Speedo-ODO correlation

[kadesean]

Recently confirmed that my '74 R90/6 speedo is waaay off. We all know these units are off, but mine is off about 10-12mph at 70mph and it gets worse the faster you go. My question is will my odometer/trip meter be off by some corresponding amount? Or could the speedo be wildly off and the trip/odo be reasonably accurate?

I tested the speedo function on a dyno. I suppose I could just go find a fixed distance and check my trip meter. But it's late and I'm into the sauce so maybe someone here will tell me before tomorrow.

[Overdog]

If the speedo is the wrong ratio for your gearing, both the speed and mileage will be off. I had two different speedos that were way off on the speed due to the needle somehow being twisted on it's shaft....I verified how much it was off, dissasembled the speedo & held the shaft & twisted the needle to what it should be. It's now accurate at all speeds.

[kadesean]

I believe instruments and rear diff to be original

W=1,144 with a 34/11 diff

Fixing the instruments are on the list, but not high on the list and the tach doesn't work at all so it'll come first.

[bpeckm]

As said above: IF your speedo has the correct "w" for your final drive, the speedo could be (and probably IS) off by up to 10%.... but the odo should be right on. BMW speedos are notorious for reading high (saying you're going faster than you are), but are basically accurate for odometer/total miles.

[kadesean]

pretty much the answer I was looking for there bpeckm. I have owned a few airheads and this speedo is the most inaccurate one that I've had. I can live with the speedo and continue to feel good about the trip/odo when calculating fuel and trips and such.

[ontic]

the speedo is reading too fast yes?
you could always try pulling the needle off the shaft and changing its position relative to the spring underneath.
First lift the end of the needle over the stop at 0mph to see how far it rests below the zero- then remove the needle up off the shaft and press it back on 10-12mph lower- then lift the end of the needle back over the stop at zero and give it a try. It is sort of like preloading the spring under the speedo face. No promises the speedo will be accurite across the full range of speed, but if it is that far out to begin with it might be a general improvement. What it should do at least is give you a more accurite speed measurement at a given speed- say for example your 70mph.
I did this to my G/S speedo and against my GPS it runs pretty true now.

[Wirespokes]

As bpeckm said, if it's the correct speedo for the final drive, the mileage will be accurate. That is, if no one's been in there and changed the gears.

The odometers are gear driven and if they're working, don't lie.

But I'm of the opinion that airhead odometers are the most likely parts to fail on these bikes. The speedometers and tachs fail too, but that's mostly from either corrosion caused by condensation or leaking lenses, or lack of lube or old and sticky lube. That's a different matter than the odo problem.

As for the speedo accuracy - that's a tricky one. Where the needle reads is an equilibrium of forces - magnetic eddy currents that drive the needle up, friction, and the hair spring that returns the needle to zero. The hair spring is a brass spiral of five or six coils.

I think the linearity depends a lot on the spring forces, and whether it's constant through the range or not. I have no idea if it exerts more force at 60 than 30, and that's something to check out. But I do know that they're not necessarily linear - at 30 it might read 35, at 60 read 62, be right on at 70 and then stay accurate from then on. There are variations - sometimes they're accurate at all speeds, other times only at one speed. You just have to know your gauge and interpret what it tells you.

I think that's one of the reasons they set these things up to read high - at different points in the scale it might read low otherwise.

[DoktorT]

All Interstates and State hiyways have mile markers. Just notice them and compare up to thousands of speedo miles to actual engineered highway miles. Typically just a t-post with a small tin sign with the mile number, set back from the shoulder aways, or sometimes at the Right of Way fence.

In addition all the interstates have 5 miles of speedo check sections with their more prominent markers designed specifically for your purpose somewhere between most cities.

Note as well the big paint markes on the shoulders used by the airplane guys pointing you out to the Gendarmes on the road ahead.

Spec that speedo and odo as you please, if you please.

[frichtie]

I have the same problem on R80G/S. Mine is in kilometers and it too is waaaaay off. It reads like 20 km/h faster.
I'm not gonna try to open the meter a see if I can fix it. All I need is some accurate speedo...
I don't mind replacing with a bicycle speedo (like a Sigma) or some sort of digital gauge.
Question is: What't the best and easiest to find replacement?

Eric