drsrst2zvh

right since ive put 17's on my rst ive felt as if the speedo must be out so ive spent the last half hour working out how to convert my reading on the speedo to my actual speed.
I think ive done it correctly as my math skills are normally pretty hot

Diameter = 2πr

17” wheel:
40mm = 1.574 inches
17+1.574= 18.574
18.574 / 2 = 9.287 = r
2 x π x 9.287 = 58.352 = Radius
1mile = 63,360 inches
In order to travel 1mile must do 1085.824 revolutions

15” Wheel:
50mm = 1.969 inches
15 + 1.969 = 16.969
16.969 / 2 = 8.4845 = r
2 x π x 8.4845 = 53.310 = Radius
In order to travel 1 mile must do 1189.083 revolutions

1189.083 – 1085.824 = 103.259 less revolutions to reach the same speed
15” wheel at 60 mph is travelling at 1189.083rpm
17” wheel at 60 mph is travelling at 1085.824 rpm
Speedo speed / 1085.824 * 1189.083 = Actual speed.
1189.083 – 1085.824 = 103.259
103.259 / 1085.824 * 100 = 9.51
9.51% increase to speed shown on speedo.
100mph * 1.0951 = 109.51mph

Let me know what you think?

jonnyboy

http://www.miata.net/garage/tirecalc.html
have a blast of this,mine is now 1.9% slow

dojj

iTrader: (3)

running 215/45/17's and a 3.14:1 diff instead of a 3.92:1 diff and 195/60/14's the speedo was reading 25 and i was doing close to 40

cossie4i+

iTrader: (15)

It will under read with bigger wheels

Steve

carpetbloke

Is it possible to adjust / calibrate this to read the speed correctly?
Mine is fitted with 17" wheels (that`s how it came from Ford) and it seems to read higher than the actual speed.
I`m under warranty so can go to the dealer if required.
Thanks.

rich123

iTrader: (3)

just get a sat nav thats what i use these days,i find that nearly all cars are out by around 5 mph,if ya doin 30mph your realy doin around 25mph and thats in all the cars/vans i drive

Lowson

Regarding your first line of calcs:

Circumference = 2πr, not "Diameter = 2πr"

In your calcs too, your "Radius" should be circumference.

S1

http://www.speedycables.co.uk/

carpetbloke

I could do with curing the error.
As ford fitted the wheels surely if a recalibration is needed they should have done it.
The cars a Ford Direct 59 plate I just want to know what i`m talking about b4 I go back 2 the dealer unless it`s an easy fix.
I was looking at the calibration of the MPG trip computer and thought it could be the same idea.
Thanks for the replies.

jaiscossie

I have 17" wheels fitted instead of 15" and when driving along at 70mph on the speedo if i check againt my garmain sat nav this says 68mph so not bad.

Oranoco

I found the fitment of 17's on my S2 brough the speedo into better accuracy than with the 15's

squizxr

On all of my mk1 fiestas im running 13's with 175/50 profile tyres!! My speedo says 80 and the Sat Nav reads 68

NoRubNoDub

Your out the tyre isn't 40mm it's 40% of the width of the tire
so a 215/40 r17 is actualy 86mm tall
so it's 43mm of tire

isn't as far out as I thought it would be