Tyre size

[joe66]

The tyres on my Ducato are 195/70R/15. Looking through the original German docs the tyre size is given as 205/70R/15. Altho my tyres are legal and ok for the van and there is not much difference price wise between 195 an 205 would I get a better MPG fitting 205s when I renew.?

[Derek Uzzell]

Replacing your Ducato’s present 195/70 R15 tyres with 205/70 R15 equivalents would increase the vehicle’s overall gearing by about 2%.

 

If your Ducato’s gearing is such that it does (say) 20mph per 1000rpm in top gear, when the motor is rotating at 3000rpm the vehicle would be travelling at 60mph. Increasing the overall gearing by 2% would mean that it would be doing 20.4mph per 1000rpm in top gear and 61.2 mph at 3000rpm.

 

Raising your vehicle’s overall gearing would affect its speedometer reading but (in my opinion) a 2% increase won’t be enough to matter. Without more details of your motorhome (age, make, model, motor) it’s impossible to say whether upping the gearing by 2% would be potentially harmful to its performance, but as you’ve said that its original German documentation suggests that it had 205/70 R15 tyres to begin with, the likelihood is that there will be no ‘technical’ difficulties (like under-wheelarch clearance problems) so changing to the wider tyre-size should be fine.

 

I don’t think fitting 205/70 R15 tyres would automatically provide improved mpg as any potential gain from upping the gearing might be offset by an increase in ‘drag' from the four wider tyres. However, a 195/70-section tyre is pretty skinny for a motorhome, so if you can get replacement 205/70 tyres for a similar price to 195/70s, you might as well opt for the wider tyres.

 

If fuel consumption is really important to you, you first need to ensure that your motorhome’s motor is functioning as well as it can and then optimise how you operate the vehicle (eg. don’t carry in it more than you need) and how you drive it.

[david lloyd]

In addition to that outlined by Derek above, the change to 205/70 tyres may well change other characteristics of your vehicke such as ride performance. Usually (but not always) I would spect a larger section tyre to give a softer ride. You may also find that the load and/or speed rating specified on the side of the tyre may differ for the 205/70.

 

David

[KeithM]

Derek Uzzell - 2016-11-02 9:07 AM

 

Replacing your Ducato’s present 195/70 R15 tyres with 205/70 R15 equivalents would increase the vehicle’s overall gearing by about 2%.

 

SNIP>>>>>>>>>>>>>>

 

.

 

Genuine question,

 

How does the extra width change the gearing ratio, is the 205 also a larger diameter overall.

[Agaric]

The 70 in the tyre markings is a ratio of the width of tread ergo 70% of 205 is greater than 70% of 195

[KeithM]

Thank you.

 

Another thing to add to my learning, only taken 66 years. *-)

[Derek Uzzell]

A tyre’s rolling circumference (RC) is the distance a point on the tyre’s tread will travel along the road when the wheel rotates once. In this instance the wheel-diameter stays at 15” and the ‘profile’ (ie the ratio of the tyre's width to the tyre's height) is “70” for both tyre sizes specified.

 

Based on an ancient Michelin technical data booklet I have, the RC of a 190/70 R15 ‘van’ tyre will be about 1985mm, while the RC of an equivalent 205/70 R15 will be about 2027mm (an around 2% increase).

 

The greater the RC value the further the vehicle will move in top gear for a given number of engine rpm. Consequently, for any given in-top-gear speed, Joe’s Ducato’s motor will run a bit slower with the bigger tyre than with the smaller size. In principle the slower the motor runs the lower the fuel consumption, but there are many other factors involved that need to be taken into account.

 

A minor tyre-size change like Joe has inquired about can be disregarded regarding fuel consumption (and probably anything else in terms of the vehicle’s on-road behaviour) but major changes need thinking about.

 

For example, there’s now the option with ‘light’ Ducato X290s to have 16”-diameter wheels with 225/75 R16 tyres, rather than the standard 15”-diameter wheels and 215/70 R15 tyres. The 16” wheel/tyre combination produces a big increase in the overall gearing and it appears that Fiat ‘gears’ the Ducato for the 16” wheel option, meaning that Ducatos with 15” wheels are under-geared.

[Agaric]

KeithM - 2016-11-02 12:32 PM

 

Thank you.

