[James T. Kirk©]

According to my TomTom GPS I have a divergence of 6 km/hour, when I drive 120 km/h on stock wheels 225x45x17. I had a divergence with my former Honda on 14" stock wheels and after putting it on 16" rims with 205x45 tires, the deviation shrank to just 1 km/h around 120 km/hour. These cars are nowadays built around the computer. Can't the speedometer be regulated somehow? It really bugs me, knowing I'm way slower then the speedometer tells me. Furthermore, now I know I certainly have less kilometers on the car, then the counter tells me.
Anyone who has insight on the matter? I certainly don't want to put larger tires on this car!

If I want to put winter tires on this car, with, say 16" rims and 205x55 tires, the deviation will be even larger...

 

[Kremmen]

As I understand it it doesn't matter what wheel size you put on because the tyre wall will increase/decrease accordingly so the tyre circumference will still be the same.

It's well known that car speedos overread so the manufacturers can't be held liable in case you are caught speeding.
My 2007 Civic was about 6mph out at 70, my 2009 Civic was 3mph out and my latest 2013 Civic is only about 2mph out.

 

[James T. Kirk©]

As I understand it it doesn't matter what wheel size you put on because the tyre wall will increase/decrease accordingly so the tyre circumference will still be the same.

I don't agree, you can change sizes and it matters:


Tire Size Calculator - Compare Tire Sizes

Tire Size Calculator - Compare Two Sizes

 

[stoney]

Have you tried folding the back seats down

 

[James T. Kirk©]

Have you tried folding the back seats down

[smilie=geesturbed.:

 

[Grayfox]

My civics overread is 4%.
Would like it to be 5% or even 6%

 

[Kremmen]

I'm going to have to backtrack here as there is a difference!

Stock Honda supplied tyres are:

16" Wheels - 205/55R16 - Overall diameter 24.88" - Circumference 78.16"
17" Wheels - 225/45R17 - Overall diameter 24.97" - Circumference 78.45"
18" Wheels - 225/40R18 - Overall diameter 25.09" - Circumference 78.81"

Presumably the standard calibration ensures that all sizes are legal.

 

[James T. Kirk©]

My civics overread is 4%.
Would like it to be 5% or even 6%

How so?

 

[James T. Kirk©]

I'm going to have to backtrack here as there is a difference!

Stock Honda supplied tyres are:

16" Wheels - 205/55R16 - Overall diameter 24.88" - Circumference 78.16"
17" Wheels - 225/45R17 - Overall diameter 24.97" - Circumference 78.45"
18" Wheels - 225/40R18 - Overall diameter 25.09" - Circumference 78.81"

Presumably the standard calibration ensures that all sizes are legal.

Yes, I know but I think it should be possible to change it in the dash computer, which wheels you're running, or update the software. My old car was 3 km/h slower then it showed on the dial, this is 5 or 6 and I think, that's just to much, I'm not driving an Astra for heaven's sake! [smilie=mad.gif]

 

[Jon_G]

How accurate is that TomTom GPS?

 

[Kremmen]

Yes, I know but I think it should be possible to change it in the dash computer, which wheels you're running, or update the software. My old car was 3 km/h slower then it showed on the dial, this is 5 or 6 and I think, that's just to much, I'm not driving an Astra for heaven's sake! [smilie=mad.gif]

All cars are the same these days, none will equal the GPS readout due to liability and they all over read.

However, if you want to make an adjustment then perform a search here for "Speedo Healer" as it can be done.

 

[baldbiker]

Speedo healers used on bikes with digital speedo. I would rather over read as less chance of getting pulled for speeding.

 

[James T. Kirk©]

Accuracy of a GPS system?

How accurate is that TomTom GPS?

How accurate is any GPS? Anyway, driving in the USA with a GPS equipped Dodge Caravan used for satellite radio, but also for the speedo, both Tom Tom (I have a EU+USA version) I saw both gave same speed all the time...

 

[James T. Kirk©]

All cars are the same these days, none will equal the GPS readout due to liability and they all over read.

However, if you want to make an adjustment then perform a search here for "Speedo Healer" as it can be done.

Now, you're talking! Thank you. But it's not as simple as I hoped for.

 

[baldbiker]

Now, you're talking! Thank you. But it's not as simple as I hoped for.

http://www.civinfo.com/forum/how/8346-calibrate-speedo.html

 

[Grayfox]

How so?

I use torque to get the VSS speed(true speed) and watch the speedometer.

There is pretty much a constant 4% over read on my car.

50km/h true, 52km/h on speedo
75km/h true, 78km/h on speedo
100km/h true, 104km/h on speed

 

[James T. Kirk©]

Speed is much closer to reality by using 18" standard wheel/tire combo!

Using several tire size calculators, I have come to the conclusion that I can easily mount 225x50x17 tires on my car. The difference in speedo reading between the standard RECOMMENDED 18" Honda wheel/tire combo and the standard 17" Honda wheel at 120 km/h is 3,79%, in general it means 4,73 km/h more then with the 17" wheel/tire combo!

Interestingly at Tire Size Calculator - Compare Two Sizes it says:

The overall diameter variance of the new tire is 25 millimeters , which is 3.94% different than the original. You should never go beyond 3% of the original tire, otherwise you run the risk of brake failure.

If I mount the 225x50x17 tire on my car, it will be 1 mm(!!) higher then the RECOMMENDED 225x45x18 tire of the 18" Honda wheel and the speed difference will be 0,15%, or 120 km/h will be 120,18 km/h.

In other words, the changing of the circumference from 45 to 50 on my 17" wheel will make the tire 1 mm higher then a stock 18" Honda wheel/tire, but brings the speedo with 4,35 km/h, way closer to the original speed as checked with GPS.

Also check: 225/50-R17 vs 225/45-R18 Tire Comparison - Tire Size Calculator

 

[Nesher666]

Just make sure you remember the more precise speedo rating to make you happy when your spine is shaking heavily with the 18" wheels. [smilie=shake.gif]

[smilie=pimp.gif]

 

[James T. Kirk©]

Just make sure you remember the more precise speedo rating to make you happy when your spine is shaking heavily with the 18" wheels. [smilie=shake.gif]

[smilie=pimp.gif]

Oh, I won't buy and mount the 18", I think the car is already way to uncomfortable on the 225x45x17 and next time I need new tires I probably go 225x50x17. Same with winter tires. 205x55x16 is way to low for the car's speedo, if you check the divergence between the 18" and the 16" recommended sizes there is a -4,1% difference between them, the 16" is 2,7 cm lower(!!!!) then the recommended 18" wheels! You can go 205x60x16 or even better 225x55x16, which makes it way less! Someone at Honda's measurements team was not paying attention! [smilie=dunce.gif]

 

[Monstermind]

Have you tried folding the back seats down

Now why didn't I think of that.