Putting larger wheels on MH

[Oldgustaf]

I hope this isn't a daft question but is it possible to replace 15in wheels (on a 2011 Ducato) with 16in wheels? Reasons being, have always though the 15in wheels look small and because we'd gain a little bit of ground clearance.

 

[Scotch mist]

I think you can BUT I’m sure someone will come along shortly to confirm

 

[XxAnthxX]

If you don’t reduce the tire height at the same time, the wheel will have a larger rolling circumference so your speedo will under read your speed.

 

[Howard H]

I replaced my 15inch steel wheels to 16 inch alloy wheels on a 2016 fiat Ducato , no problem at all .

 

[Howard H]

If you don’t reduce the tire height at the same time, the wheel will have a larger rolling circumference so your speedo will under read your speed.

I found the speedo always read a bit fast on the 15inch wheels but upon changing to 16 inch wheels it read correct according to the gps speedo .

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[mikebeaches]

I found the speedo always read a bit fast on the 15inch wheels but upon changing to 16 inch wheels it read correct according to the gps speedo .

Interesting - and presumably you gain an extra half inch ground clearance and slightly change (raise?) the driving gear ratio?

 

[Two on Tour]

I change ours from 15" steels to 16" alloys, but there is not a lot of choice if your wheels are 5x118 PCD that are rated for commercial and motorhome weight.


I fitted these. https://www.wheelbasealloys.com/alloy-wheels/supermetal/hammer/grey-polished/16-inch

 

[Emmit]

It will also affect the gearing on the van.
Just as the speedo which may have been optimistic will become either accurate or illegally pessimistic.
(That can be circumvented by installing a digital sat. nav regulated v.cheap speedo)

The gearing difference can be more problematic. It depends on the power to weight ratio of the 'van.
You may find the performance is barely changed when on the flat but on hitting the inclines, you may find that hills that could be taken in a certain gear will not longer be achievable.

I noticed that on our 15" tyred 'van just by putting larger diameter tyres on it with the same profile (ie 70%).
The difference made just 3% difference to the circumference.

 

[Howard H]

Interesting - and presumably you gain an extra half inch ground clearance and slightly change (raise?) the driving gear ratio?

The ground clearance wasn’t noticeable or the gear ratio in forward gears but was very pronounced in reverse .

 

[NorthernSands]

Assuming you have the actual wheel clearance in the wheel arches, the following will occur:

1. You’ll slightly raise the ride height
2. Your speedo reading will lower
3. Your engine will rev. lower at the same speed, in the same gear
4. You’ll have less power (especially at higher speeds in top gear)
5. Fuel consumption may get worse

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[Lucky]

Talk of (seemingly) dramatic and adverse changes arising from a 16" diameter wheel begs the question, how do Fiat get away with it when they offer both 15" and 16" wheels on their vans?

You're surely not suggesting Fiat make all manner of changes to gearbox ratios, speedos and engine builds to compensate.

Or maybe they fit lower profile tyres to offset the change in wheel diameter?

 

[NorthernSands]

Talk of (seemingly) dramatic and adverse changes arising from a 16" diameter wheel begs the question, how do Fiat get away with it when they offer both 15" and 16" wheels on their vans?

You're surely not suggesting Fiat make all manner of changes to gearbox ratios, speedos and engine builds to compensate.

Or maybe they fit lower profile tyres to offset the change in wheel diameter?

They may well fit lower profile tyres to keep wheel diameter the same,I don’t know, but changing the gearbox final drive ratio is relatively trivial. Mechanical speedos also had different gearing to compensate, though most / all modern vehicles will be electronic and have ratios programmed into the ECU, I’d imagine.

Historic example: LR Defender and Discovery had the same engine, gearbox and diffs. Defenders had larger diameter wheels and a 1.4 gearbox final drive. Discos had smaller diameter wheels and a 1.2 gearbox final drive (I say ‘gearbox’ final drive as the actual final drive was the front / rear diffs, both of which 3.54 - I think).

 

[Two on Tour]

I thought the speedo readout on the later Ducato's and Boxers was the ECU matching engine revs to wheel turns per mile.

 

[tacr2man]

Just changing rim diameter does not make any real comment without taking the tyre size into account at the same time , you can get an overall gearing change by either fitting a taller tyre to the fifteen rims or fit 16 rims and a suitable profile tyre . Which route you go may well be governed by the clearances both in diameter and section width for the tyre/wheel combination you are thinking of using . HTSH
The scope of change possible is unlikely to make any real difference to speedo accuracy , as they are usually optimistic on the high side anyway .

 

[Bobby-gg]

Vehicle speedometers are allowed to "over read" by upto 10% but never under read, so when doing an indicated say 70mph, you might actually only be doing 63mph and quite often going up a tyre size, be it a rim size or a aspect ratio quite often brings it closer towards a true reading

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[NorthernSands]

I thought the speedo readout on the later Ducato's and Boxers was the ECU matching engine revs to wheel turns per mile.

Indeed, but it has to know the diameter of the wheels to work out distance covered / speed. This will be pre-programmed based on the rims / tyres fitted. Change the diameter and the ECU won’t know...