 

Another thing to add to my learning, only taken 66 years. *-)

 

I'm glad a fellow 66 year old has passed some knowledge on. ;-)

[KeithM]

Ah! yes Derek but no but yes.

 

What about when the tyre is worn and lost a few millimetres. ;-) B-)

 

 

 

 

 

 

 

 

Flippant remark/question and not meant to be taken seriously. :-D

Thanks for the extra info.

[joe66]

To answer a couple of questions

Van is a 1996 2.5 Ducato

before I look at the German log book and seen the tyre size I had spoke to a tyre company and the speed rating should be 102/104 the 205 gives a rating of 103/104.

The van handles fine with no issues regarding steering, braking ect.

[Steve928]

Derek Uzzell - 2016-11-02 1:51 PM

 

For example, there’s now the option with ‘light’ Ducato X290s to have 16”-diameter wheels with 225/75 R16 tyres, rather than the standard 15”-diameter wheels and 215/70 R15 tyres. The 16” wheel/tyre combination produces a big increase in the overall gearing and it appears that Fiat ‘gears’ the Ducato for the 16” wheel option, meaning that Ducatos with 15” wheels are under-geared.

 

That's interesting, Derek, because it's the scenario that I suggested back in this thread in November 2015;

http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Alloy-wheels-Ducato-X290/40045/

- at which point you disagreed;

 

"As only a small percentage of ‘light chassis’ Ducato-based motorhomes could be expected to have 16” wheels, it seems unlikely that Fiat would have ‘down-geared’ the X290 to accommodate the few rather than the many".

 

I'm genuinely not looking to score points here but would be interested to hear what you've learnt subsequently that has changed your mind on this issue?

 

 

[aandncaravan]

Deleted duplicate

[aandncaravan]

Joe66, If you want better fuel consumption choose a tyre that has a rating closer to A than G for Fuel Consumption. See chart below.

Apparently there can a marked difference between the best and worst, but this is achieved by reducing rolling resistance. That sometimes means reduced Grip as well, so check that the Wet Grip rating is not an abysmal G?

 

One advantage of a wider tyre is better overall grip.

 

If you can get a tyre with A for Fuel and A for Wet grip plus go back up to 205 width you should win all round.

Not sure you will find a tyre that meets all that, though!!

[Derek Uzzell]

Steve928 - 2016-11-03 7:30 PM

 

Derek Uzzell - 2016-11-02 1:51 PM

 

For example, there’s now the option with ‘light’ Ducato X290s to have 16”-diameter wheels with 225/75 R16 tyres, rather than the standard 15”-diameter wheels and 215/70 R15 tyres. The 16” wheel/tyre combination produces a big increase in the overall gearing and it appears that Fiat ‘gears’ the Ducato for the 16” wheel option, meaning that Ducatos with 15” wheels are under-geared.

 

That's interesting, Derek, because it's the scenario that I suggested back in this thread in November 2015;

http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Alloy-wheels-Ducato-X290/40045/

- at which point you disagreed;

 

"As only a small percentage of ‘light chassis’ Ducato-based motorhomes could be expected to have 16” wheels, it seems unlikely that Fiat would have ‘down-geared’ the X290 to accommodate the few rather than the many".

 

I'm genuinely not looking to score points here but would be interested to hear what you've learnt subsequently that has changed your mind on this issue?

 

 

A year on I believe you were right - that motorhomes based on a light Ducato X290 Euro 5 camping-car chassis have ‘compromise' gearing. I’ve read nothing to suggest that Fiat alters the gear ratios according to whether a light Ducato has 15” or 16” wheels, so middle-of-the-road gearing seems to be the only logical alternative theory.

 

I think my Rapido (with its 150hp motor) could cope well enough with the significant step up in overall gearing that moving to 225/75 R16 tyres would produce, but I’m less sure about equivalent motorhomes with the 130hp powerplant and 16” wheels.

[david lloyd]

Derek Uzzell - 2016-11-04 9:08 AM

 

Steve928 - 2016-11-03 7:30 PM

 

Derek Uzzell - 2016-11-02 1:51 PM

 

For example, there’s now the option with ‘light’ Ducato X290s to have 16”-diameter wheels with 225/75 R16 tyres, rather than the standard 15”-diameter wheels and 215/70 R15 tyres. The 16” wheel/tyre combination produces a big increase in the overall gearing and it appears that Fiat ‘gears’ the Ducato for the 16” wheel option, meaning that Ducatos with 15” wheels are under-geared.

