17" to 18"- speedometer reading affected?

[Megabucks38]

my 320I, just change tyre from 17" to staggerred 18".Feels like the car is moving faster but speedometer remains the same. eg 120km/hr feels like 130km/hr!Am I hallucinating or bigger wheels affests the speedometer accuracy?

 

[astroboy]

Depending on the tyre size. From what to what? Provide me the measurement, I calculate for u.

 

[bimmerdude]

yes of course bigger wheels affects ur speedo reading. go to tirerack.com to find out more.

 

[t2ribena]

maybe the tyre you bought is not the correct size to stay within the 3% rule when going 1 size up...larger diameter mean larger circumference thus affecting the fixed speedo meter...

based on your observation your speedo is under-reading and this means ur wheel/tyre overall diameter/circumference larger than the original wheel it designed for...check the thread width and series...normally if u get the same width, the series should be lower then for bigger rims...

 

[astroboy]

Just give me the tyre size (before and after) and I will give u your exact variance in rolling circumference in mm.

 

[Mouse]

use online tyre calculator make sure u stay within the 3% rule. however, its better to over-read the speedo than under-read. u dont want to be travelling at an actual speed of 120km/h but your speedo telling u 110km/h only, then kena saman lor

 

[astroboy]

Ok. Let's simulate,

Specification Sidewall Radius Diameter Circumference Revs/km Difference

225/45-17 101mm 317mm 634mm 1993mm 502 0.0% (OEM SIZE)

225/40-18 90mm 319mm 637mm 2002mm 500 0.5%
235/40-18 94mm 323mm 645mm 2027mm 493 1.7%
245/35-18 86mm 314mm 629mm 1975mm 506 -0.9%
255/35-18 89mm 318mm 636mm 1997mm 501 0.2%
265/35-18 93mm 321mm 643mm 2019mm 495 1.3%
275/30-18 82mm 311mm 622mm 1955mm 512 -1.9%
285/30-18 86mm 314mm 628mm 1974mm 507 -1.0%

225/35-19 79mm 320mm 640mm 2011mm 497 0.9%
235/30-19 70mm 312mm 624mm 1959mm 510 -1.7%
245/30-19 74mm 315mm 630mm 1978mm 506 -0.7%
255/30-19 76mm 318mm 636mm 1997mm 501 0.2%
265/30-19 80mm 321mm 642mm 2016mm 496 1.2%
275/30-19 82mm 324mm 648mm 2034mm 492 2.1%
285/25-19 71mm 313mm 625mm 1964mm 509 -1.5%

225/30-20 68mm 322mm 643mm 2020mm 495 1.4%
235/25-20 59mm 313mm 626mm 1965mm 509 -1.4%
245/25-20 61mm 315mm 630mm 1981mm 505 -0.6%
255/25-20 64mm 318mm 636mm 1996mm 501 0.2%
265/25-20 66mm 320mm 640mm 2012mm 497 1.0%
275/25-20 69mm 323mm 646mm 2028mm 493 1.8%
285/25-20 71mm 325mm 650mm 2044mm 489 2.6%

225/25-21 56mm 323mm 646mm 2029mm 493 1.8%
235/25-21 59mm 325mm 651mm 2045mm 489 2.6%
245/25-21 61mm 328mm 656mm 2061mm 485 3.4% (OUT!!)

Where's our speedo sensor? Gearbox? or drive by wire, using our DSC wheel revolution sensor to determine our speed/odo? If its like JDM, its at the gearbox, so only the driving (not steering) wheel size will affect the speedo/odo. FWD, then its front wheel size. RWD, then its rear wheel size, that will affect our speedo/odo reading.

Once your odo is affected, the FC calculation and oil change interval differs accordingly, base on the above percentage.

 

[turbology]

Provide me the measurement, I calculate for u.

Just give me the tyre size (before & after)

Ok. Let's simulate

LOL...
Good Job AstroBoy

One more thing, changing your tires rolling diameter (outter size) will affected your final gear ratio too.
Going bigger rims will add more rotating mass, which decrease your acceleration time.
Since the tires sidewalls got thiiner, you will feel more bumps, and could suffer more tramlining

 

[Megabucks38]

Ok. Let's simulate,

Specification Sidewall Radius Diameter Circumference Revs/km Difference

225/45-17 101mm 317mm 634mm 1993mm 502 0.0% (OEM SIZE)

225/40-18 90mm 319mm 637mm 2002mm 500 0.5%
235/40-18 94mm 323mm 645mm 2027mm 493 1.7%
245/35-18 86mm 314mm 629mm 1975mm 506 -0.9%
255/35-18 89mm 318mm 636mm 1997mm 501 0.2%
265/35-18 93mm 321mm 643mm 2019mm 495 1.3%
275/30-18 82mm 311mm 622mm 1955mm 512 -1.9%
285/30-18 86mm 314mm 628mm 1974mm 507 -1.0%

225/35-19 79mm 320mm 640mm 2011mm 497 0.9%
235/30-19 70mm 312mm 624mm 1959mm 510 -1.7%
245/30-19 74mm 315mm 630mm 1978mm 506 -0.7%
255/30-19 76mm 318mm 636mm 1997mm 501 0.2%
265/30-19 80mm 321mm 642mm 2016mm 496 1.2%
275/30-19 82mm 324mm 648mm 2034mm 492 2.1%
285/25-19 71mm 313mm 625mm 1964mm 509 -1.5%

225/30-20 68mm 322mm 643mm 2020mm 495 1.4%
235/25-20 59mm 313mm 626mm 1965mm 509 -1.4%
245/25-20 61mm 315mm 630mm 1981mm 505 -0.6%
255/25-20 64mm 318mm 636mm 1996mm 501 0.2%
265/25-20 66mm 320mm 640mm 2012mm 497 1.0%
275/25-20 69mm 323mm 646mm 2028mm 493 1.8%
285/25-20 71mm 325mm 650mm 2044mm 489 2.6%

225/25-21 56mm 323mm 646mm 2029mm 493 1.8%
235/25-21 59mm 325mm 651mm 2045mm 489 2.6%
245/25-21 61mm 328mm 656mm 2061mm 485 3.4% (OUT!!)

