Wheel and tyre question

[merl]

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.

Strange . Mechanicals on vehicles aren't my forte but unless the vehicle has some form of tracking to ascertain the actual real speed I cant (at the moment) see how the speedo would know the 'new' speed with the change of tires. As far as I know the final output on the gearbox drives some sort of sensor (hall effect/opto/etc (on Trevs Skoda it still probably drives a cable) and that effectively governs the speed displayed after taking into account any other fixed gearing and tire circumference.

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

Mark, Do you mean speedo and GPS Vs Revcounter.
or speedo vs GPS on one tire and then speedo vs GPS on the other tire?

 

[mark61]

Strange . Mechanicals on vehicles aren't my forte but unless the vehicle has some form of tracking to ascertain the actual real speed I cant (at the moment) see how the speedo would know the 'new' speed with the change of tires. As far as I know the final output on the gearbox drives some sort of sensor (hall effect/opto/etc (on Trevs Skoda it still probably drives a cable) and that effectively governs the speed displayed after taking into account any other fixed gearing and tire circumference.

Mark, Do you mean speedo and GPS Vs Revcounter.
or speedo vs GPS on one tire and then speedo vs GPS on the other tire?

Speedo vs GPS.

 

[merl]

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

Online calc says there's virtually no difference in circumference between your new wheels and originals (about half a percent) so nothings changed anyway!

 

[mark61]

Strange . Mechanicals on vehicles aren't my forte but unless the vehicle has some form of tracking to ascertain the actual real speed I cant (at the moment) see how the speedo would know the 'new' speed with the change of tires. As far as I know the final output on the gearbox drives some sort of sensor (hall effect/opto/etc (on Trevs Skoda it still probably drives a cable) and that effectively governs the speed displayed after taking into account any other fixed gearing and tire circumference.

Mark, Do you mean speedo and GPS Vs Revcounter.
or speedo vs GPS on one tire and then speedo vs GPS on the other tire?

It doesn't, that's why there is an error, but then the speed shown with original tyres/wheels has a built in error too.

 

[merl]

Speedo vs GPS.

Of course, I would expect that to change.
Do you have a rev counter and if so can you say if the relationship between the 2 has changed? (I'd expect that to remain exactly the same, I don't see how that can be affected)

 

[Tim120]

Might have a knock on effect for your insurance.
I asked my insurance few months back about fitting an inverter and lithium battery, not a problem.
Asked what they would be interested in and wheels was of interest to them, amongst a few bits that really wouldn't apply to a MH.
My guess is, if you've got a hot hatch or whatever and want to make it hotter then wide/lower profile etc would be of interest.
Not applicable in this case, but they may be interested.

 

[mark61]

Of course, I would expect that to change.
Do you have a rev counter and if so can you say if the relationship between the 2 has changed? (I'd expect that to remain exactly the same, I don't see how that can be affected)

Been a long time since I've has standard size tyres on, but I would expect the relationship to be the same too, I think.

 

[wildebus]

Of course, I would expect that to change.
Do you have a rev counter and if so can you say if the relationship between the 2 has changed? (I'd expect that to remain exactly the same, I don't see how that can be affected)

Not sure exactly what you are meaning. I will give you a particular example from a past experience. This is from memory, so the numbers will not be 100% accurate,but close enough to show what I am trying to say ....

VW T4. Was fitted in the past with some Audi Saloon Car Alloys (under load rated) and standard tyres.
I used the vehicle as was for a time and from the Sat Nav, knew what the speedo displayed for a given true - GPS - Speed, and what the Tachometer said.
I changed the Wheels for more appropriate ones with the right load rating and fitted larger Continetal AT Tyres (so in-between road-orientated and off-road tyres) for a greater rolling radius very deliberately
Photo of old and new so you can see the difference ....
Old
old by David, on Flickr
New
imagejpg1-5_1 by David, on Flickr
(you can see the difference between the tyres by looking at the gap front and back (top means nothing as changes with weight in rear))

With the new - bigger - tyres, for a given true speed (I think we can agree a GPS speed is going to be accurate?), I noticed the following:
The Speedo would read lower than before (IRRC, it actually became spot on rather than exaggerating the speed)
The Rev Counter at Motorway cruising speed - around 65 for this old girl - was around 300 RPM than previously seen (The reduced engine noise was very nice when travelling for 8+ hours from Scotland to South-East England!).
No difference in economy (Should maybe have been slightly better given lower engine revs, but new chunkier tyres probably countered that)

 

[colinm]

No difference in economy (Should maybe have been slightly better given lower engine revs, but new chunkier tyres probably countered that)

There are a lot of variables to consider, but even if the tyres had the same rolling resistance the energy needed to travel at any given speed will be the same, so it would depend more on how efficient the engine was at any given speed, this can mean that in some circumstances a lower engine speed could use more fuel, but as you say in your case it's more likely the tyres.

 

[merl]

Not sure exactly what you are meaning

I think I know where things got confusing. I originally said "Using the dash tacho / speedo there will be no difference whatsoever"
What I trying to say to RSD7a was this:
Dont expect to see a change in relationship between what the rev counter says Vs what the speedo says because those 2 devices are inextricably linked by the mechanics of the gearbox.
So if your rev counter reads 2500rpm when the speedo says you're doing 70mph (for example) then that WONT change when you change tire diameter.
Right bummer at times this ere tinternet fingy stuff but I think we're all on the same page at last

 

[mark61]

I think I know where things got confusing. I originally said "Using the dash tacho / speedo there will be no difference whatsoever"
What I trying to say to RSD7a was this:
Dont expect to see a change in relationship between what the rev counter says Vs what the speedo says because those 2 devices are inextricably linked by the mechanics of the gearbox.
So if your rev counter reads 2500rpm when the speedo says you're doing 70mph (for example) then that WONT change when you change tire diameter.
Right bummer at times this ere tinternet fingy stuff but I think we're all on the same page at last

Yes I think that is right.