 

That's interesting, Derek, because it's the scenario that I suggested back in this thread in November 2015;

http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Alloy-wheels-Ducato-X290/40045/

- at which point you disagreed;

 

"As only a small percentage of ‘light chassis’ Ducato-based motorhomes could be expected to have 16” wheels, it seems unlikely that Fiat would have ‘down-geared’ the X290 to accommodate the few rather than the many".

 

I'm genuinely not looking to score points here but would be interested to hear what you've learnt subsequently that has changed your mind on this issue?

 

 

A year on I believe you were right - that motorhomes based on a light Ducato X290 Euro 5 camping-car chassis have ‘compromise' gearing. I’ve read nothing to suggest that Fiat alters the gear ratios according to whether a light Ducato has 15” or 16” wheels, so middle-of-the-road gearing seems to be the only logical alternative theory.

 

I think my Rapido (with its 150hp motor) could cope well enough with the significant step up in overall gearing that moving to 225/75 R16 tyres would produce, but I’m less sure about equivalent motorhomes with the 130hp powerplant and 16” wheels.

 

This is also very interesting Derek. When I uprated my Fiat Ducato light chassis from 3500kg to 3859 kg SVTech advised (in addition to the rear air suspension) that I change the 215/70R15 tyres on the rear axle to 225/70R15 tyres with a higher load index. Would it have been possible, and perhaps given any additional benefits, if I had changed to 16" wheels/tyres?

 

My Chausson also has the 2.3 150bhp engine upgrade with a comfortmatic gearbox though.

 

David

[aandncaravan]

David, Moving from 15" to 16" wheels does not itself endow any extra load carrying capacity, however, larger wheels generally have a better selection of 'widths' and therefore higher potential load capability.

 

Increasing the width of a Tyre, like moving up from 195 to 205 almost always results in an improvement in load carrying capability. This is because the weight of the vehicle is spread across a wider footprint.

 

A larger wheels prime advantage can be a better ride quality as the larger the wheel the greater the chance of 'bridging' a pot hole where a 10" wheel might drop right into it.

 

Going up one size in width can also improve the ride quality for exactly the same reason in that the wider the footprint the more likely to ride over pot holes that a narrow tyre might drop into.

This does mean that you can 'pick -up' more bumps, like Cats Eyes, but I would guess the majority of modern road quality issues these days are 'indentations' in the road surface rather than 'protrusions'.

 

Additionally the Tyre wall is taller (it is a percentage of the width) so has more flex to absorb shock.

 

Generally dry road braking is also safer the wider the tyre as the greater tyre contact with the road results in better grip so less risk of 'Skidding'.

 

Hence the advice to go up one width after your weight Upgrade.

 

One disadvantage of wider tyres might be a greater tendency to 'sway' more due to the taller tyre sidewall, which because it is a percentage of the width is effectively higher so may flex more.

This may be counteracted by also going down a corresponding sidewall size, say from 75 to 70.

 

The other disadvantages of a wider tyre can be a greater tendency to aquaplane and slightly less grip in Snow, but not normally noticeably with one or two size changes.

 

 

 

 

 

[Derek Uzzell]

david lloyd - 2016-11-04 9:25 AM

 

This is also very interesting Derek. When I uprated my Fiat Ducato light chassis from 3500kg to 3859 kg SVTech advised (in addition to the rear air suspension) that I change the 215/70R15 tyres on the rear axle to 225/70R15 tyres with a higher load index. Would it have been possible, and perhaps given any additional benefits, if I had changed to 16" wheels/tyres?

 

My Chausson also has the 2.3 150bhp engine upgrade with a comfortmatic gearbox though.

 

David

 

I believe Chausson offered Fiat’s 16” wheel + 225/75 tyre-size option for your model of motorhome, so retrofitting that size of wheel/tyre should not present any technical problems.

 

Assuming that you were fitting the 16” wheels front and rear (Putting 16” wheels/tyres on just the rear axle would produce a very odd-looking tail-up stance!) the motorhome’s height and ground-clearance would increase a bit, as would its weight.

