Relevent Speeds 15-16 Inch Wheels

[biffstergray]

As I have changed from 15's to 16's, when my speedo reads 30mph,will my actual speed be above or below 30mph, or will there not be much change?

[Woody27]

I wouldn't have thought it would be a huge difference! If my basic maths calculations in my head are right the speed should be slightly underestimated as the larger wheel takes longer to rotate by the same amount (thats just me doing some basic maths in my head so may be wrong!) Out of interest, if it is out by a lot? can the computer be reconfigured to record at the new diameter?

[selfbuild]

http://www.rochfordtyres.co.uk/TyreCalc.asp

[Clueless]

Bigger wheels = as much as 10% out on the speedo.

Drive on a motorway beside a friend and drive a 70 mph then ask your friend what his car is reading at. Know the difference and keep it in mind.

Or

invest in Apexi RSM and set it up for your wheel size

[chrisb]

http://www.carbibles.com/tyre_bible.html

Try this!Three quarters of the way down the page!Presumably you would have had lower profile tyres on your 16's so tthe difference isnt going to be much!

The rolling radius on my 16's was actualy less than the standard 15's!

CB

[clarky]

Ok - the standard t-sport rims are 185,55,R15s and the usual 16s are 195,45,R16s - which means the rolling diameter is 1.1% different.

According to my satnav my standards (have them on for winter) are just under accurate whereas my 16s are out a little more (slower actuall than the speedo)

Hope this helps mate

[Woody27]

Wow i was right :D is there anyway to reconfigure the original speedo though?

[Viper_7]

Aye, the speedo can be reconfigured...and it's not just this that's mis-representative...but the odometer will be out too! One for Mr T, I'm afraid though.

As said though..."usually" larger rims also means lower profile tyres..so the overall diameter isn't much different.

Knowing driving with a defective speedo ( which it will be) will not get you a chrimbo card from pc plod though!

( oh I'm getting old)

Vipes

[chrisb]

Here you go

185/55/r15 Circumference = 1836.26mm

195/45/r16 Circumference = 1828.09mm

The r16s have a smaller circumference of -0.44%

So assuming your speedo is accurate when your speedo reads 70mph you are travelling at 69.69 mph!

So its realy nothing to worry about!

CB

[Woody27]

Yeah isnt there already a certain inaccuracy in them so its nothing more then usual really!

[chrisb]

Nothing worth having your speedo recalibrated for anyway!

Tyre wear would have more of an affect than that!

Speedos are never very accurate anyway!

CB

[Blaze]

My speedo is out, depends how fast im going to what speed, if i travel at 70 then im actually about 65/66 and i think mine are 15"

[Yaris WRC]

how much would toyota charge to get it calibrated? and are you sure they will do it?

[ronnyboy696]

i dont think it would change, would it, because you have a rotor and magnetic sensor, sensing how fast the wheel is turning.

so whatever the size wheel, it will still read the same speed.

is that right?

ill have to find out when i go back to wotk, because we allways get cars in with bigger wheels

[Woody27]

Thats what i said when this was discussed before ronny but apparently its wrong as its calibrated to be more accurate by keeping the wheel size as a standard! As you need to measure the rolling of the outside of the wheels contact with the road (the rolling diameter) [this is the miles in mph if you like :P] So the speed is measured at a place on the shaft that isnt affected by Wheel size and then maths is done on it correct to your wheel size to get the MPH speed! (basically the actual reading isnt done on the wheel like you said but its the maths afterwords between the reading and your read out that has the maths specific to that rolling size) Apparently this is one of the most accurate ways to get speed readings cheaply as its how done on most (if not all cars). Does seem a bit silly i suppose but i guess they dont expect you to change the wheels around that much!

How about if you opt for the larger wheel option at a garage when buying the car? do they reconfigure it or just forget about it? (not sure if you can chose a larger wheel from the garage for a yaris but i meant in general)

[Flat Pack]

Basically your speedo works by measuring the number of revolutions of the wheel per second and then converts that into your speed.

If your combination of wheel and tyre has a larger rolling diameter than standard, then you'll travel further forwards when the wheel rotates once. This will make a previously accurate speedo underread (it'll say 30 when you're actually doing 32 or something).

Between 30 and 70mph your speedo is allowed to overread by up to 10%, but it must not underread. Most speedos overread by a fair whack as standard so as to avoid the 'must not underread' problem, so you'll probably be fine. Best to check using GPS or a known good speedo though.