Wow...Speedo way off

So I always wonder why car be passing me when I'm already doing 75mph on the highway.
Saturday for the first time a used a GPS on my 78 z1r.
At 55 ..gps said 46mph
At 70..gps said 63mph
And at 80..gps said 73
So I have to drive at 85 on my speedo to actually be doing 76.
Anyone else have found this to be?

What size tire do you have up front?
Steve

The original 18" for the z1r. What sucks too is that the GPS said 54 miles to distination and the bike recorded 64 miles.

I'm talking about the size of the tire not the rim. I thought the front was 19" like most of them at that time. Ether way it could be a larger tire but now a smaller rim?
Steve

All equipment is original.

spdygon wrote: The original 18" for the z1r. What sucks too is that the GPS said 54 miles to distination and the bike recorded 64 miles.

That's interesting. I can understand the speedometer being inaccurate since it is not directly driven from the cable and sending unit, but I thought the odometer had gearing that is directly connected to the cable coming from the sending unit. Apparently I misunderstood the setup. Ed

spdygon wrote: So I always wonder why car be passing me when I'm already doing 75mph on the highway.
Saturday for the first time a used a GPS on my 78 z1r.
At 55 ..gps said 46mph
At 70..gps said 63mph
And at 80..gps said 73
So I have to drive at 85 on my speedo to actually be doing 76.
Anyone else have found this to be?

Motorcycle speedometers have for a long time been notoriously optimistic.

Am vaguely recalling that Triumph speedometers and maybe BMW speedometers more accurately reflect true road speeds, this being from the days of cable-driven speedometers.

Good Fortune!

So if I do the math on mileage.
64 divided by 54 = 1.19
11000 divided by 1.19 = 9244miles
So in reality my 78 z1r has 9244 miles
Not 11000.

New that 35 years ago! Even before they wore out! Kaw speedos always sucked! at least 10 mph off.
So you get a friend or ? whomever in a decent car, drive with them on a highway, next to each other @55 , @65 etc. go by your rpm's.
At least that's what I did when I drove these cycles on the street 35 + years ago.
Pretty simple. For me anyway.

If you know the circumference of the tire, some calculations can be done.
Also, the number of teeth on the speedo drive gear & on the pinion.

Or it would be handy to know what tire or the tire size. Or the radius of the front wheel/tire --- measure from garage floor to center of axle, with front tire properly aired up & resting on the garage floor; bike vertical; measurement within 1-2mm or 1/16".

Up to 10% error is pretty common for the speedometer, but like 650Ed, I've not heard of an odometer that far off.

I couldn't find my magazines with 1978 Z1R tests to see what they said of the speedo and gave up looking for now, but as mentioned a lot of them were optomistic, but the normal for kawasaki was about 4-5mph at 60mph. The odometer thing I don't get at all as your figure shows close to 20% off. I've never heard of this, and the normal was 3-4%. I rode my 79 KZ1000MKII 10,800 indicated miles on my last big trip, and actually was 10,300 or so according to my calculated miles using maps in the old days before GPS.