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[Field Marshal Dr. Opti]

Some time ago I replaced  215/50/17 tyres with 225/45/17. This saved me about £30 a corner.

However, the total diameter of the new wheel and tyre combination is roughly 2% less than with 215/50 section tyres. The car has, in effect, lower gearing which should improve acceleration

I know this makes the speedo read slightly fast but will this also make the odometer in the trip computer show more miles than the car has actually driven?....making the car appear more economical than it really is.



 

[Mr Skrunts]

Am sure there are some usefull web pages you can type both sizes in to find out.

[Mr Skrunts]

https://tiresize.com/comparison/

[05omegav6]

What is the correct size as per the manual? If both are viable then don't worry about it

[Broomies Mate]

That sort of change wouldn't really matter.  You are semi correct.  The circumference is 2% less, so the odometer will record 2% more mileage.

I certainly wouldn't be concerned about it.

[zirk]

Unless your driving a explod the speedo will be more than +/- 2% out anyway.

[Broomies Mate]

Unless your driving an explod the speedo will be more than +/- 2% out anyway.

Correct, but his speedo is now a further 2% out. 

[zirk]

Unless your driving an explod the speedo will be more than +/- 2% out anyway.

Correct, but his speedo is now a further 2% out. 

Or it may be more accurate, as it could have been out the other way in the first place, the point Im making here is, the 2% Calculated error on the Tyre size is probably irrelevant when compare to the unknown +/-  error within the Speedo, Oddmeter, MID Tolerance's anyway.

I think we are all in agreement here, its not something to worry about.

[Broomies Mate]

Unless your driving an explod the speedo will be more than +/- 2% out anyway.

Correct, but his speedo is now a further 2% out. 

Or it may be more accurate, as it could have been out the other way in the first place, the point Im making here is, the 2% Calculated error on the Tyre size is probably irrelevant when compare to the unknown +/-  error within the Speedo, Oddmeter, MID Tolerance's anyway.

I think we are all in agreement here, its not something to worry about.

Yes, we are all in agreement.

Pedantic point:  Manufacturers calibrate the speedometers.... They have to by law.  They calibrate them to under-read by a small amount, NEVER over-read.  Just sayin'.

[Phil]

Just to clarify, assume you are talking about the Signum?

In which case (for those that don't know) the 225/45/17 is an OEM option to the 215/50/17 and is clearly stated as such in the owners manual

There is a setting via Tech2 on the Vectra C/ Signum where you can alter the tyre group to match the OEM size - 16, 17, 18 & 19" rims with varying diameters are all OEM options on the Vectra C/ Signum - funnily enough the 18" option is also 225/45 profile

The 225 & 215 17s are in the same group, so according to Vauxhall, the manufacturer, there should be no noticeable difference, guessing the ABS sensor would throw up a fault if the rolling circumference was a mile out compared to what it would expect, as you could have 225 on one end and 215 on the other (but obviously not on the same axle)

[05omegav6]

Ok, so as suspected nothing to fret about as both are 'in spec'

[Field Marshal Dr. Opti]

I'm only asking because my car seems quite economical according to the trip computer.


So I did a 'brim test' to see..........not 100% accurate but it should give a better idea.


Trip computer                                   Brim Test


242.8 miles.     

8.3 gallons.                                      38.55 litres (8.47 gallons)

29.0 MPG.                       28.7 MPG 


Who needs a horrible smoky diesel?