Tyre sizes - calculations and compatibility?!?

[andywood]

OK, so been doing some number crunching thanks to this:

 m http://www.etyres.co.uk/tyre-size-calculator.htm m

...am planning a change to 225/40/16 on the back from the stock 215/45/16, this will result in a 2.3% reduction in effective diameter which will roughly equate to a 7.2% reduction in distance covered every wheel revolution   s:shock: :shock: s:shock:  

Now, this is where I am trying to get my head straight (it is really hurting after last night!   s:( s:(  ), by doing this change in tyre size I am going to be over-reading on the speedo by 7.2% plus whatever it was already over-reading as standard.
Not a major problem as safer than under-reading with all the Gatso's around, but it would be nice to keep the reading as close to standard as possible. Tyre size limitations seem to be that you pay the money for going with stock sizes or switch to the alternative Falken or Toyo sizes and take this slight speedo hit.

Is it just me who is sad enough to think about this or has anybody else considered or checked this previously???

Should I really be bothered?   s:? :? s:?

[Tem]

7.2% reduction in distance

I am going to be over-reading on the speedo by 7.2% plus whatever it was already over-reading as standard.

Yes. Say you're speedo is showing 100mph. If you change the tyres and make the speedo show 100mph again, you will travel 7.2% slower than you used to with the same speedo reading. The speedo just knows the axle revs, not what's between the axle and ground.


Or in other words, driving at the same speed, you have to rev the engine 7.2% higher than you used to. Might be someting to consider if you do long runs at highway speeds.

Good side is that this will also equate to faster gearing, which means you'll accelerate faster than you used to...but have to change gears sooner than you used to. Might also affect your top speed, probably downwards. Smaller tyres are usually lighter, so that's a bonus too.

(yes, many do consider these, it's not just you  s;) s;)  )

[andywood]

nice to know that it is not just me then   s:wink: :wink: s:wink:  

If the difference was only a couple of percent then wouldn't be too worried, but this difference is quite substantial - not that there seem to be any critical negatives though. Would mean that I could get cheaper tyres which is always a bonus   s:D s:D  

Might get the missus to take my garmin handheld gps out with us when they are fitted to see what the true speed is when the car speedo is showing a steady speed   s:shock: :shock: s:shock:

[Anonymous]

Might get the missus to take my garmin handheld gps out with us when they are fitted to see what the true speed is when the car speedo is showing a steady speed   s:shock: :shock: s:shock:

Try it now before fitting the new tyres, you'll probably find the speedo is already over-reading by 7-10%.

[Anonymous]

Going through the same headache myself about this.

Can I ask why lots of people prefer to change the profile at the back from 45 to 40?

My plan is to keep the profile the same front and back and just go up a step in width.
Front = 195/55/15 gives 1.88% difference to stock radius
Rear = 225/45/16 gives 1.5%

Or am I being an arse.  s:lol: :lol: s:lol:

(There's sooo much posts on here and spyderchat about this that it just becomes a nightmare to make a decision)

[markiii]

tyre availability and rolling radius

[Anonymous]

I shouldn't worry about it too much.  I'm still using identically sized Advan Neova LTS all round which has dropped the max speed down a little - but the speedo was well out beforehand anyway so I just use GPS for speed (a previous car had a non functional speedo altogether so I used a bike computer, so kind of use to this kind of nonsense now!).  s:P s:P

[VVT-i]

Going through the same headache myself about this.

Can I ask why lots of people prefer to change the profile at the back from 45 to 40?

The profile isn't an exact size.. ie  40mm or 45mm.. it is a percentage of the given width.... 40% or 45%, so when you go to a wider tyre you have to drop the % profile to keep the same aspect ratio (rolling radius)

[andywood]

Might get the missus to take my garmin handheld gps out with us when they are fitted to see what the true speed is when the car speedo is showing a steady speed   s:shock: :shock: s:shock:

Try it now before fitting the new tyres, you'll probably find the speedo is already over-reading by 7-10%.

Have been out with the gps and got the following results:

Speedo: 30, 40, 50, 60, 70
GPS: 27, 37, 46, 57, 67

Seems that it is not offset by a percentage across the range, but by approximately 3-4mph over-read across the speed range - not what I was expecting as thought it would be a percentage, based on the fixed mechanical ratios.
If I go for the Toyo's then I could expect this offset between speedo and actual to look like the following:

Speedo: 30, 40, 50, 60, 70
Actual: 25, 34, 43, 53, 62

Starts to get quite a gap then   s:shock: :shock: s:shock:

[Anonymous]

does it depend as well how much tread is remaining on your current tyres?

[andywood]

....and how much dirt has adhered to the tyre   s:wink: :wink: s:wink:

[Anonymous]

so no going off road and no drag style wheel spins   s:lol: :lol: s:lol:

[andywood]

does it depend as well how much tread is remaining on your current tyres?

....but on a serious note, the tyre change will reduce effective diameter by 14mm. You could expect around 8mm or so reduction in effective diameter due to tyre wear over lifetime so probably isn't helping the current situation.

Maybe I need to re-do the calculations................or not!  s:wink: :wink: s:wink:

[Anonymous]

[quote="andywoodHave been out with the gps and got the following results:

Speedo: 30, 40, 50, 60, 70
GPS: 27, 37, 46, 57, 67
[/quote]

Curious...

I'll check mine tomorrow but, going by when I've bothered to look, it has been in the region of 7-10% off.

[Anonymous]

does it depend as well how much tread is remaining on your current tyres?

Whilst the amount of tread remaining is going to make a difference it's probably not as much as the amount the tyre grows with speed.

[Anonymous]

i would have thought this is something toyota would have thought of, so tyre tread probably isnt a main issue in terms of what the speedo shows