Speedometer Issues

My 1987 VFR700 has a speedo that is waaaaaayyyy off. I'm talking like 20% off. I haven't verified with my GPS yet, but glancing into car windows, seeing 70MPH and I'm reading 85. The bike is stock/original, including tires. Why would this be so far off, is this a common issue, is there a fix for it?

Thanks,

 

Being off to 7-10% probably not a big deal. Maybe the sprockets were changed at some point. There should be a number stamped into the sprockets. If not, count the front and rear. But of course, I don't know how many teeth were on a stock sprocket for that year.

 

Pliskin,
The spedo is front-wheel driven so sprockets won't matter. But just for reference, they are stock.

 

Shows you how much I know about that generation. Or any other for that matter. Doh!

 

Like Pliskin mentioned, I would not concern myself about 10%. You are pushing that though.

Regardless of where you get your readings from, you should expect them to be out of whack. For one reason, maybe not the biggest reason though, on a car or truck, the speed is calculated giving consideration to the number of times the wheel turns in a given distance. Change the tire size, or circumference, the speed reading changes. That is on a flat profile tire.

Now on a bike, your tire does not have a flat profile. For example, when you are cornering, your tire will complete more revolutions per linear foot than if you were going in a straight line, due to you leaning the bike.

So a bikes speedometer is only so accurate. They would be set up to error in your favour to avoid such things as tickets.

 

BTW. IO have read something about speed healers but confess, I know absolutely nothing about them. If you are interested, I am sure you can find something from Google. I would definitely check out the accuracy with your GPS first though. The car you were looking at could easily be wrong. It is amazing the difference in speed when you put on a different profile of tire.

 

Hmm, didn't think about the leaning thing but so far I've been riding fairly straight. I will find a mount for my GPS and hit the freeway. At least I'll know where I'm at. My OCD kicks in when I think of racking up artificial miles on the bike...LOL.