 

The vehicle’s overall gear ratio would increase by about 9%, meaning that its speedometer would probably need recalibration (which I presume Fiat takes care of when the Ducato chassis is being built and the 16”-wheel option has been chosen). I’ve no idea if anything is done by Fiat regarding the ComfortMatic transmission’s ‘programming' to reflect the significant increase in tyre size and overall gear ratio when the 16”-wheel option is specified.

 

Obviously if you went for the 225/75 profile for the tyres for the 16”wheels you’d gain an extra 10mm of tread-width over your motohome’s original 215/70-section tyres, but as you had wider tyres fitted to the rear axle when the motorhome was uprated, you could fit the same wider tyres to the front axle now if you so chose.

 

Because the circumference of a 225/75 R16 tyre is greater than that of a 215/70 R15, the former tyre’s ‘footprint’ on the road would be potentially larger. This should provide improved braking, grip and road-holding, but it’s very doubtful that the amount of improvement would be measurable (or noticeable) except on a test track with the vehicle being driven to its limits. (If control of an over-3-tonne motorhome is lost in a big way, it would be better to pray to Saint Christopher than think “I wish I’d had 16” wheels fitted”.)

 

A Ducato ‘Maxi’ chassis has a heavier load-carrying capability than the ‘light’ chassis and consequently has large front brake-discs that need 16” wheels to obtain the necessary clearance. The 16”-wheel option for Ducato X290s is referred to in this 2014 link

 

http://www.fiatprofessionalpress.com/press/detail/118108

 

but no benefits are mentioned. When 16” wheels (steel or alloy) are factory-fitted to a light Ducato chassis, the standard brake-discs aren’t changed so there’s no obvious ‘technical’ need for the larger-wheel option. Admittedly 16” wheels/tyres do ‘fill’ a Ducato’s wheel-arches more than the 15” combination, so the option may be more about cosmetics than anything else. The (few) motorhomes I’ve seen with a light Ducato chassis and the 16”-wheel option don’t appear to have had any suspension alterations and (to me) seem to be standing on tip-toe.

 

The real disincentive for retro-fitting 16” wheels/tyres would be the cost. I’d anticipate approaching £1000 for a set of four new Fiat steel 16” wheels, suitable tyres and wheel trims, though you could, of course, offset that expenditure by flogging the original parts.

 

I’ve always thought it odd that the DVLA/VCA is evidently happy with ‘uprating’ a motorhome’s gross vehicle weight via the bigger rear tyres/air-bags approach that SvTech commonly recommends. This would not be acceptable in France (where the relevant authorities tend to be paranoid about technical homologation) and I’m pretty sure Fiat will consider such modifications as unauthorised with a potential impact on the vehicle’s warranty. But if you had wanted to have the 16”-wheel/tyre combination fitted when your motorhome was uprated you almost certainly could have, though I’m doubtful that the cost would be justified.

[david lloyd]

Thanks Derek. I'm not sure it is something I would consider now but it may have given me an alternative when we were considering the uprate - especially if having 16" wheels and tyres (with higher load index) would have meant not having to have the air suspension - then the cost would have been much the same but, of course, we would have had to recalibrate the speedo. This already reads about 8% faster than the speed shown on the gps.

 

As you say, it's odd that Fiat would readily accept the change in tyre/wheel size but would have problems with the larger tyres/air suspension modifications we have made.

 

David

[Derek Uzzell]

I believe you’d find that SvTech would have advised that rear ‘air assist’ be added even if you had chosen to retro-fit 16” wheels and bigger tyres, as the air-bellows are there to beef up the rear springing which merely fitting bigger wheels/tyres won’t do.

 

Fiat warrants a vehicle in its ex-factory condition, so if it leaves the factory with 15” wheels that will be OK warranty-wise, or if it leaves the factory with 16” wheels that will also be OK. But if you ask Fiat whether they will authorise retro-fitting bigger wheels (even if they are genuine Fiat parts), it’s likely that the answer will be No.

