place your bets!

[BBROWN1664]

OK.

The law is that speedos are allowed to over read but are not allowed to under read. The maximum over read allowed is 10%. Speedos can never be 100% accurate.

On a car its easy to see why. A brand new tyre on the car has around 8mm of tread. A worn out one has 1.6mm. The difference in the rolling distance of the two is about 2%. This means physically travelling next to each other, two identical new cars with the new and worn out tyres will show different speeds of about 2%. Add in other factors like going round corners which makes one wheel turn faster than the other and it gets very confusing.

Now look at bikes. Which point on the tyre will the speed be accurate. When you are totally upright going in a straight line, or leant over going round a bend where you reduce the rolling radius even more?

Sat navs are more accurate but only in a straight line, on level ground, at a constant speed. Change any of the points here and the SATNAV is play catchup as it only calculates your speed between two points and assumes you are going at a steady speed in a straight line etc.

[fazersharp]

OK.

The law is that speedos are allowed to over read but are not allowed to under read. The maximum over read allowed is 10%. Speedos can never be 100% accurate.

On a car its easy to see why. A brand new tyre on the car has around 8mm of tread. A worn out one has 1.6mm. The difference in the rolling distance of the two is about 2%. This means physically travelling next to each other, two identical new cars with the new and worn out tyres will show different speeds of about 2%. Add in other factors like going round corners which makes one wheel turn faster than the other and it gets very confusing.

Now look at bikes. Which point on the tyre will the speed be accurate. When you are totally upright going in a straight line, or leant over going round a bend where you reduce the rolling radius even more?

Sat navs are more accurate but only in a straight line, on level ground, at a constant speed. Change any of the points here and the SATNAV is play catchup as it only calculates your speed between two points and assumes you are going at a steady speed in a straight line etc.

So how does all that effect the mileage read out - total have I actually done more or less miles than when my clock says

[darrsi]

OK.

The law is that speedos are allowed to over read but are not allowed to under read. The maximum over read allowed is 10%. Speedos can never be 100% accurate.

On a car its easy to see why. A brand new tyre on the car has around 8mm of tread. A worn out one has 1.6mm. The difference in the rolling distance of the two is about 2%. This means physically travelling next to each other, two identical new cars with the new and worn out tyres will show different speeds of about 2%. Add in other factors like going round corners which makes one wheel turn faster than the other and it gets very confusing.

Now look at bikes. Which point on the tyre will the speed be accurate. When you are totally upright going in a straight line, or leant over going round a bend where you reduce the rolling radius even more?

Sat navs are more accurate but only in a straight line, on level ground, at a constant speed. Change any of the points here and the SATNAV is play catchup as it only calculates your speed between two points and assumes you are going at a steady speed in a straight line etc.

So how does all that effect the mileage read out - total have I actually done more or less miles than when my clock says

You've only actually done 100 miles in 19 years 

[darrsi]

Is a sat nav more reliable than a speedo though?

I reckon it is.

I've read before that it is against laws/regulations for speedos to read less than the actual speed and therefore manufacturers 'play it safe'.

When my car speedo reads 30mph the sat-nav reads 28mph.

At 40 on the speedo it is 37mph on the sat-nav.

At 70 on the speedo it is 65mph on the sat-nav.

Just been having a look about and sure enough it mentions everywhere that manufacturers deliberately alter the speedo by "approximately" 8% to allow for certain things, ie: tyre wear, using tyres that started with quite high tread, which would eventually result in the wheel spinning quicker when just above the legal limit as the reading would change from when the tyre was new.
It's done to keep you under the limit by default because of the percentage discrepancy.


So on those maths, we should be getting 150mph, at a push, if the bike was new. I'd imagine that ship has sailed now, due to the age of them though, and wear and tear.

So David Ike is right when he says we are all living in an altered reality, what else is not real.

No, he's just a nutter!

[BBROWN1664]

So how does all that effect the mileage read out - total have I actually done more or less miles than when my clock says

In reality, you have probably done 5-10% less than shown in the odometer. The odo gets its feed from the speedo.

[Bretty]

So what should the service interval be then?! 

[darrsi]

So what should the service interval be then?! 

For what?

[BBROWN1664]

So what should the service interval be then?! 

As far as everyone cares, the odometer is correct and servicing is based on the reading there . Reality is a bit different as most bikers don't do manufacturer mileage based servicing, they do it more often so the odo reading is pointless.

[fazersharp]

So how does all that effect the mileage read out - total have I actually done more or less miles than when my clock says

In reality, you have probably done 5-10% less than shown in the odometer. The odo gets its feed from the speedo.

Great - I dont do enough and now your telling me I have done even less !

[Bretty]

So what should the service interval be then?! 

That was a silly irrelevant joke and reference to the fact if the speedo is out 10%, you might be doing a 3000mile service, when you had only done 2700. 


Anyway, I service my motorbike every 15,000 miles whether it needs it or not! :-p