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[Shackeng]

I am running 225/55 16's on my TD Estate. As the winter tyres vary considerably with size, what size can I usefully - and legally - fit to these rims?

[Shimmy]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

[MV6Matt]

Your tyre size is fine, my winter tyres are 225/55/16 on a 16" rim

Matthew

[birdman]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

how can the speedo under read when the profile is the same!?

[Jimbob]

Because 55 is a percentage, not an actual measurement.

[Osprey]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

Shouldn't that be over read?  Circumference is smaller so the car will be going slower for a given rpm. 

[Entwood]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

how can the speedo under read when the profile is the same!?

As Jimbob says .. it is a percentage.

So 55 % of 225/55/16 = 123.75 The width of the tyre is 225mm, the height of the sidewall is 123.75 mm

55% of 205/55/16 = 112.75 The width of the tyre is 205mm the height of the sidewall is 112.75mm

As they are both on 16" rims the difference in radius of the two wheels is 11mm. The larger wheel radius is 203.2 mm (16"/2) + 123.75 mm = 326.95 mm The smaller wheel radius is 203.2 + 112.75 = 315.95 mm

The circumference of a wheel - the distance it travels in one rotation is 2*Pi*r  so for the larger wheel it is 2054.358 mm, for the smaller wheel 1985.24 .. a difference of 69.18 mm

So for every rotation of the wheel the smaller tyre travels about 7cm less than the  bigger one... but your speedo is measuring the RPM of the wheel and "assuming" the larger ones circumference ..

The error is (69.18 / 2054.358) * 100 = 3.37% ... about what Birdman said

HTH

[omegod]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

how can the speedo under read when the profile is the same!?

As Jimbob says .. it is a percentage.

So 55 % of 225/55/16 = 123.75 The width of the tyre is 225mm, the height of the sidewall is 123.75 mm

55% of 205/55/16 = 112.75 The width of the tyre is 205mm the height of the sidewall is 112.75mm

As they are both on 16" rims the difference in radius of the two wheels is 11mm. The larger wheel radius is 203.2 mm (16"/2) + 123.75 mm = 326.95 mm The smaller wheel radius is 203.2 + 112.75 = 315.95 mm

The circumference of a wheel - the distance it travels in one rotation is 2*Pi*r  so for the larger wheel it is 2054.358 mm, for the smaller wheel 1985.24 .. a difference of 69.18 mm

So for every rotation of the wheel the smaller tyre travels about 7cm less than the  bigger one... but your speedo is measuring the RPM of the wheel and "assuming" the larger ones circumference ..

The error is (69.18 / 2054.358) * 100 = 3.37% ... about what Birdman said

HTH

You just gave me a migraine trying to follow that !!

[aaronjb]

Or.. this page does all the maths for you: http://www.miata.net/garage/tirecalc.html

(The maths, of course, isn't perfect as it doesn't take into account things like squish, but it's as good as you'll get without a piece of string and a tape measure..)

[Rods2]

As you'd expect 225/55/16 will be fine.

205/55/16 is fine and cheaper but will make the speedo under read by 3%.

how can the speedo under read when the profile is the same!?

As Jimbob says .. it is a percentage.

So 55 % of 225/55/16 = 123.75 The width of the tyre is 225mm, the height of the sidewall is 123.75 mm

55% of 205/55/16 = 112.75 The width of the tyre is 205mm the height of the sidewall is 112.75mm

As they are both on 16" rims the difference in radius of the two wheels is 11mm. The larger wheel radius is 203.2 mm (16"/2) + 123.75 mm = 326.95 mm The smaller wheel radius is 203.2 + 112.75 = 315.95 mm

The circumference of a wheel - the distance it travels in one rotation is 2*Pi*r  so for the larger wheel it is 2054.358 mm, for the smaller wheel 1985.24 .. a difference of 69.18 mm

So for every rotation of the wheel the smaller tyre travels about 7cm less than the  bigger one... but your speedo is measuring the RPM of the wheel and "assuming" the larger ones circumference ..

The error is (69.18 / 2054.358) * 100 = 3.37% ... about what Birdman said

HTH

Rock solid piece of maths and logical thinking, well done, I really like it. 

It has answered a question, where I have been thinking about fitting 205/55/16 winter tyres to my Miggy and wondering what the speedo difference would be, will look at 225's just to keep speedo accurate, even though they will give poorer grip in snow due to lower psi due to bigger contact area. 

I've got a clean licence and want to continue to keep the speed camera mafia at bay. 

[aaronjb]

You could probably look at 205/60R16 as an alternative to 225/55R16 - the difference in overall circumference is only 0.2% (so diddley, really) and they ought to fit just fine as they are a tiny weeny bit smaller.

Obviously no point if they're more expensive than 225/55

[Shackeng]

The handbook states that the TD permitted  tyre sizes can be 205/65 15's, so they would be preferable for winter as narrower AND about 25% cheaper, and no insurance issues. I'd buy them with the steel wheels, but have already got a spare summer set on alloys in the garage, so no more storage for another set.

[Shackeng]

However, as you all have just spotted, I still need the smaller rims to put them on! Ah well, daughter's MG will have to spend winter on the drive.

[2woody]

the maths on all of these websites and in the hand calcs is worng, sorry.

the tyre business and the auto business doesn't use them because they're wildly inaccurate. The problem stems from the static figures vs. what actually happens on the road. It's for that reason that every profile is tested and the measured figures are what's used. There are two important figures, the wheel revs per mile ( which gives you the gearing ) and the hub height, around which the suspension geometry is designed. Using measured figures takes out the installed sag and the effects of slip.

Omega B is designed around 817 wheel revs per mile. Which is what the standard tyres are. 225/55 x 16 is 811 wrpm for example.

So if you need to change tyre sizes, then stick only to something that's as near as dammit to 817 wrpm. Just for reference, that's all of the standard sizes including 205/65 x 15, plus 215/50 x 17 and 235/40 x 18 and 245/40 x 18

205/55 x 16 is borderline acceptable, as it has a wrpm of 837 - an error of 2.4%

as a guide, and because I know you're going to ask, 225/55 x 16 has a wrpm of 817, which means a circumference of 1969mm. The "calculated" figures make that 2054mm, or a 7.2% error. Generally speaking, I find that the calculated figures are between 5% and 15% inaccurate.

[Shackeng]

the maths on all of these websites and in the hand calcs is worng, sorry.

the tyre business and the auto business doesn't use them because they're wildly inaccurate. The problem stems from the static figures vs. what actually happens on the road. It's for that reason that every profile is tested and the measured figures are what's used. There are two important figures, the wheel revs per mile ( which gives you the gearing ) and the hub height, around which the suspension geometry is designed. Using measured figures takes out the installed sag and the effects of slip.

Omega B is designed around 817 wheel revs per mile. Which is what the standard tyres are. 225/55 x 16 is 811 wrpm for example.

So if you need to change tyre sizes, then stick only to something that's as near as dammit to 817 wrpm. Just for reference, that's all of the standard sizes including 205/65 x 15, plus 215/50 x 17 and 235/40 x 18 and 245/40 x 18

205/55 x 16 is borderline acceptable, as it has a wrpm of 837 - an error of 2.4%

as a guide, and because I know you're going to ask, 225/55 x 16 has a wrpm of 817, which means a circumference of 1969mm. The "calculated" figures make that 2054mm, or a 7.2% error. Generally speaking, I find that the calculated figures are between 5% and 15% inaccurate.