how fast am I going

[Isamemon]

yes another speedo question

I know that this 85 rt I am working on is not accurate. ( not my bike)

So its an 85 R80 rt. and I weigh just under 200lbs

at 4000 rpm the speedo says I am doing about 70

the local radar at the sie of the road siad I was doing 23 when my speedo was just barely under the 30 mph marker

havent had a way to check otherwise, and I dont want our local "friend" asking me
.........son......you know how fast you were going

when I know where a speedo is working, then I can mentally adjust form there, and dont need to mount a gps or other unit


my 78 R80 is within about 5mph at 70, but I cant ride them both at the same time to see :scratch

[20774]

Use Anton's chart to get a handle on what the speed "should" be. At least you can make a comparison. And remember that the ratio of the speedometer needs to be correct for the final drive, otherwise it's apples-oranges.

http://www.largiader.com/articles/gearing/

There's also running down the interstate and timing yourself between mile markers, say 5 miles worth. Then do the math...

[Isamemon]

yes I will be on the interstate today
I looked at the link, does it mean at 5000 rpm wiht a 336 gear that I should be at 70mph
if so , my speedo is not far off

thanks for the link

[rpeckham136133]

If you ride the interstate, it is easy enough to time yourself. Depending on traffic, if you can safely go 60mph (around here you would cause a traffic jam!) the math is real easy: one mile in one minute.

The point is go a STEADY mph per your bike's speedo, for a number of miles, while timing yourself. Then figure out your AVERAGE TIME PER MILE. The formula to convert that to miles per hour is 60/time= speed. For example let's say it actually took you 65 seconds PER MILE. You convert that to algebraic way of saying it and you get 1.08 minutes per mile 65/60). Next, plug in that time into the formula - 60/1.08=55.5 i.e. you were going 55.5 mph.

It's a real life application of those great high school math problems!

Remember that the correction is a percentage, so your speedometer will be pretty much constantly "off" by the same percentage. In other words, if you figure it is 10 percent off, it will be off by about 5mph at 50, and 10mph at 100mph.

By the way, it is not at all unusual to be 10% off!

T

[rinty]

Isamemon:

Borrow a GPS and stick it in a clear top tank bag; then you'll know exactly how fast you're going. Many BMW speedometers are way off, and, as you say, may result in unwanted attention from the constabulary.

I just bought a U.S. spec sports car with a dual speedometer, but I can't read the inner kilometre scale when wearing my polarized sunglasses. So I just place a Garmin Nuvi on the dash (on a friction gravity mount) and use its digital speed read out as my speedometer. This saves me having to spend $400 converting the speedo to a readable metric one. And when we tour in the U.S., I can switch it to miles, for back up.

If I had another BMW bike, the first thing I would do is put a removable Garmin on it. Those speedos are useless.

Rinty

[shire2000]

Normally the Airhead Speedo's will read faster than you actually going. They were just a little optomistic? Over the years, the ones that only went to 80MPH were considered the most accurate. That being said, we are talking about bikes that are 25+ years old. The Speedos tend to have issues.

The GPS or a bicycle speedo are the best way to get an accurate reading and you can guage how accurate your speedo is from that. If you really want to spend money, you can have it rebuilt by someone such as Palo Alto, but eventually it will go haywire again.

[Isamemon]

yesterday, I did the old hiway trick, and at at what appears to be a true 60mph my speedo is reading just under 70.
I had lots of clear hiway miles ahead of me, and ran the bike for a couple of miles at speedo said 60, then again at 65 and then again at 70. at between 65 and 70 on the speedo took 1 minute to cover 1 mile. I actually did it for several miles at each speed.
Plus it gave me a good excuse to ride :dance

what is really interesting is my 78 is just the oppisite. If it says 70 it is dong about 75 according to a friend that was in a car behind me ( did that one months ago) and there car is accurate according to drive by radar

[rinty]

The GPS...[is] the best way to get an accurate reading....shire2000

And if you're riding at a speed that's close to being at the edge of the local constabulary's tolerance zone, you don't want to accidently stray out of it. And if your bike has cruise or throttle control of some sort, you can dial in that exact speed.

I just brought a "new" car home from Salt Lake City, and drove the interstate the whole way. The limit was 75, and I just tapped the speed up an indicated 81 on the Nuvi with the cruise control, then sat back and enjoyed the ride.

Rinty