Speedo calibration

[LukeZS96]

Just noticed coming down the motorway that at 120 KM/H the revs sat at about 3.5K RPM. Normally in or around the 4K mark so I got my girlfriend to get the speed readout on the GPS on her phone and it read 110.4 KM/H when the car was reading 120. I did get the front tyres changed and I had to go from tyres with a 40 sidewall to a 45 sidewall, same as what's on the back. Would this throw the speedo out by so much or am I looking at something else? Speedo is normally out by maybe 1 KM/H+/-

[Roverlike]

I would disagree. Speedo is calibrated to have discrepancy of - 1km/h at low speeds, but as you climb to higher speeds, it would have much greater discrepancy, of even 10% at highway speeds. From my point of view your speedo is reading correct.
In case of factory setup, including tyres, your speedo will always show greater speed then it usualy is.

With change of tyres diameter you might have greater descepancy then before.

GPS reading is also with discrepancies, because public GPS must not have precision like military one. So GPS reading will be more correct than speedo reading, but still it will not be accurate.

[talkingcars]

And shouldn't the tyres be 45 profile anyway?

[LukeZS96]

They are 45s on the back alright but I was told by an ex Rover dealer that the 1.4s had 40s on the front and that's what was always on the front wheels. I know speedos aren't totally accurate but when I seen how much the road speed was differed in relation to the engine speed I thought the tyres might be affecting it

[Mark S]

speedos can overread by as much as 10% and underead by 1%, rev counters lie also, my itr rev counter revs to 9k but it overreads by 400rpm!!

[talkingcars]

Despite having the correct size tyres my Espace used to read 30mph when it was only doing 22mph.