Rear Tyre Width Confusion

[boosted]

The wheel (and hence tyre) is too far out from the hub , the movement is a radius...et 20 is wrong should be 35 offset ideally so under suspension compression the tyre can go up inside arch without catching lip.

[probedb]

Just to show what you can do and what fits, these are 18x10 et20 with 255/35

http://www.flickr.com/photos/johnrenna/5722750998/

[Metalorama]

Sorry guys, haven\'t had acces to a computer the last few days and my phone was being a dick. The car is quite low, not any lower than yours though fil (those wheels are NICE by the way!) I have read before now that the offsets aren\'t ideal, the wheels look nice, but if I were to buy wheels for it I\'d have gone with something closer to 35.

18x10 255/35 looks just right actually so I should be ok with a 255 then, are the arches rolled at all? Since I\'m running 40 profile on the front would it be retarded to run 35\'s on the rear only? I don\'t have the cash right now to buy 4 tyres but I really need rears.

[Grant Monkhouse]

if you have the same width wheels all round then you need the profiles to be the same, if theyre thinner on the front then you will require a different profile. theres a way to work it out bu i dont know how, but if you run the same profile you end up with the car looking like its leaning forward because the rears are actually bigger in circumference.
 
i have had 3 people explain to me about why different width tyre/wheel combos require different profiles and i still dont really understand why
 
Your rears should be fine though, if not a slight roll of the inner ip can be done easily.

[Will66]

It\'s all to do with rolling circumference.  That is the distance around the edge of the tyre, or the distance the wheel and tyre will roll if you push it for one revolution.

On a car like the RX7 you need to have the same rolling circumference on all four tyres as otherwise the ABS (if fitted) will think one axle is rotating at a different speed under braking and cause ABS faults under braking.  On an RX8 it\'s even more important because of the DSC (b***h switch) as this is calibrated based on identical rolling circumferences on all wheels.

The way to check is the following:

First off you need to know the overall diameter of the tyre and wheel combination.

The wheel is easy, that\'s just the diameter of the rim.  So on a 17 inch it\'s 17 inches. (Obvious).  However because the imperial system gives me a headache I like to convert it into millimetres.  There are 25.4 mm to every inch so a 17 inch rim has a diameter of 17 x 25.4 mm = 431.8 mm

You then need the extra diameter added by the profile of the tyre.  Say you\'ve got a 235/40R17 tyre.   The profile is a percentage of the width of the tyre, so on a 40 profile tyre the profile is 40% of the width.  The width of the tyre is given in mm.

So the profile of the tyre in mm is 235 x 40% (or 0.4) = 94 mm

Because you\'ve got tyre at the top and bottom of the rim you have to multiply that by two.

2 x 94mm = 188mm

Therefore the overall diameter of the tyre is the diameter of the rim + 2 x the size of the profile.

So in the case of our example:

The diameter of the wheel and tyre is:

431.8mm + (2 x 94mm) = 619.8 mm

To get the circumference you simply multiply the diameter by pi.  If you\'re doing this on a calculator then use the pi button (π).

619.8 x π = 1947.16 mm

If you want to compare it to a 255/35R17 tyre you need to find the diameter of that tyre.

The rim diameter is the same, 431.8 mm

The tyre profile is 255 x 0.35 = 89.25

Therefore the overall diameter is 431.8 + (89.25 x 2) = 610.3

The rolling circumference is therefore:

610.3 x π = 1917.31 mm

It\'s immediately obvious that the 255/35R17 has a smaller rolling circumference but to find the percentage difference you need to divide the smaller number by the larger number and multiply the result by 100.

1917.31/1947.16 x 100 = 98.46%.  The smaller 255/35R17 tyre is 98.46% of the size of the 235/40R17 Tyre.  Or it\'s 1.54% smaller.  Generally most ABS systems will tolerate a difference of 1-2% so you\'d be okay.

If you don\'t have ABS then it\'s not a problem you just have to fit the same size tyre across the same axle, particularly on the rear if it\'s fitted with an LSD, a difference in size on these will wear out the LSD very quickly.

[Metalorama]

if you have the same width wheels all  round then you need the profiles to be the same, if theyre thinner on  the front then you will require a different profile. theres a way to  work it out bu i dont know how, but if you run the same profile you end  up with the car looking like its leaning forward because the rears are  actually bigger in circumference.
 
i have had 3 people explain to me about why different width tyre/wheel  combos require different profiles and i still dont really understand why
 
Your rears should be fine though, if not a slight roll of the inner ip can be done easily.

I was worried that the car might look like it was leaning back with the front sitting higher and just look stupid, I might bite the bullet and go with 35 profile all round. I get confused by the whole thing, that\'s why I thought I\'d best ask before I spend hundreds on unuseable tyres.

It\'s all to do with rolling circumference.  That is the distance around the edge of the tyre, or the distance the wheel and tyre will roll if you push it for one revolution.

On a car like the RX7 you need to have the same rolling circumference on all four tyres as otherwise the ABS (if fitted) will think one axle is rotating at a different speed under braking and cause ABS faults under braking.  On an RX8 it\'s even more important because of the DSC (b***h switch) as this is calibrated based on identical rolling circumferences on all wheels.

The way to check is the following:

First off you need to know the overall diameter of the tyre and wheel combination.

