Speedometer

[John GRAHAM]

I recently bought a Lexus is250 but very quickly discovered the speedo was widely optimistic to the tune of 8% When I contacted Lexus for there opinion I got the impression that it was deliberate and could not be adjusted. I am therefore considering altering the tyre diameter. Actually a 245/55 R17 has exactly an 8% increase in circumference and would bring the speedo in to line with reality. I would invite comments on this plan. John HG

[Linas.P]

Hello John and welcome,

Up-to 10% over real speed is standard deviation and it is mandated by international law - that means speedometer must overstate the speed. There are complex formulas to calculate what exact margins are acceptable, but I guess for this discussion it is enough to say 8% is normal. And you right, from my memory my IS250 used to have margin of ~8% @~50MPH, up-to 10% at lower speeds and then something completely out of order over 100MPH.. I think it was indicating ~160MPH, when according to GPS I was doing 225km/h on autobahn (which is ~139).

This is as well the case on any car I owned, but you right to say Lexus is above average. Most of my other cars were closer 5% and not 10%.

If that is big issue for you, then I guess having bigger tyres could make it closer to real speed. 245/55 will definitely be fine for rear, but what is your plan for front? Ideally, you should keep them 225, maybe maximum 235 as they may scrub on full lock. Unless you planning to keep front as standard and have "very positive" rake angle - not sure how much it matters for you, but it may look little bit odd from the appearance (especially considering overall IS250 design is already on positive side). This may even affect car balance in cornering a little bit and will impact your fuel consumption - expect the car to use more fuel in general.

[Herbie]

You don't want it to be too accurate. A sneeze or cough and an accidental pressing of the loud pedal at the wrong moment may just get you caught by those ever-watchful cameras.

[Texas]

I rather it over read by a bit as opposed to being bang on also!

[Britprius]

Increasing tyre size "circumference" by 8% will also have the effect of increasing brake pedal pressure by a similar amount. This is because you are increasing the leverage on the brakes by increasing the radius of the wheel without increasing the radius of the brake disc.
Not suggested by you but fitting larger tyres on the rear only will likely cause ABS problems.

Three percent is the maximum recommended tyre rolling radius increase. Going over this "using non standard tyre sizes listed by the car manufacturer" could affect any insurance claim.

John.

[Guest bigbullhead]

I go through speed camera at 43 as my gps says it’s 40

[NemesisUK]

They are speed limits, not targets ...

[Linas.P]

3 hours ago, NemesisUK said:

They are speed limits, not targets ...

Based on completely arbitrary and unjustified rules and thus cannot be respected.

[NemesisUK]

1 minute ago, Linas.P said:

Based on completely arbitrary and unjustified rules and thus cannot be respected.

You don't have to respect them, just don't exceed them...

[Linas.P]

Just now, NemesisUK said:

You don't have to respect them, just don't exceed them...

I think we all can decide here what we want to do and are prepared to face consequences of it?

The topic is about speedometer accuracy and not what we think about limits. You were off-topic, I was off-topic - I hope we can agree we could end this here.