Wheel and tyre question

[alwaysared]

I'm currently running 215/70 R 15C Michelin Agilis Cross Climate tyres on my 2006 MK2 Fiat Ducato based Hymer T594 coachbuilt motorhome and I'm thinking of upgrading to 16" wheels so how important is it to keep the rolling radius the same? I'm told the equivelent 16" tyre is a 215/65 R 16C but when I check them out they have a lower load rating than my current 15" ones I can get 215/75 R 16C but they told me that the difference in rolling radius could effect my speedo, I realise that but I'm not too bothered but more worring is they said it could bring on my ABS warning light I was also looking at the 225/75 R 16C but yhe told me they would be too wide. Has anyone done this and did it cause any problems? I'm beginning to think it may not be worth the bother and to just stick with the 15" wheels, anyone got any thoughts please?

Regards,
Del

 

[trevskoda]

Should no effect abs, but will alter speedo slightly, also watch for wheel rub on full steering lock, my proton savvy car has this problem afte filling a slightly taller tyres.
Strange about the abs as mine went wonky short time after changing tyres, cannot see why unless g box speed is tied to wheel speed.

 

[colinm]

Can't comment on ABS, but with 215/75-16 actual speed will go up by around 7% compared to indicated speed, for many vehicles this will still be within limits. You may find it has a marked affect on acceleration.

 

[mark61]

Haven't worked it out, but that sounds like a pretty sizeable increase radius. It will affect performance on hills significantly, depending on weight and power of vehicle that could be a PIA.
Even if there is enough room for wider tyres, you'll need to bear in mind if you ever use chains.

 

[merl]

Should no effect abs, but will alter speedo slightly, also watch for wheel rub on full steering lock, my proton savvy car has this problem afte filling a slightly taller tyres.
Strange about the abs as mine went wonky short time after changing tyres, cannot see why unless g box speed is tied to wheel speed.

Dont understand that either Trev, but surely wheel speed MUST be tied to gearbox speed, the 2 are simply connected together so one cant turn without the other. So whatever revs the gearbox (output shaft) is doing then the wheels must do the same. I wonder if the tyre fitter was talking about fitting different sizes across front and rear maybe? Dunno, I'm no vehicle ex-spurt.

 

[merl]

Can't comment on ABS, but with 215/75-16 actual speed will go up by around 7% compared to indicated speed, for many vehicles this will still be within limits. You may find it has a marked affect on acceleration.

I think it'll be less than half of that Colin, don't you need to use difference in size of the OD of the inflated tire not the rim?

 

[Boris7]

I'm beginning to think it may not be worth the bother and to just stick with the 15" wheels, anyone got any thoughts please?

That’s the most sensible sentence in this thread tbh, changing wheel assembly overall diameter effects ride height, gearing ratio, suspension effect etc, Fiat group make many vans and trucks and I’m pretty sure they looked at the options before building yours, if Hymer had reason to think wheel/tyre sizes needed changing they would have done so.

 

[Caz]

Just curious as to why you want to change the wheel size?

 

[colinm]

I think it'll be less than half of that Colin, don't you need to use difference in size of the OD of the inflated tire not the rim?

Been a long day, but it should be 215x 0.75x 2+16x25.4 vs 215x0.7x2+15x25.4, leaving out Pi as it's common.

 

[witzend]

Just curious as to why you want to change the wheel size?

I'm wondering that to seems like a pointless exercise fit larger wheels with smaller dia tyres to keep them the same

 

[RSD7a]

I increased from steel 15 inch to 16 inch alloys a while ago. I dutifully tabulated revs against various indicated road speeds in various gears with original wheels. Then re-did the measurements with the bigger wheels and all season tyres. Of course, I was expecting to find a higher speed indicated at given revs with the bigger wheels. In fact there was virtually no difference that the Ducato (analogue) speedo would measure. My conclusion is that the gearing difference in practice makes v little difference. Neither have I noticed fuel consumption change (I calculate every after every fill up) or any feeling that the thing is overgeared.

Cosmetically the van looks much better.to my eyes, but as for all the other effects ... nothing to report.

Ps no ABS issues either

 

[wildebus]

There are numerous reasons to change wheel size.
Appearance; Tyre Availability; Load Ratings; Handling; Ride and no doubt others not listed.

On a Motorhome, changing from 15" to 16" is often done to get a tyre with a high enough Load Rating. There are loads of Tyre Calculators that will, when the original wheel/tyre combo is entered, will give you multiple options to tyre sizes and what degree the speed is affected. That would be the first step.
Fiat Ducatos can come with 15" or 16" Wheels even with the same engine and gearbox. Plenty of vehicles will have pressure info on the doorjamb sticker for all the different wheels and tyres the vehicle can come with, and sometimes there is not just a 1" difference, but 2" for the wheel size options.

