speedo accuracy
Post by Bigzuk on 06/30/10 at 07:36:07

I just got back from hundreds of miles checking mine with my GPS.

With the 140/90 I started out with the speedo matched the GPS very well.

With the 150/90 I changed to mid trip the speedo is about 1 mph slow for ea. 10 mph.
40 mph speedo = 44 mph GPS
50 mph speedo = 55 mph GPS
60 mph speedo = 66 mph GPS

It didn't take but a few miles to get used to this and adjust my speeds, but then I've ran many miles watching time/speed/mileage charts in off road rallies/enduros with bikes, buggies, & 4x4's and sportscar rallies. ;D ;D

Bigzuk

Title: Re: Speedo accuracy
Post by babyhog on 06/30/10 at 07:56:57

Thanks all. When hubby first commented that I was driving a little fast, he was on his Vulcan, so I didn't think much about it, not knowing its accuracy either. Which one was correct, right? Then with his C90, it still seemed that I was running a little faster than speedo showed. Then he followed me with 2 different vehicles, same result. Then when the guy from work followed me and he commented, that's what prompted my question.

Never paid much attention to the odometer, and I don't ride much Interstate with mile markers, but I do know of an easy straight stretch that would work nicely for these tests. I'll do it.

But I'm still confused with a couple responses. My speedo "seems" to be reading lower than I am actually going. If I go from the stock size to the 140/90, Charon said it will make the speedometer read lower, but Bot said it will make it more accurate. If the speedometer will read lower, isn't that less accurate since its already lower? (Sorry, blonde moment, like daylight savings time.... I can never figure out whether its gonna get darker earlier or later... lol)

Title: Re: Speedo accuracy
Post by Bubba on 06/30/10 at 07:58:53

just days ago I switched to a 140/90 rear tire. Every morning I drive past a radar speed sign. With my stock tire (which was very close to bald) I was consistently 3-4 miles per hour over on my speedo. Now, with the taller tire it appears to be spot on.
That's travelling at roughly 35-40 MPH... I'm happy.

Title: Re: Speedo accuracy
Post by Charon on 06/30/10 at 08:50:47

Odd as it sounds, both Serobot and I could be right. The larger tire will make the speedometer (and odometer) read lower when travelling at the same road speed. If the speedometer started off by reading high, then the resulting lower reading might be more accurate. Note that the speedometer and the odometer, even though housed in the same case and driven by the same cable, are two separate and independent instruments. It is very common for them to have different errors, and actually fairly unusual for them both to be right. The speedometer measures how fast its drive cable is turning, and the odometer counts how many times it has turned.

GPS does not actually measure speed. It measures and calculates its position on the Earth's surface. It repeats the measurement fairly often. It calculates the difference in position between successive measurements, and knowing the times of the measurements, it calculates a speed. Because there is a small error in position measurements, it "filters" its results to smooth the readings. Because a finite amount of time is required to measure, calculate, and display results, a GPS always lags by a short time. GPS gives excellent results when you are "straight and level," but if you are accelerating or braking, or turning sharply, the GPS unit's filtering might give odd results for a moment.

Title: Re: Speedo accuracy
Post by T Mack 1 - FSO on 07/01/10 at 08:17:45

Just about every vehicle I had read faster than I was going. So , if someone is telling you you're going 60 when you thought you were going 57, I would suspect that they should have they car (truck ?) checked too.

Easiest way to check youself is to go on the interstate or highway with mile markers, ride 60 mph and it should take you one minute to go from mile marker to mile marker. If it takes longer than a minute ( 1min 2 sec, 1 min 5 sec etc etc) than your speedo reads Fast (Your 60 is actually less). If it takes you less than a minute ( 59, 58, 57 etc etc ) then your speedo reads fast (your 60 is actually faster and you may want to watch out for speed traps!).

What I normally do when I get a new vehicle (car, truck) is to have someone ride along with a digital watch or stop-watch and we time it for a few miles. My 2004 Chevy Colorado is going 57 when the speedo says 60. 60 mph is about 63 on it's speedo.

I never checked the Savage or my current Sporty this way, but the local town nearby has one of those "Your Speed is" gizmo's and both were withing 1 mph of what it showed. Just don't know how well the gizmo was calibrated.. :-/

Title: Re: Speedo accuracy
Post by verslagen1 on 07/01/10 at 09:10:55

In general, all speedo's are calibrated under actual speed.
Just think about it, 'sorry officer, I didn't know I was speeding. This is a brand new car and the speedo should be accurate' and how many suits will arise from that.