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Mark
Mark, Peugeot Certified Technician
Category: Peugeot
Satisfied Customers: 1504
Experience:  Fully qualified technician and MOT tester.
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Sir, I have an Elddis motorhome on a boxer base. The

Customer Question

Sir,
I have an Elddis motorhome on a boxer base. The vehicle was new in April of this year.
Engine is a 2.2, 110bhp and has 15'' wheels.
Registration No is S777 APH
My question is the vehicle speedo seems to read 13.33 % fast at 30 mph therefore should the wheels be 16''?
I have concluded by simple maths i.e. wheel plus tyre circumference if the wheels were 16'' the speedo would read correct. Also I am of a mind that if the wheels were bigger the vehicle would perform better because at the moment it is seriously sluggish and underpowered. Frankly I have never seen a Fiat/Peugeot with 15'' wheels.
Regards
G HILL
Submitted: 2 years ago.
Category: Peugeot
Expert:  Mark replied 2 years ago.
Mark :

Hello, my name is ***** ***** I will try help you with this.

Mark :

All speedometers have an inbuilt tolerance of 10% =- 2mph fast.

Mark :

This means when you are showing thirty on a speedo, your actual speed can be 30 -10%(3) =- 2. So your speed can be anywhere between 25 and 29mph.

Mark :

It can never read higher than your actual speed. If it did it would be faulty and illegal.

Mark :

If you also fit larger wheels then you have to take into account the tyre profile. Suppose you currently have 225/60/15 and you fit 225/60/16 then top speed might be marginally increased but the torque would be reduced. This means acceleration would be worse.

Mark :

It would also affect the calibration of the speedo and could make it illegal.

Mark :

I assume you are reading the speed from a satnav at thirty and your speedo is showing about 35mph. This is normal.

Mark :

It also actually protects you from ever getting a speeding ticket.

Mark :

If you use the speedo to drive at thirty then you can never be going faster than thirty.

Mark :

The same applies on motorways.

Mark :

You can never be going faster than the speed indicated on the speedo.

Mark :

If you rely on a satnav for speed indication then it is possible you could get a speeding ticket as satnavs don't have this factoring correction.

Mark :

It could also land you with a ticket where they use average speed cameras over a set distance.

Mark :

My advice would be to leave well alone.

Mark :

If you have another vehicle you can check this with the satnav and you will see that it too will have some margin for error.

Mark :

Providing the margin is within the prescribed band then the manufacturer will not accept it has a fault.