Ideally, a new tyre should be selected that is within around 3% of the original. Bear in mind that as the tread of a tyre wears, the rolling circumference changes anyway, and different tyres are different anyway even when they are the same quoted size. I used to have a set of 'normal' tyres and a set of Winter tyres on on of my RAV4s and there was a 3% difference in speed between the two even though they were the same size officially (Winters were Continental and Regular were Bridgestones FWIW).
I got new wheels and tyres for my Motorhome shortly after I bought it. New tyres needed to get a higher load rating. I chose to keep the 15" wheel size rather than got larger and end up with thinner tyres as I though they would be more compliant in ride and also better protection for the wheels if encountering big potholes.

Sometimes changing the rolling circumference can make a difference to the enjoyment of the vehicle. On my VW T4, I deliberately bought oversized tyres in order that the engine revs would be lower at motorway speeds. Dropping the revs by 300RPM at around 65MPH was quite a nice noise difference on what was a highly-geared van. The acceleration was affected no doubt, but this is not a factor I cared about. The fuel economy would also have been improved (due to the lower revs).
There were no adverse effects in terms of any sensors

 

[merl]

I increased from steel 15 inch to 16 inch alloys a while ago. I dutifully tabulated revs against various indicated road speeds in various gears with original wheels. Then re-did the measurements with the bigger wheels and all season tyres. Of course, I was expecting to find a higher speed indicated at given revs with the bigger wheels. In fact there was virtually no difference that the Ducato (analogue) speedo would measure. My conclusion is that the gearing difference in practice makes v little difference. Neither have I noticed fuel consumption change (I calculate every after every fill up) or any feeling that the thing is overgeared.

Cosmetically the van looks much better.to my eyes, but as for all the other effects ... nothing to report.

Ps no ABS issues either

Out of interest what size tires are you using on the larger rims and any noticeable change in cab noise?
Using the dash tacho / speedo there will be no difference whatsoever (If this is what you meant by indicated road speed?) Because the speed indicated on the speedo is calculated from the rpm at the gearbox and the engine RPM vs gearbox RPM hasn't changed. You'd need to have ,measured the time taken over a set distance at a set engine RPM before and after to know the difference in speed. (I remember my old mad driving at a steady 60 on the M1 while I timed how long it took between phone boxes which (he said) were set 1m apart.

 

[wildebus]

Out of interest what size tires are you using on the larger rims and any noticeable change in cab noise?
Using the dash tacho / speedo there will be no difference whatsoever (If this is what you meant by indicated road speed?) Because the speed indicated on the speedo is calculated from the rpm at the gearbox and the engine RPM vs gearbox RPM hasn't changed

I don't know if you are saying this knowing the technical details of the specific vehicle he has? But generally I would say you are incorrect.
I mentioned my VW T4 and changing tyres ... the indicated speed absolutely had changed after fitting them.
I also did the same thing - bigger tyres on same sized wheels - on my VW LT, as they are very poorly geared for motorway driving and the speedo went from slightly overreading to noticably underreading.
Nowadays it is a lot easier to check the comparisons when you have a GPS system such as a SatNav that shows the speeds, so I know absolutely changing tyres changed the speedo reading when travelling at a given known (by GPS) speed.

. You'd need to have ,measured the time taken over a set distance at a set engine RPM before and after to know the difference in speed. (I remember my old mad driving at a steady 60 on the M1 while I timed how long it took between phone boxes which (he said) were set 1m apart.

 

[RSD7a]

Out of interest what size tires are you using on the larger rims and any noticeable change in cab noise?
Using the dash tacho / speedo there will be no difference whatsoever (If this is what you meant by indicated road speed?) Because the speed indicated on the speedo is calculated from the rpm at the gearbox and the engine RPM vs gearbox RPM hasn't changed. You'd need to have ,measured the time taken over a set distance at a set engine RPM before and after to know the difference in speed. (I remember my old mad driving at a steady 60 on the M1 while I timed how long it took between phone boxes which (he said) were set 1m apart.

Oh. If you are right, no wonder there was no difference. Should have used the garmin sat nav vs Speedo.

Tyres:
Goodyear Cargo Vector 2 215/65R16 109T All Season (EC70)
No difference in road noise that I have detected, or fuel consumption.

It's a Citroen Relay PVC by the way

 

[mark61]

Changed tyre size on a handful of vehicles, indicated speed has always changed.

 

[mark61]

Oh. If you are right, no wonder there was no difference. Should have used the garmin sat nav vs Speedo.

Tyres:
Goodyear Cargo Vector 2 215/65R16 109T All Season (EC70)
No difference in road noise that I have detected, or fuel consumption.

It's a Citroen Relay PVC by the way

What where the original tyres on 15" rims?


If you had same tyres as OP, 215/70 R 15C and changed to 215/65 R 16, that’s an increase of only 0.4%, you won’t notice that.

The OP is enquiring about going from 215/70 R 15 to 225/75 R 16, an increase of 8.9%, massive difference.
Yes, I did use a tyre size calculator.

 

[RSD7a]

What where the original tyres on 15" rims?

The original 215/70R15C. Can't remember the manufacturer.

 

[RSD7a]

Changed tyre size on a handful of vehicles, indicated speed has always changed.

How did you measure it?

 

[mark61]

How did you measure it?

Speedo and GPS. Not 100% accurate, but close enough.
All of the online calculators will give you the speedometer error.