Where's our speedo sensor? Gearbox? or drive by wire, using our DSC wheel revolution sensor to determine our speed/odo? If its like JDM, its at the gearbox, so only the driving (not steering) wheel size will affect the speedo/odo. FWD, then its front wheel size. RWD, then its rear wheel size, that will affect our speedo/odo reading.

Once your odo is affected, the FC calculation and oil change interval differs accordingly, base on the above percentage.

mine is

(Rear)255/35-18 89mm 318mm 636mm 1997mm 501 0.2%

 

[astroboy]

hehe... so oversize by 0.2%, meaning speedo is 100kph, u r actually doing 102kph.. .. same with the odo reading, after doing 10,000km on the odo, you have actually covered 10,200km in distance.. same with FC calculation.. :wink:

See... as simple as that.. u asked a simple question, so simple answer is all u need.. all the gear ratio la, rotating mass la... another story for another day, let's not complicate our live any worses than it is now.. :wink:

 

[Megabucks38]

thanks bro,

But it feels like extra 10 km/hr when doin 120km/hr !!!

 

[astroboy]

Okay... there are actually other factor to it. Your tread thickness would also contribute to the over circumference of the tyre. Were your old tyres very botak compare to your current set?

.. and psychologically, when the car is stable, it should not felt "fast", maybe your new tyre needs some run in to settle down before more stability is felt, then feeling of "fast" may reduce... by 10kph..

 

[thule]

What about 245/40 R18 change to 245/45 R18???
Sorry... just changed recently

 

[astroboy]

What about 245/40 R18 change to 245/45 R18???
Sorry... just changed recently

Specification Sidewall Radius Diameter Circumference Revs/km Difference
245/40-18 98mm 327mm 653mm 2052mm 487 0.0%
245/45-18 110mm 339mm 678mm 2129mm 470 3.8%

You should gain 3.8% in top speed.. but pickup should be slower.. try using a GPS to measure your speed versus the speedo.. :wink:

If your E60 N52 continue to max out at 242kph on the speedo, you are actually covering 251.196kph... and can overtake the M5 who's electronically intervene to max out at 250kph..

Time for u to get the M5 emblem..

 

[t2ribena]

hehe... so oversize by 0.2%, meaning speedo is 100kph, u r actually doing 102kph.. .. same with the odo reading, after doing 10,000km on the odo, you have actually covered 10,200km in distance.. same with FC calculation.. :wink:

See... as simple as that.. u asked a simple question, so simple answer is all u need.. all the gear ratio la, rotating mass la... another story for another day, let's not complicate our live any worses than it is now.. :wink:

AB...0.2% of 100kph is not 100.2kph mer? so afta 10,000km it is 10,020km nia ler...

 

[t2ribena]

Specification Sidewall Radius Diameter Circumference Revs/km Difference
245/40-18 98mm 327mm 653mm 2052mm 487 0.0%
245/45-18 110mm 339mm 678mm 2129mm 470 3.8%

You should gain 3.8% in top speed.. but pickup should be slower.. try using a GPS to measure your speed versus the speedo.. :wink:

If your E60 N52 continue to max out at 242kph on the speedo, you are actually covering 251.196kph... and can overtake the M5 who's electronically intervene to max out at 250kph..

Time for u to get the M5 emblem..

slight diff nia...on the stock e60 525 N52 with 245/40/18, the odometer over read by almost 10kmh at speed of 240kmh. Using the GPS, at meter read of 240kmh, GPS only show 230kmh...so in this case thule reduces his over reading on the meter by approx 3.8% so at 240kmh the actual speed on GPS shud be around 239.6kmh...thule try it and let us know...

 

[astroboy]

AB...0.2% of 100kph is not 100.2kph mer? so afta 10,000km it is 10,020km nia ler...

kakaka! my math sucks, like my driving.. ... or was it my calculator kaput..

slight diff nia...on the stock e60 525 N52 with 245/40/18, the odometer over read by almost 10kmh at speed of 240kmh. Using the GPS, at meter read of 240kmh, GPS only show 230kmh...so in this case thule reduces his over reading on the meter by approx 3.8% so at 240kmh the actual speed on GPS shud be around 239.6kmh...thule try it and let us know...

Oh man! thule, scrap the M5 emblem.. I seriously need a young female elementary math home tuition teacher..

.. but wait, can I request to hold judgement and appeal at the same time?

 

[t2ribena]

kakaka! my math sucks, like my driving.. ... or was it my calculator kaput..



Oh man! thule, scrap the M5 emblem.. I seriously need a young female elementary math home tuition teacher..

wah i heard young female elementary math teacher....count me in also la...