Adventure Bike Rider Forum

My garmin satnav warns you when a camer is coming up and speed limit changes.
Car is +/- 2.5mph under true speed at all speeds.
700 Tenere 10% under at all speeds so 44 is 40 and 77 is 70 ish.
Just about to fix that though

My ktm 1190 also 10% fast, until I changed the ABS rings which read, true to actual speed. It now agrees with the Sat nav at all speeds.

I just realised that I should have said speedos are calibrated by manufacturers to read 10% over, not 10mph ... doh ... early onset dementia, being a young oap lol .

What perplexes me is do the police allow 10% +1 over this reading, or 10% +1 over the true GPS speed?

If doing 50mph on speedo, you can do actually 56mph on speedo before nip,
Or
At 50mph on speedo you're actually doing 45mph GPS, so police may allow 45 + 4.5 = 49.5 .... plus 1mph. So 50.5 mph ?

I guess they use GPS speeds ... I dunno?

OK well as a maths fan I have a question about this.

My old DR650SE came with a set of SuperMoto wheels, the original front was 21" (90/90-21) and the speedo was set for that. The SM front was 17" (120/70-17). Roughly what difference would it make to the speedo as a percentage? I never tried it with satnav, they made the steering feel really weird so after a trial I just left them in the garage.

Use 3.14 x rolling diameter of each tyre.
With the 17" supermoto wheels on the speedo will be overreading by just over 15%.
Calculated by using the rolling diameter of my 21" Himalayan (690mm) and my Wife's 17" MT07 (590mm).

Edit*providing that the speedo reads off the front wheel

garyboy wrote: ↑Fri Aug 05, 2022 1:53 pm
What perplexes me is do the police allow 10% +1 over this reading, or 10% +1 over the true GPS speed?

They use the 'true speed' from a calibrated device and the '10% plus 1 or 2 miles' is only a guideline rather than a law, so theoretically you can still get nicked for travelling 43mph in a 40mph limit.

p.s. the 'true speed' is probably very close to what shows on a good GPS device.

Cheers Scott,
So we Do need to make 2 calculations, if no GPS..
1 to calculate the true speed, and
1 to calculate a hopeful 10% plus one .. if you're feeling lucky [punk] lol

Pondered this the other day, its my understanding that a GPS shows your lateral speed across the ground and doesnt take any vertical component into account so if riding a steep gradient up or down then GPS will show slightly lower reading than actual speed on the road (very small amount unless you're riding down the side ben nevis.

I doubt itll make a practical difference but one of those things you ponder when sat in traffic in a metal box

It's just a simple speed device to get more money out of us ... ok.. it involves Einstein s theory of relativity, the Dopler effect, atomic clocks, earth clocks, orbiting satellites, the spin of the earth, power emitter stations, electricity, the ionosphere, microwave ovens and death rays, .. but what did the Romans ever do for us? Apart from carriages, roads and speed bumps?

The 10% rule purely a guidance and different forces have different rules so dont get fooled like alot do and think its ok. I know a few people who have had tickets for 3mph over in a 50 for instance. Another was let off for doing 85 in a 60 so there is no common approach however it depends on conditions, location, attitude and what the current campaign is I find.

I dont know many that can honestly say they have never gone over the speed limit. Take Miss Daisey I saw the other day which it was impossible to safely pass. She was doing 45mph in a 60 and driving very carefully, however when we reached a 30 she remained at 45 so it was the speed she drove everywhere regardless