Quick Q. - How does tyre size effect the speedo?

[Gibbo]

Circumference is lower, so for every revolution of the wheels of the car travels less distance. In your case Gibbo, when the speedo reads 70 you'll actually be doing ~65.2mph.

***edit***

I used the calculator here - http://www.carbibles.com/tyre_bible.html - since I couldn't bothered at this time on a Sunday to do any arithmetic.

So by basically fitting lower profiles I may think my car is quicker as it will hit an indicated 70mph quicker and over a shorter distance but in reality my real speed is only 65mph.

So fitting lower profile tyres makes the speedometer read higher then, is that right?

 

[JRS]

So fitting lower profile tyres makes the speedometer read higher then, is that right?

Yep. If you decrease the rolling radius of the wheel+tyre by fitting lower profile tyres to the same size wheel, your speedo will read faster than your actual road speed.

 

[Liverpool-Lad]

No, your speedo will lead LOWER

 

[JRS]

No, your speedo will lead LOWER

Gibbo's old tyre size - 235/55/R17
Rolling Radius - 345.15mm
Circumference - 2168.64mm

Gibbo's new tyre size - 235/45/R17
Rolling Radius - 321.65mm
Circumference - 2020.98mm

Difference - 147.66mm, or 6.81%

So when his speedo reads 70mph, he'll be doing 65.23mph.

Did the maths myself this time, and got the same answer as Car Bibles.

 

[Gibbo]

Hi there

So when I say hit 60mph indicated now with the 235/45/17 tyres on that indicated would have been more like 56mph with the 235/55/17 tyres on?

Is that right?

 

[Scarfacé]

Smaller profile = smaller radius = smaller circumference.


So for each rotation of the axle you will be covering less distance.


So compared with a larger circumference, yes the speedo will think you are travelling greater distance than you are.


Really simple if you take about 5 seconds thinking about it.

Speedometer reading with non-stock tyre is 6.8% too fast.
When your speedo reads 60mph, you are accutally travelling 55.9mph.

 

[JRS]

Hi there

So when I say hit 60mph indicated now with the 235/45/17 tyres on that indicated would have been more like 56mph with the 235/55/17 tyres on?

Is that right?

Yep. 55.914mph to be reasonably exact.

***edit***

Your acceleration will have improved by a small amount with the new tyres, top speed will have been reduced ever so slightly. Also, your mileage reading will be out as the speedo is reading slightly wrong.

 

[Liverpool-Lad]

Correctamundo.