Revs would drop in relation to the (true'ish) speed shown on GPS, but not to the speed shown on the vehicles speedo.

Doesn't matter where the speed is calculated from, drive shaft or ABS sensor, whether electronic or mechanical, barring any slack in torque converter etc that relationship will remain constant.

I know some vehicles like 2001 onwards G Wagen, have an option to change tyre size via the command system (or whatever they called their computer stuff)

 

[merl]

There are a lot of variables to consider, but even if the tyres had the same rolling resistance the energy needed to travel at any given speed will be the same, so it would depend more on how efficient the engine was at any given speed, this can mean that in some circumstances a lower engine speed could use more fuel, but as you say in your case it's more likely the tyres.

And of course 1st gear is now slightly higher so I'd imagine pulling off will be slightly less efficient?
I must admit the drop in revs when cruising is quite appealing, bloody fiats are so noisy. I borrowed a mates Movano at the weekend, just a builders van but way quieter than our Ducato

 

[wildebus]

Yes I think that is right.

Revs would drop in relation to the (true'ish) speed shown on GPS, but not to the speed shown on the vehicles speedo.

Doesn't matter where the speed is calculated from, drive shaft or ABS sensor, whether electronic or mechanical, barring any slack in torque converter etc that relationship will remain constant.

I know some vehicles like 2001 onwards G Wagen, have an option to change tyre size via the command system (or whatever they called their computer stuff)

something that I found interesting on my Toyota RAV4 looking at the difference between my Winter Tyres and Standard Tyres ....
The speedo reading used to change slightly compared to the real (GPS) speed due to the change in rolling diameter as we have discussed. But .... when I looked at the Vehicle Speed using the data from the EOBD (so coming from the vehicle) it knew exactly what the speed was (and was not the same as the speedo). I did wonder why they could use not that information for the speedo?

 

[RSD7a]

I think I know where things got confusing. I originally said "Using the dash tacho / speedo there will be no difference whatsoever"
What I trying to say to RSD7a was this:
Dont expect to see a change in relationship between what the rev counter says Vs what the speedo says because those 2 devices are inextricably linked by the mechanics of the gearbox.
So if your rev counter reads 2500rpm when the speedo says you're doing 70mph (for example) then that WONT change when you change tire diameter.
Right bummer at times this ere tinternet fingy stuff but I think we're all on the same page at last

.... Phew

 

[1 Cup]

Good tyres them michelin crossovers / Michelin Agilis Cross Climate tyres but get the ones with C on them as your size are used on 4x4. but my sprinter came with × 4 = 235 65 16.they (internet) want £400.00 ish for 2.agilis quote started a £150 each for 4x4.

 

[Nabsim]

Good tyres them michelin crossovers / Michelin Agilis Cross Climate tyres but get the ones with C on them as your size are used on 4x4. but my sprinter came with × 4 = 235 65 16.they (internet) want £400.00 ish for 2.agilis quote started a £150 each for 4x4.

I did okay on my Conti Four Seasons up on top car park on Cairngorms yesterday and today. Not a single spin and they have been on 5 years now lol. Good job really seeing as I can only fit Conti’s

 

[rugbyken]

my b544 originally came with 15” wheels as part of the upgrade in weight 16” wheels were advised along with bellows etc, my van always ran about 8% lower speed according to the sat nav v speedo with the new 16” wheels this has almost completely corrected itself , there was a german guy selling the wheels on e bay brand new supposed from a converter who changed the wheel size down to improve performance, all i know is a set of wheels with brand new tyres and wheel trims shipped from germany were considerably cheaper than a set of tyres locally

 

[Robmac]

I did okay on my Conti Four Seasons up on top car park on Cairngorms yesterday and today. Not a single spin and they have been on 5 years now lol. Good job really seeing as I can only fit Conti’s

Is it ok overnighting on the top car park Neil?

 

[alwaysared]

my b544 originally came with 15” wheels as part of the upgrade in weight 16” wheels were advised along with bellows etc, my van always ran about 8% lower speed according to the sat nav v speedo with the new 16” wheels this has almost completely corrected itself , there was a german guy selling the wheels on e bay brand new supposed from a converter who changed the wheel size down to improve performance, all i know is a set of wheels with brand new tyres and wheel trims shipped from germany were considerably cheaper than a set of tyres locally

Hi Ken, (@rugbyken) What size tyres did you fit?

Thanks for all the replies, I called Michelin today and they said I needed to speak to either Hymer or Fiat about changing from 15" to 16" and they also said I should use CP not C tyres but I'm thinking he was only trying to convince me to buy their Cross Climte Camping tyres.

Regards,
Del

 

[Nabsim]

Is it ok overnighting on the top car park Neil?

Yes Rob, insummer it’s a motorhome stop over and you pay, there are bins, drinking water and elsan point. Bins were locked up and everything else shut off while I was there. Wasn’t after services though so didn’t matter