Ducato speedo inaccuracies.

[Happy Jack]

Think most only go to 116 in that size

Think I saw the Fallken 225/75/16 at 121.
But, as reply couple minutes ago, 15" under consideration.
Axles are 1600 front, 1500 rear.
Don't need 121 rating.
108 minimum I think but 112 would be good to have slightly lower tyre pressures

 

[Happy Jack]

My 225/70/15 s are 112/110R

View attachment 967311

Would be good for me.
Is the rude less bumpy?

 

[Happy Jack]

Speedo information is normally taken from a driveshaft sensor.

So that means speed info is independent from tyre size?

 

[Carpmart]

So that means speed info is independent from tyre size?

360 degrees is always 360 degrees… so one rotation of a drive shaft is one rotation of a drive shaft. The distance you travel varies based on the rolling radius of the tyre. Hence final drive and gearing and speedo reading are all linked to the rolling radius. Have a look at my previous post as it explains it!

 

[Landy Andy]

Would be good for me.
Is the rude less bumpy?

Yes. A lot better ride, and a LOT more grip. Even v good with standing water.

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[Happy Jack]

If in no rush, plenty of people swap across to alloys, and sell their 16” wheels if that is the direction you wish to go. I notice the gearing change when swapped from 215 15, to 225 15, so I don’t know if I would now swap to 16” wheels.

Been on tyre size calculator website.
215/70/15 (my present tyre), 752 revs/mile.
225/70/15. 736. 2.1%
215/75/16. 703. 6.5%
225/75/16. 689. 8.4%
My percentage difference from speedo to sat nav is 7.7%.
Not really bothered with overeading speedo.
Think I will be going for 225/70/15 or 215/75/16.

If in no rush, plenty of people swap across to alloys, and sell their 16” wheels if that is the direction you wish to go. I notice the gearing change when swapped from 215 15, to 225 15, so I don’t know if I would now swap to 16” wheels.

 

[Happy Jack]

360 degrees is always 360 degrees… so one rotation of a drive shaft is one rotation of a drive shaft. The distance you travel varies based on the rolling radius of the tyre. Hence final drive and gearing and speedo reading are all linked to the rolling radius. Have a look at my previous post as it explains it!

Rolling radius in practice makes a minor difference.
as long as the speedo never undereads.
Happy that the suppliers have confirmed that bigger diameter tyres will pull the speed discrepancy nearer to correct.

 

[Carpmart]

Rolling radius in practice makes a minor difference.
as long as the speedo never undereads.

Where do you get that from? Rolling radius ‘in the real world’ makes a noticeable difference, particularly to final drive, so your gearing ratios! It’s most important in larger heavy vehicles (Motorhomes) and performance/race cars. I accept in the average car it makes little difference. Ask a biker if they can tell the difference with a couple of cogs either way on a rear sprocket! All of them without exception will tell you that they absolutely can!


Why is it so important that the speedo doesn't under read? All manufacturers had this baked into their vehicles. You’ve a satnav giving you accurate speed! Why is it so imperative you have a speedo showing 70 when you’re doing 72?

 

[Happy Jack]

Where do you get that from? Rolling radius ‘in the real world’ makes a noticeable difference, particularly to final drive, so your gearing ratios! It’s most important in larger heavy vehicles (Motorhomes) and performance/race cars. I accept in the average car it makes little difference. Ask a biker if they can tell the difference with a couple of cogs either way on a rear sprocket! All of them without exception will tell you that they absolutely can!


Why is it so important that the speedo doesn't under read? All manufacturers had this baked into their vehicles. You’ve a satnav giving you accurate speed! Why is it so imperative you have a speedo showing 70 when you’re doing 72?

Was referring to rolling radius compared to tread depth, mistakenly thought you were referring to "as the tyre wire down".
Didnt say it was important to me that it doesent underead.
Was simply quoting a previous post where someone said it was not allowed to underead.
All I said was that it would be good if the speedo read correctly.
Happy that bigger will get it nearer, not imperative for it to be spot on.

 

[gus-lopez]

It's an intriguing look into others' thought processes as I've never even thought about it. In any case it really wouldn't concern me that although my speedo read 70 I was actually travelling at 65mph.

Apart fro m when you are in 116 miles of road works & in the 50's they manage to drop speed to 32mph & in the 40 limit we were in second gear at one point.No traffic, 3 open lanes, no works of any description in the Uk this summer. what was a hour & 40 minute journey ended up at nearly four hours. I set the sat nav/dash cam/heads up at the limit on the gps.

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[Carpmart]

Was referring to rolling radius compared to tread depth, mistakenly thought you were referring to "as the tyre wire down".
Didnt say it was important to me that it doesent underead.
Was simply quoting a previous post where someone said it was not allowed to underead.
All I said was that it would be good if the speedo read correctly.
Happy that bigger will get it nearer, not imperative for it to be spot on.

Your mixing your under and over reading up.

Under reading is baked into speedometers. Increase the rolling radius with bigger tyres and you read closer to true speed. Over reading is where the speed displayed is lower than actual speed!

 

[Happy Jack]

Your mixing your under and over reading up.

Under reading is baked into speedometers. Increase the rolling radius with bigger tyres and you read closer to true speed. Over reading is where the speed displayed is lower than actual speed!

Woops.
I genuinely thought overeading was when the speedo showed higher than actual speed