[keninpalamos]

If your speedo is reading 8% faster than your GPS with 15" wheels then with 16" wheels your speedo will be more accurate, say around 2-3% faster than your GPS

[Derek Uzzell]

Replacing 15”-diameter wheels carrying 215/70 R15 tyres with 16”-diameter wheels and 225/70 R16 tyres would be a helluva expensive way to produce a more accurate speedometer. ;-)

 

Besides which - from what Steve928 has said elsewhere - it may be practicable to remove/reduce the inaccuracy of the speedometer of an X290 Ducato/Boxer by tweaking its software.

 

As the speedometer display of X290s is notoriously difficult to read in daylight (and the official ‘fix’ to have the instrument-cluster’s backlighting on with the ignition is not much cop in my view) it probably doesn’t matter much whether the speedometer over-reads a fair bit or just a little.

[david lloyd]

Derek Uzzell - 2016-11-06 8:46 AM

 

Replacing 15”-diameter wheels carrying 215/70 R15 tyres with 16”-diameter wheels and 225/70 R16 tyres would be a helluva expensive way to produce a more accurate speedometer. ;-)

 

Besides which - from what Steve928 has said elsewhere - it may be practicable to remove/reduce the inaccuracy of the speedometer of an X290 Ducato/Boxer by tweaking its software.

 

As the speedometer display of X290s is notoriously difficult to read in daylight (and the official ‘fix’ to have the instrument-cluster’s backlighting on with the ignition is not much cop in my view) it probably doesn’t matter much whether the speedometer over-reads a fair bit or just a little.

 

Yes I think you are right Derek - I have not bothered with the 'fix' and use the gps reading when I have it but if not I now have a fair idea of the actual speed rather than the indicated speed on the dial.

 

Although I would not now go for changing the wheels tyres, as you say, the work already done may preclude me claiming on the Fiat warranty but, luckily, I am not reliant entirely on that as the motorhome has a 3 year RAC warranty for base and conversion. Hopefully, I won't need to test this in the future.

 

David

[keninpalamos]

Derek Uzzell - 2016-11-06 8:46 AM

 

Replacing 15”-diameter wheels carrying 215/70 R15 tyres with 16”-diameter wheels and 225/70 R16 tyres would be a helluva expensive way to produce a more accurate speedometer. ;-)

 

Besides which - from what Steve928 has said elsewhere - it may be practicable to remove/reduce the inaccuracy of the speedometer of an X290 Ducato/Boxer by tweaking its software.

 

As the speedometer display of X290s is notoriously difficult to read in daylight (and the official ‘fix’ to have the instrument-cluster’s backlighting on with the ignition is not much cop in my view) it probably doesn’t matter much whether the speedometer over-reads a fair bit or just a little.

(lol) I agree with you there, Just pointing out that, that would be a consequence as apposed to making it even more "optimistic" as they say

[Steve928]

Just a wee correction to a couple of the posts above that refer to the tyre size and tread width. The width element of a tyre's vital statistics doesn't refer to the tread width but rather the section width i.e. the widest point of the casing and it doesn't always follow that a wider tyre will have wider tread.

 

I've given this example before but when I switched from 215mm section width Conti Vanco CPs to 'wider' 225mm section width Falkens I lost circa 15mm of tread width. Also a 215mm section width Michelin Agilis CP has visibly narrower tread than the same size Conti. Tread width isn't specified in a tyre's measurements and will vary from manufacturer to manufacturer and tyre model to tyre model.

[Rayjsj]

Derek Uzzell - 2016-11-05 2:27 PM

 

I believe you’d find that SvTech would have advised that rear ‘air assist’ be added even if you had chosen to retro-fit 16” wheels and bigger tyres, as the air-bellows are there to beef up the rear springing which merely fitting bigger wheels/tyres won’t do.

 

Fiat warrants a vehicle in its ex-factory condition, so if it leaves the factory with 15” wheels that will be OK warranty-wise, or if it leaves the factory with 16” wheels that will also be OK. But if you ask Fiat whether they will authorise retro-fitting bigger wheels (even if they are genuine Fiat parts), it’s likely that the answer will be No.

Someone had better tell Autotrail then, because my 2014 V Line 600, has 17 inch Ronal alloy wheels, 215/70/ Goodyear tyres.(all fitted as standard from new). It has had several Fiat Professional services, with no comments about the wheels or tyres or Warranty problems .