Ducati Rear - 55 or 50 section??

**Rev DJ** · 7th January 2010, 09:34

Happy New Year all...

I have to replace the rear tyre on my 999. I have been told that 55 section tyres are better than 50 section tyres - I'm looking at 190/50/ZR17's vs 190/55/ZR17's... Any help on why a 55 is better than a 50?

Cheers DJ

**Cajun** · 7th January 2010, 09:53

in theory a 55 tire is taller than a 50(alot of this depends on tire profile shape etc), so will give the bike better tip in, and reason they say go with a 190/55 over the 180/55
its one reason people use to run 180/55 instead of factory 190/50 tires.

We have run 190/50,190/55, 180/55 on our aprilia rsv1000r, and running a 55 helps greatly with it turn ability.

**Marmoot** · 7th January 2010, 11:42

and what about speedometer inaccuracies? Are the rolling diameters the same for 50 and 55 profile tyres?
Or would the 55 profile tyres have larger rolling diameter and cause the speedo to give slightly lower reading?

**Cajun** · 7th January 2010, 11:52

Originally Posted by Marmoot

and what about speedometer inaccuracies? Are the rolling diameters the same for 50 and 55 profile tyres?
Or would the 55 profile tyres have larger rolling diameter and cause the speedo to give slightly lower reading?

well the ape goes off the bolts on the rear wheel discs, so changing sprockets/tires has no effect on that bike, no idea what ducatis use for speedo

**Marmoot** · 7th January 2010, 11:53

gearbox sensor. So if the tyre rolling diameter changes then the speedo reading would be innacurate.

Larger rolling diameter = speedo shows slower speeds than actual.

**cs363** · 7th January 2010, 11:56

Originally Posted by Marmoot

and what about speedometer inaccuracies? Are the rolling diameters the same for 50 and 55 profile tyres?
Or would the 55 profile tyres have larger rolling diameter and cause the speedo to give slightly lower reading?

Depends where the speedo pick up is, but the rolling diameters aren't the same as the 50 or 55 figure denotes the percentage of the tread width. Meaning that on a 190/50 the sidewall height is 50% of the tread width or 95mm, whereas a 190/55 would be 104.5mm and a 180/55 would be 99mm. So, in theory a 190/55 will be 19mm taller than a 190/50. Bear in mind though that some manufacturers measure the width at different points of the tyre, so not all tyres of the same size are actually the same if you know what I mean....

But as you can see from the above, ideally if you went to a 55 section tyre you would go to a 180 if you were currently running a 190/50 - assuming that rolling diameter was important to you.

On a personal note I prefer to stick with the manufacturers recommended size as they have obviously put a lot of time into testing the ideal combination for that bike, however as I pointed out above different brands can differ in 'real' sizing and profile shape so there's probably more mileage in experimenting with brands than sizes IMHO.

Edit - This might be of interest: http://www.ducati-upnorth.com/tech/tyresize.php

**Rev DJ** · 7th January 2010, 12:11

great info - thanks...

**bucket boy** · 7th January 2010, 13:55

same for the 999 cajun

**98tls** · 7th January 2010, 15:09

Fwiw if your bike has a hugger on it there might be clearance issues with the 55.

**crash harry** · 10th January 2010, 19:31

Originally Posted by cs363

Depends where the speedo pick up is

No it doesn't. I get your point, if the pickup is at the rear sprocket then it makes no difference if you change sprocket ratios, but if you change the wheel rolling radius it will still make the speedo read incorrectly.
Think about it, the only thing the speedo knows is that the wheel turned x number of revolutions in a given time. It doesn't know how far you went for each of those revolutions.

Only way around that one is if the speedo pickup is on the front wheel (but then front tyre profile will obviously affect it...)

To the OP - FWIW I believe that a 180x55 has approximately the same rolling radius as a 190x50 - the "profile" number is (nominally) the height of the tyre expressed as a percentage of the width so:
190 x 50 = 95mm
190 x 55 = 104.5mm
180 x 55 = 99mm

Given that the rim is 17" and these profiles will be added to each side of the rim, you should get approximately 3% speedo error with a 190x55 or 1% speedo error with a 180x55, if the speedo was 100% correct with a 190x50. I wouldn't worry about it, tyre wear affects the speedo about that much or more anyway.

**speed63** · 13th January 2010, 07:31

You are right in that the speedo will indicate lower speed than actual with a 55. Majority of speedometers are inaccurate anyway. But it is worth it for the better sidegrip with a 55 profile tyre.