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Speedo Reads High

3K views 13 replies 8 participants last post by  ealanm 
#1 ·
Everyone knows that factory speedometers typically read on the high side - mostly for liability. So I mounted a digital speedometer on my Super III as a comparison. After calibrating the digital speedo, down to the 100th of a mile, I was shocked by how far off the factory one was. When I'm going 55 mph (actual speed - according to the digital) the factory speedo reads about 65 mph. That's about 18% high!
Has anyone else verified their speedometer and was yours this far off?
Thanks.
 
#2 ·
I have utilized my wife following in a vehicle to determine my speedometer is off by about 12%, meaning it reads higher than the actual speed. I have a 2006 Thruxton. I just had to accept the fact that it's off and constantly run calculations in my head to adjust my speed when I'm in certain areas.
Is there a fix?
 
#3 ·
If you're using another production vehicle as your reference, that vehicle's speedo is probably also reading a bit high, so the 12% difference between your Thruxton and your wife's vehicle is probably closer to the 18% difference that I'm seeing when compared to actual.
 
#5 ·
That's really egregiously off, are you sure the digital speedo is correctly calibrated?!?! My Trident was a little under 10% off, IIRC, or maybe just under 9%? It's been a while... My Sprint RS says 107 on the speedo when the GPS says 100.

Cheers,
-Kit
 
#6 ·
Yes, I've taken quite a bit of time to calibrate the digital device. I measured it against the highway department's 4-mile marked section and my device was reading 4.04 miles...so I reprogrammed the wheel diameter reducing it by 1% and now it's spot on - to within one 100th of a mile.
Perhaps I should speed past a highway patrol car, and then compare my speedo reading to the ticket...
 
#8 ·
<...>
Perhaps I should speed past a highway patrol car, and then compare my speedo reading to the ticket...
A GPS would probably be cheaper! :eek: Not to mention, are you sure you trust the PD's equipment to be properly calibrated? :D

<...>
What I've not thought of checking on my bike is the odometer. If the speedo is off by 10%, is the odo also off? If it is, that just dropped my fuel mileage! Yikes!
Generally speaking, at least with recent Hinckley bikes, the speedo reads high but the odo is accurate. It's a bit of a problem for people on the newer bikes who want to use a speedo healer: speedo and odo are read from the same input, so making the speedo correct makes the odo incorrect.

By the way, wasn't there a thread a while back talking about speedometer accuracy and using add-on electronic "computers" to check actual speed? That might be useful.
I believe that's exactly what Mozart 2 has done.

Cheers,
-Kit
 
#7 · (Edited)
According to my GPS unit, both my cars' speedometers are within 1 kph of the GPS's indicated speed. I believe that most North American market (4 wheeled) vehicles tend to have reasonably accurate speedometers (This has held true for several other cars I've owned - never tried checking a truck) so I think doublecafe's test should have been reasonably accurate. My Sprint 900's speedometer seems to be about 10% optimistic from the tests I've run checking it. 18% sounds like there might be another problem lurking about (Speedometer, it's drive gear?). What I've not thought of checking on my bike is the odometer. If the speedo is off by 10%, is the odo also off? If it is, that just dropped my fuel mileage! Yikes!

By the way, wasn't there a thread a while back talking about speedometer accuracy and using add-on electronic "computers" to check actual speed? That might be useful.
 
#9 ·
Using two different GPS units, my car read about 2 MPH faster at 70 than the GPS says we’re doing, and I’ve verified that with radar, so I think my GPS unit is pretty close ;)

Using my GPS my 04 Thruxton shows 5 MPH off at 55, 7 MPH off at 70, and almost 12 MPH off at just over a Ton. Now I’m running taller gearing on my bike, not that that should make a difference. Wish there was a way to calibrate it since we’re all racking up more mileage on our bikes (on the odo) than we actually are going.
 
#11 ·
I'm not sure I've ever had a vehicle with stock speedo that I really trusted.

Every time I've compared the speedo against 1 or more GPS's I've been between 5 and 10% high on the speedo. All of my Triumphs have been near 10% high. Heck, even my '08 BMW 335 reads a bit high - though closer than the bikes.

I've never understood this - if they can constantly make it within a few percent, but always fast, why not just re-adjust the base line? Are they simply worried about somebody fitting a slightly larger tire or something?

I'm having a SpinalTap flash back. 'but this one goes to 11...'
 
#12 ·
Okay, I have to admit as a mechanical engineer I should have known better...
If you're trying to check the accuracy of a gauge reading, the larger the reading the better precision you'll have. A 5% error in the gauge reading at 40 mph is only 2 mph and on a gauge that's marked every 5 mph, 2 mph is pushing it for precision...especially when a slight change in angle-of-sight makes a difference of several mph. So...if you take the reading up to say 140 mph, now a 5% error in reading is suddenly readable (7 mph). In other words, the gauge is more precise at the higher reading.
So...faster I did go!:D
I also increased the distance over which I was calibrating the odometer. Previously I had calibrated it over a 4-mile distance. Once again, my reading would have to be off by a very large percentage just to be visible over such a short distance.
So...I took a long road trip and recalibrated accordingly. Now My factory speedometer is reading about 10% above the digital reading which seems to be in line with what you guys were saying all along.
KitNYC was right, my original 18% overage reading was way off. Sorry for waving the caution flag too quickly.:eek:
 
#13 ·
My understanding is that 8% high speedometer readings are pretty standard. My Ducati's manual explains that the speedo reads 8% high but the factory data logger is accurate!
 
#14 ·
Shortly after I got my 95 Speed Triple I started to notice that traffic always seemed faster than when I drove my car. I attributed it to riding the bike at different times of the day than when I usually drive my car. Where I live there's a huge difference between, say, morning go-to-work traffic and evening or weekend traffic -- as much as 15 mph mean traffic speed. It had occurred to me that the bike's speedo might read high, but I never checked it. Think I'll get out the GPS next time and see how it compares.
 
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