 

[Two on Tour]

Indeed, but it has to know the diameter of the wheels to work out distance covered / speed. This will be pre-programmed based on the rims / tyres fitted. Change the diameter and the ECU won’t know...

A case of reprogramming the ECU then which would seem to be the way to go on a lot of vehicles.

 

[NorthernSands]

A case of reprogramming the ECU then which would seem to be the way to go on a lot of vehicles.

If you can. That would solve the speed / odometer change, but not the loss of power at the wheels (if that’s even noticeable). That can only be fixed with a more powerful engine or a gear ratio change.

As for what manufacturers do, they do make it right. I’ve just remembered that we have two nigh on identical cars, but with different wheel diameters: 2009 Subaru Outback and 2009 Subaru Legacy. Same - beautiful - Diesel engine, same gearbox, same rims, different tyre profiles. Both read the same speed in comparison to GPS, both show the same revs at the same speed in the same gear. This can only happen if a) the speedo ratio is different and b) the gear ratios / final drive is different. The Legacy, however, accelerates better.

 

[peterc10]

Indeed, but it has to know the diameter of the wheels to work out distance covered / speed. This will be pre-programmed based on the rims / tyres fitted. Change the diameter and the ECU won’t know...

Speedos can read up to 10% too fast, but they are not allowed to read too low. That gives manufacturers the ability to design their speedo such that it achieves that range whatever wheel diameter or tyre depth (another factor) they specify for the range.

People who uprate their motorhome MGW often have to change their 15" wheels to 16" to get the extra they want. Never heard any of them say that they had to change their speedo, and I am not even sure that Fiat actually have different speedos.

 

[Emmit]

I've posted this elsewhere but before we went to France earlier in the year we bought one of the below as I had changed the profile of the tyres and the speedo was very accurate.
I didn't want to give the french Gov. one cent in fines.
It worked well and and is adjustable to give a tolerance.

Bought it for single figures on Ebay.

It was fitted right in front of my sight on the steering wheel cover immediately in front of the Fiat speedo

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[63720]

Would you advise your insurers and the next owner about the modification?

 

[Howard H]

Would you advise your insurers and the next owner about the modification?

I told the insurance company ,they weren’t bothered at all . They noted it on their files and surprisingly made no extra charge . If we sold it privately I would tell the new owner of the change as I would of all the extras I have fitted .

 

[NorthernSands]

Speedos can read up to 10% too fast, but they are not allowed to read too low. That gives manufacturers the ability to design their speedo such that it achieves that range whatever wheel diameter or tyre depth (another factor) they specify for the range.

True about the under read, but not so different wheel diameters can be fitted. It is partly due to the potential for new tyres, of the same size, to have a slightly larger diameter.

and I am not even sure that Fiat actually have different speedos

Not different speedos, just a change to ratio logic in the ECU (or a change of speedo gear in the gearbox if cable driven). The speedometers are all the same.

 

[Oldgustaf]

Thanks for the comments. What concerns me most is the potential loss of power and potential increase in fuel consumption. I'm surprised manufacturers put 15in wheels on such big vehicles. Is that why most new MHs now have 16in wheels?

 

[Lofty1]

On my last van I fitted 215 65x16 instead of 215 70x15 the speedo registered 1mph more acurate against the sat-nav (still 3mph under the actual speed at 70 mph) and no change in gearing power or fuel economy. I will do the same on this van when the tyres wear out.

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[Two on Tour]

Thanks for the comments. What concerns me most is the potential loss of power and potential increase in fuel consumption. I'm surprised manufacturers put 15in wheels on such big vehicles. Is that why most new MHs now have 16in wheels?

I would not get to concerned about the potential loss of power and potential increase in fuel consumption as I have not noticed one iota of difference since fitting our 16" wheels last year.
As for the speedo reading, ours still reads higher that the actual road speed going by the satnav and dashcam which both show the same speed and I would trust to more accurate being GPS.

 

[Howard H]

As Two on Tour says don’t worry about loss of power , I have not noticed any and we have done 2 big tours with them on . Go for it

 

[Cheshirecat57]

Maybe controversial.....I prefer the ride on 15” wheels, bigger tyre profile = smoother

Notinterested in aesthetics or handling when Im cruising in a van

 

[PeteH]

As for Changes in Power/Fuel consumption? How about re-mapping?. I dont think the change of Wheels will make the Speedometer illegal. And could be offset by a change (possibly) of profile?.

If you go on the Alko website. The Ducacto standard, chassis, can be uprated to its maximum, with a change to 16" wheels. The Maxi Chassis comes with 16" wheels. The code number on the Front Crossmember denotes the Load Range, dependant on the axles.

EDIT:- I just reprised that. And the change to 16" wheels is for the Heavy Chassis. (35H). Changing the wheels to 16" on the light chassis would not uprate any further.

 

[XxAnthxX]

Those running 16” rims, what tyre profiles are you running?

Dropping to 65’s with 16” rims would keep the diameter to within .1” of the original 15” with 70’s on.

Hence no real world difference......That is what I’d be doing.

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