Hi,

If I'm correct (does happen sometimes ) then for a tyre that's say, 205/55, the sidewall is 55% of the width, meaning that the overall diameter of the tyre (forget tread depth for now) is :=

2x(205x0.55) + rim diameter. For a 16" wheel this is 406.4mm meaning that the overall diameter of the wheel is 631.9mm

and using the same formula a 205/45/r16 wheel is 590.9mm diameter.

Assuming that this is all correct, how much difference would it make to my speedo reading? As far as I can tell, I would be overreading by 6.4% (i.e. going slower in reality than indicated).

Is this correct? And is approx 6% a significant difference? After all, at an indicated 100mph I'd "only" be doing 94mph!

I've been offered some wheels which are 205/45R16 as opposed to my "normal" 205/55R16

Thanks,

Martin

Looks about right, but for an easier way use a tyre calculator

Plus tbh most speedos I've known have always been a little bit slower than true anyway (according to gps).

That's correct, but it's illegal for a speedo to over-read by more than 10%, so your new tyres would make your speedo illegal if it presently over-reads by more than 4%.

Edited 27 September, 201114 yr by KenONeill

Speedos don't under read, they over read - i.e. if my speedo says I'm doing 30mph in reality I'm doing 28, if the speedo says I'm doing 65mph, then in reality I'm doing 60ish.

I thought that they weren't allowed to under read i.e. indicate a slower speed than actual?

I thought that they weren't allowed to under read i.e. indicate a slower speed than actual?

True but only VOSA I think can test them. I've seen speedos be over 10mph out at 70mph!!!

Also satnavs don't always tell the truth, I've had a satnav on the go with a fully calibrated VASCAR system and the satnav some times said it was faster and then said it was slower than the actual speed.

For sat nav to be accurate it needs to calculate your speed on a completely level and straight road.

True but only VOSA I think can test them. I've seen speedos be over 10mph out at 70mph!!!

Also satnavs don't always tell the truth, I've had a satnav on the go with a fully calibrated VASCAR system and the satnav some times said it was faster and then said it was slower than the actual speed.

It wasn't actually a pratnav unit, but I've had a GPS reporting 12 satellites, and a sink rate of 20m/s. This whilst it was sat on a triangulation pillar on top of a 300' high plug of Lewisian Gneiss. Does anyone still think that pratnavs are always accurate?

It wasn't actually a pratnav unit, but I've had a GPS reporting 12 satellites, and a sink rate of 20m/s. This whilst it was sat on a triangulation pillar on top of a 300' high plug of Lewisian Gneiss. Does anyone still think that pratnavs are always accurate?

Sorry, I have no idea what that means!

Sorry, I have no idea what that means!

Pratnav - GPS(qv)-based device used by prats to replace maps. Notorious for directing them to make illegal and/or impossible manoevres.

GPS - Global Positioning System receiver.

reporting 12 satellites - number of GPS satellites which the receiver is receiving a signal from, and using to fix its present position. 12 is "a lot" in this context; IIRC it's actually the maximum number possible.

sink rate - speed of movement straight downwards.

Triangulation pillar - accurately (conventionally) surveyed point used by the OS in map-making. Also often used to check the calibration of a GPS.

plug - In context, a hill made entirely of a certain type of rock.

Lewisian Gneiss - One of the oldest, hardest types of rock on the planet. Other than due to plate tectonics (typical speed of movement about 5cm per year) this type of rock just plain does not move.

Speedos don't under read, they over read - i.e. if my speedo says I'm doing 30mph in reality I'm doing 28, if the speedo says I'm doing 65mph, then in reality I'm doing 60ish.

I thought that they weren't allowed to under read i.e. indicate a slower speed than actual?

I just put figures into the Tyre calculator on Tyrebibles website http://www.carbibles.com/tyre_bible_pg4.html#ixzz1ZH8gA8oU

Difference in circumference: -123mm. (41mm in diameter) or 6.49% smaller. Tyre websites recommend the max change is +/- 2.5%.

So when your speedo reads 70mph, you're actually travelling at 65.46mph.

Edited 28 September, 201114 yr by Jim H