Speedo reading

[Barkz71]

Hi all, I have just gone to s 245 75 16 from 235 70 16 on my 02 Td5 Disco. Has anyone got an idea how much the speedo will be out if any? I thought there was a chart but couldn't find one when searching. Cheers Brett

[mrs]

About 5% from memory

[Slunnie]

Yep, 5% (5.2%)

[sierrafery]

5.2% according to this tool Tire Size Comparison (https://tiresize.com/comparison/)

[DiscoMick]

A GPS used while driving should show the actual speed.

[trout1105]

A GPS used while driving should show the actual speed.

Be careful when relying solely on a GPS for your speed because they Can be wrong at times.
I recently fitted a HUD GPS unit in my Navara and this gives me a reading that is higher ( about 2-3% faster) to the speedo reading in the truck But the HN7 unit is saying that I am travelling 5-6% slower.
So at a speedo reading of 100kph the HUD is saying 102-103kph and the HN7 is saying 94-95kph.
I have 265/75/16 tyres fitted to the Navara But a 70 profile is stock which should give me a faster speed than the speedo is telling me so I am pretty sure that the HUD unit is more accurate than the GPS readout on the HN7 unit.
I have a spare Garmin street GPS that I might hook up and see what that reads at out of curiosity.

If you are travelling AT the speed limit then it is critical to make sure you KNOW exactly what speed you are travelling at But as I generally travel about 5kph slower than the posted limit I haven't had a problem with the boys in blue.

[AK83]

also be careful to differentiate between theoretical, and actual % differences.

Using the calculator, it calculated that when I went from 255/70 to 245/75's I'd have a 1.5% drop in indicated speed, whereas I a 1% increase in the speedo reading.
There will be differences between tyre manufacturers .. most likely due to the design of the tyre.

If you have a smartphone, there are many apps that can use the GPS in the phone(if it has one) to give you an idea of your speed differences.
For Android I use GPS Test.
I think some iPhones don't have GPS built in tho.

[V8Ian]

Be careful with all aspects, tyre diameter will vary from model and brand to model and brand, making theoretical calculations iffy. Tyre pressure will also effect this.
GPS is historical information, generally close enough to keep you out of trouble, but particularly unreliable in heavily treed or steep (cuttings etc.) terrain. Tall buildings and tunnels can and do interupt the signal.
The most reliable way to mentally calibrate the speedo is on a level road note its reading after the GPS has remained constant for at least 5 seconds.

[rick130]

also be careful to differentiate between theoretical, and actual % differences.
There will be differences between tyre manufacturers .. most likely due to the design of the tyre.



Totally, and sometimes differences between batches of the same make/model.

The indicated the size of nominal, not actual.

[DiscoMick]

Yes, all true. Another factor is the margin of accuracy in GPS. I read that the military signal is much more precise than the civilian version, but I can't remember the details.

[pgsaapl]

Hi,


I made the same tire transition on my 2000 D2 TD5 3 months ago (to Pirelli Scorpion AT255 70R16's, and it has resulted in a ~ 3% increase in actual speed, relative to the speedo readout (checked using GPS speedometer apps). Being a nerd, I created a spreadsheet to estimate the like change in rolling radius, and came up with a similar number (diameter increase of about 1"). I do a lot of country kms, and reckon it has made cruising at 100-110km a bit easier for all concerned.


Paul

[grizzz]

I made the same tyre change and it appears the speedo now reads correct.
This is done by comparing with a GPS and the speed indictator signs that display your speed. Actually more than 1 of these
With all 3 being with in 1 or 2 km/hr they cant all be equally wrong can they?

[AK83]

I made the same tyre change and it appears the speedo now reads correct.
....

That's normal.
The std tyres will give a higher speedo reading that actual(say 5% or whatever) .. and the increase in tyre diameter(eg. 235/70 to 245/75) will correct that.
So the end result is usually correcting the speedo to read more accurately.

The thign to watch for tho is the expected difference to actual difference .. which is accounted for by the actual tyres fitted.
I think Rick said, these variations could be from the same manufacturer on different batches of tyres of the same model too.

My brothers D2 has 245/75 BFG KO2s and it reads 5km/h slower on the speedo than actual now.
Whereas my D1 on it's new Chinese Hifly 245/75s' now shows 0-1km/h faster than actual .. both at 100k/h.

[discodude94]

Hi all, I have just gone to s 245 75 16 from 235 70 16 on my 02 Td5 Disco. Has anyone got an idea how much the speedo will be out if any? I thought there was a chart but couldn't find one when searching. Cheers Brett
148617