The wheel is easy, that\'s just the diameter of the rim.  So on a 17 inch it\'s 17 inches. (Obvious).  However because the imperial system gives me a headache I like to convert it into millimetres.  There are 25.4 mm to every inch so a 17 inch rim has a diameter of 17 x 25.4 mm = 431.8 mm

You then need the extra diameter added by the profile of the tyre.  Say you\'ve got a 235/40R17 tyre.   The profile is a percentage of the width of the tyre, so on a 40 profile tyre the profile is 40% of the width.  The width of the tyre is given in mm.

So the profile of the tyre in mm is 235 x 40% (or 0.4) = 94 mm

Because you\'ve got tyre at the top and bottom of the rim you have to multiply that by two.

2 x 94mm = 188mm

Therefore the overall diameter of the tyre is the diameter of the rim + 2 x the size of the profile.

So in the case of our example:

The diameter of the wheel and tyre is:

431.8mm + (2 x 94mm) = 619.8 mm

To get the circumference you simply multiply the diameter by pi.  If you\'re doing this on a calculator then use the pi button (π).

619.8 x π = 1947.16 mm

If you want to compare it to a 255/35R17 tyre you need to find the diameter of that tyre.

The rim diameter is the same, 431.8 mm

The tyre profile is 255 x 0.35 = 89.25

Therefore the overall diameter is 431.8 + (89.25 x 2) = 610.3

The rolling circumference is therefore:

610.3 x π = 1917.31 mm

It\'s immediately obvious that the 255/35R17 has a smaller rolling circumference but to find the percentage difference you need to divide the smaller number by the larger number and multiply the result by 100.

1917.31/1947.16 x 100 = 98.46%.  The smaller 255/35R17 tyre is 98.46% of the size of the 235/40R17 Tyre.  Or it\'s 1.54% smaller.  Generally most ABS systems will tolerate a difference of 1-2% so you\'d be okay.

If you don\'t have ABS then it\'s not a problem you just have to fit the same size tyre across the same axle, particularly on the rear if it\'s fitted with an LSD, a difference in size on these will wear out the LSD very quickly.

Wow! I had no idea so much was involved. Thanks for the in depth explanation, at first glance I thought my head was going to explode with all the information, but you have broken it down so it is useable for plebs like me, ha ha. Thanks man, really appreciate it :god

[Grant Monkhouse]

Smarty pants award goes to Will. I knew it was about the circumference but that\'s it.

It\'s all to do with rolling circumference.  That is the distance around the edge of the tyre, or the distance the wheel and tyre will roll if you push it for one revolution.

On a car like the RX7 you need to have the same rolling circumference on all four tyres as otherwise the ABS (if fitted) will think one axle is rotating at a different speed under braking and cause ABS faults under braking.  On an RX8 it\'s even more important because of the DSC (b***h switch) as this is calibrated based on identical rolling circumferences on all wheels.

The way to check is the following:

First off you need to know the overall diameter of the tyre and wheel combination.

The wheel is easy, that\'s just the diameter of the rim.  So on a 17 inch it\'s 17 inches. (Obvious).  However because the imperial system gives me a headache I like to convert it into millimetres.  There are 25.4 mm to every inch so a 17 inch rim has a diameter of 17 x 25.4 mm = 431.8 mm

You then need the extra diameter added by the profile of the tyre.  Say you\'ve got a 235/40R17 tyre.   The profile is a percentage of the width of the tyre, so on a 40 profile tyre the profile is 40% of the width.  The width of the tyre is given in mm.

So the profile of the tyre in mm is 235 x 40% (or 0.4) = 94 mm

Because you\'ve got tyre at the top and bottom of the rim you have to multiply that by two.

2 x 94mm = 188mm

Therefore the overall diameter of the tyre is the diameter of the rim + 2 x the size of the profile.

So in the case of our example:

The diameter of the wheel and tyre is:

431.8mm + (2 x 94mm) = 619.8 mm

To get the circumference you simply multiply the diameter by pi.  If you\'re doing this on a calculator then use the pi button (π).

619.8 x π = 1947.16 mm

If you want to compare it to a 255/35R17 tyre you need to find the diameter of that tyre.

The rim diameter is the same, 431.8 mm

The tyre profile is 255 x 0.35 = 89.25

Therefore the overall diameter is 431.8 + (89.25 x 2) = 610.3

The rolling circumference is therefore:

610.3 x π = 1917.31 mm

It\'s immediately obvious that the 255/35R17 has a smaller rolling circumference but to find the percentage difference you need to divide the smaller number by the larger number and multiply the result by 100.

1917.31/1947.16 x 100 = 98.46%.  The smaller 255/35R17 tyre is 98.46% of the size of the 235/40R17 Tyre.  Or it\'s 1.54% smaller.  Generally most ABS systems will tolerate a difference of 1-2% so you\'d be okay.

If you don\'t have ABS then it\'s not a problem you just have to fit the same size tyre across the same axle, particularly on the rear if it\'s fitted with an LSD, a difference in size on these will wear out the LSD very quickly.

[Will66]

I did make an excel page that would work it all out for you but unfortunately I lost it when a hard drive died and I haven\'t got round to doing it again.  But once you know the maths it\'s not difficult.

[boosted]

I knew it 80 % ...but could not explain that way......top marks!

[bimbleuk]

Nice explanation above but if you want a quick tyre size calculator I\'ve been using one for years. Doesn\'t cover offsets and and rim widths but an easy way to compare % changes to the speedo and overall tyre diameter changes.
 
Search for Miata Tyre Calculator
 
PS I\'m a standard alloy fan so I can\'t help with fitting big tyres.