Wheel and tyre size changes

[Malc1]

Just reflecting on my " new "  Mk1 Ls400 and the wheel and tyre changes from the original 15" some time back 

She's now on 16" wheels and 225/55/16 tyres ..............  and drives fine

The 16" wheels are with appropriate sized bolt shanks I'm advised 

HOWEVER I'm not at all convinced that the speedo is reading quite correct miles covered 

Does anyone know with any certainty what the formula must be for this change of wheel size etc ?

At the moment I'm simply relying on the prior tyre advisers and providers to have sold the correct stuff .......  I'm a bit loathe to visit any old tyre place for their thoughts .....  confidence wouldn't be inspirational !

Is there a magic but correct formula ?

Thanks for your thoughts everyone

 

Malc

 

[lex - eleven]

Can't you check your speed against a mobile phone sat nav? There's bound to be a slight difference anyway

[Malc1]

2 minutes ago, lex - eleven said:

a mobile phone sat nav

my very recent experience last Friday of this distance monitoring tells me it can be simply quite  " vague "

But thanks !

Malc

[BigBoomer]

This is a good site for comparing tyre sizes.
https://tiresize.com/comparison/
It looks like the 225/55-R16 is the closest to the OEM size on a 16" rim.
With the 225/55-R16 you are actually reading slightly less speed and miles than with the OEM tyre size.

[Malc1]

Thank you so much George  ........  I'll take super care in the new 20mph speed restriction areas popping up around here then  🥲       but won't worry too much about the MOT speedo shortfall reading 😃

Malc

[lex - eleven]

I can't believe the originals were 15's 

 

[BigBoomer]

In 1989 15" was considered a large wheel size. My 1993 Carlton Diplomat had 15" wheels.
More sidewall gives better comfort but affects handling as the sidewalls also move sideways.
On later cars we have MUCH better suspension so can use larger wheels and smaller sidewalls to give better handling but still keep the comfort,... mostly. 😁

[scythe]

As Mark suggested above, there are free apps for Android and iOS that will measure and display speed very accurately.

: o )

[Rabbers]

7 hours ago, Malc1 said:

HOWEVER I'm not at all convinced that the speedo is reading quite correct miles covered 

Does anyone know with any certainty what the formula must be for this change of wheel size etc ?

I would guess the best people to ask if and by how much you would need to have the speedo recalibrated after fitting bigger tyres would be Michelin’s customer service via email.  Not that any understatement of speed is likely to exceed the small tolerance you are allowed over a given limit.

[ReflexSonic]

Unfortunately i checked my speedo against a gatso last week and found that i had a 11mph discrepancy 😞

[BigBoomer]

I use a TomTom SatNav and it displays a much more accurate speed than the LS430 speedo on a straight road at constant speed. My speedo consistently reads high compared to the SatNav by about 2Mph at 30Mph and 5Mph at 70MPH and I am on factory wheels and tyre sizes. If you fit wheels and tyres that have a different rolling radius from the factory size, then you REALLY need to check the calibration of your speedo and drive appropriately. As others have said, you can get speedo apps for Android and apple phones that are pretty accurate when at a constant speed on a straight road so you can find out how accurate your speedo is, if you don't have an aftermarket SatNav. Almost all speedos over-read (eg. at a real speed of 70Mph the speedo shows 75Mph) and the older ones tend to be worse.

[lex - eleven]

So really if you just keep to your speedo reading you should always be under the speed limit

[Malc1]

Lucky I'm not worrying too much about my fuel consumption with speed these days

She goes from zero to lots of mph very very quickly and stops pretty quickly too  .....  not quite as fast as my Mk3 and with new Mk3 pads all round, well, the Mk3 is a " stopper " for sure 

Malc