Converted My 86 VFR700 to Sprocket Driven Speedo

Due to my front end swap I haven't had a working speedo or odometer in a while. I really miss the odometer, It's like having a really accurate gas level guage.

Anyway, I had a second sprocket cover laying around and I already bought a 90-93 speedo drive and cable on ebay months ago for $25 total as well as a new white drive gear for $9.

I had some free time so I figured I'd get this project done. I decided to space out the speedo drive so I could run the clutch line behind it and make things look nice and factory. I bought a piece of 1/2" aluminum for $5 and cut it up to shape. I scalloped the back to match the curve of the sprocket cover. I did a few mock ups, a lot of cutting grinding, filing, drilling, tapping, etc... and had a finished mount for the speedo drive. I had a local shop weld the mount to the sprocket cover for $30 then painted the whole thing. I cut the drive gear in half and extended it about 5/8". Total cost was $70.

 

Everything works great, but my speedo is not accurate. It reads quite a bit too fast. According to Iphone GPS app, which seems to match traffic really well, at 60mph, the speedo reads 80mph.

My gearing and tire size matches the 90-93 bikes really closely.
1990-93 - 16/43 sprockets and a rear 25" tire
My 1986 - 16/44 sprockets and a rear 25.1" tire

Maybe the dash speedometers are internally geared differently between the 86 and 90 bikes?

Other than installing a much smaller rear sprocket, do you guys know of any way to improve the accuracy?

 

About the only thing I can think of is to put the 90-93 speedometer on your bike and hope that would work.

 

No chance of driving speedo off the Kawi front end?

 

Install an aftermarket digital gauge that uses a magnetic pickup mounted to the front rotor

 

The Kawasaki drive isn't even close to matching the Honda cable. Plus, I don't like the look of that big cable.

Back in the day you could buy little gear boxes that went inline off your car or truck transmission and it had swappable gears in it to change the input output ratio. I looked everywhere and it's like they never existed.

 

No way I want to do that. Sorry. I want the oem speedo working properly.

 

The 90-93 speedo's doesn't look anything like my square one with the odometer reset through the lens. I'll do some research on it, but it may still be inaccurate.

I'm hoping to find a tiny gear reduction box I can install inline with the cable.

 

Do a search for something called a "speedometer ratio adapter". There still some shops that specialize in speedometers.

 

I took another long ride and used both my gps speedometer apps and now they are saying something a little different. They show 60 mph and my speedo is reading about 71 mph. That's better, but it's hard to trust them. I might travel a certain distance and see how close my odometer is.

If the speedo reads fast, that means the odometer reading is greater than actual, correct?

From a topped up tank, my fuel light came on at 125 miles, I needed to switch to reserve at 136 miles. My miles per gallon were 37.3. That sounds about right to me. What do you guys think?

 

You're mpg was based on the odo recorded mileage or the GPS? Your speedo/odo is reading about 18% higher than the GPS from what you posted, so if the 37.3mpg is a bike based calc, your actual mileage is 18% less, 31.5.

 

I did an 82 mile run last night, but the gps that was tracking my distance stopped halfway so I didn't get that data. I only had the bikes odometer to go by.

Now keep in mind that oems design the speedo to read a certain percentage higher than actual for liability reasons, but the odometers are more accurate. So 18% too high in speed is not 18% too high in mileage. The odometer is probably closer to 12% too high.

I think I'll do an exact 20 miles on the bikes odometer and then compare that with the gps distance. I'd rather the odometer be exact than the speedo.

 

I did some odometer testing with my phone gps distance app.

17 miles on odometer was 15 on gps.
55 miles on odometer was 48.28 on gps.

That's a consistent 14% difference on odometer readings.

The gps speedometer might not be all that accurate, but seems to show a 18% difference.

 

I made a thread on swapping to a later model vfr dash. The 94-97 dash looks pretty nice and it has an electric speedo. I could easily swap my new speedo drive for the electric version and buy a speedo healer and calibrate it how ever I like. It would be cool having a fuel level gauge as well. My stock dash has a few small cracks on each side and is looking a bit ragged.