I have just compared gps (aftermarket) unit speed and actual speedometer in the car and notice offset of 5 km how to correct that ?

does the increase in tire size could compensate for this?
right now I have stock on it 215/55/r16.

speedometer reading 100 km/h
gps reading 95 km/h

 

http://www.miata.net/garage/tirecalc.html

 

looks like tire size 225/30 R 20
(20 inch rims) are right on money reading 60 km/h and actual speed is 60 km/h

will try those and see how it goes.

 

compared to 215/55R16 it will be the same...

It's not unusual to get an offset of about -5km/t

215/55R16 is stock dimension and increasing size may cause problems, at least if the car is lowered.


IMO you should just live with the offset like everybody else

 

run 225/30/20 tires.

 

option #1
according to website calculator for tires I do need size 225/45 R16
to compensate for for speed .
this size running a bit slower spedometar reading 60 mph
actual speed :58.9 mph

diametar diference :1.68 % (which is good because limit will be 3%)

this is table to show diference between stock size (left) and new size(right side of table)
<TABLE style="BORDER-COLLAPSE: collapse" borderColor=black cellSpacing=0 cellPadding=4 background=images/spacer.gif border=1><TBODY><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Section Width: the width of a tire; the width at its widest point from sidewall to sidewall.';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD><TABLE width=280><TBODY><TR><TD align=right width=80>8.85 in</TD><TD align=right>225 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Diameter: height of the wheel this tire will fit on';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Rim Diameter:</TD><TD align=right width=80>16 in</TD><TD align=right>406.4 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Rim Diameter:</TD><TD align=right width=80>16 in</TD><TD align=right>406.4 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Width Range: recommended rim widths that this tire will fit on';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width="100%"><TBODY><TR><TD width=100>Rim Width Range:</TD><TD align=right width=80>6 - 7.5 in</TD></TR></TBODY></TABLE></TD><TD><TABLE width="100%"><TBODY><TR><TD width=100>Rim Width Range:</TD><TD align=right width=80>6 - 8 in</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Diameter: overall height of the tire';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Overall Diameter:</TD><TD align=right width=80>25.31 in</TD><TD align=right>642.87 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Overall Diameter:</TD><TD align=right width=80>25.74 in</TD><TD align=right>653.79 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Sidewall: the height of one tire sidewall';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Sidewall Height:</TD><TD align=right width=80>4.65 in</TD><TD align=right>118.11 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Sidewall Height:</TD><TD align=right width=80>4.87 in</TD><TD align=right>123.69 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Radius: height from the center of the tire to the outside edge';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Radius:</TD><TD align=right width=80>12.65 in</TD><TD align=right>321.31 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Radius:</TD><TD align=right width=80>12.87 in</TD><TD align=right>326.89 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Circumference: distance around the outside edge of the tire';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Circumference:</TD><TD align=right width=80>79.51 in</TD><TD align=right>2019.5 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Circumference:</TD><TD align=right width=80>80.86 in</TD><TD align=right>2053.8 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Revs per Mile: number of times this size tire rotates when travelling one mile';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE><TBODY><TR><TD width=100>Revs per Mile:</TD><TD align=right width=130>821.8</TD></TR></TBODY></TABLE></TD><TD><TABLE><TBODY><TR><TD width=100>Revs per Mile:</TD><TD align=right width=130>808.0</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

those tire size runs to slow .

option #2 215/35 R20

<TABLE style="BORDER-COLLAPSE: collapse" borderColor=black cellSpacing=0 cellPadding=4 background=images/spacer.gif border=1><TBODY><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Section Width: the width of a tire; the width at its widest point from sidewall to sidewall.';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Section Width:</TD><TD align=right width=80>8.46 in</TD><TD align=right>215 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Section Width:</TD><TD align=right width=80>8.46 in</TD><TD align=right>215 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Diameter: height of the wheel this tire will fit on';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Rim Diameter:</TD><TD align=right width=80>16 in</TD><TD align=right>406.4 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Rim Diameter:</TD><TD align=right width=80>20 in</TD><TD align=right>508 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Width Range: recommended rim widths that this tire will fit on';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width="100%"><TBODY><TR><TD width=100>Rim Width Range:</TD><TD align=right width=80>6 - 7.5 in</TD></TR></TBODY></TABLE></TD><TD><TABLE width="100%"><TBODY><TR><TD width=100>Rim Width Range:</TD><TD align=right width=160>Unrecognized tire size.</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Diameter: overall height of the tire';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Overall Diameter:</TD><TD align=right width=80>25.31 in</TD><TD align=right>642.87 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Overall Diameter:</TD><TD align=right width=80>25.92 in</TD><TD align=right>658.36 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Sidewall: the height of one tire sidewall';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Sidewall Height:</TD><TD align=right width=80>4.65 in</TD><TD align=right>118.11 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Sidewall Height:</TD><TD align=right width=80>2.96 in</TD><TD align=right>75.184 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Radius: height from the center of the tire to the outside edge';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Radius:</TD><TD align=right width=80>12.65 in</TD><TD align=right>321.31 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Radius:</TD><TD align=right width=80>12.96 in</TD><TD align=right>329.18 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Circumference: distance around the outside edge of the tire';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=100>Circumference:</TD><TD align=right width=80>79.51 in</TD><TD align=right>2019.5 mm</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Circumference:</TD><TD align=right width=80>81.43 in</TD><TD align=right>2068.3 mm</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Revs per Mile: number of times this size tire rotates when travelling one mile';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE><TBODY><TR><TD width=100>Revs per Mile:</TD><TD align=right width=130>821.8</TD></TR></TBODY></TABLE></TD><TD><TABLE><TBODY><TR><TD width=100>Revs per Mile:</TD><TD align=right width=130>802.4</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'; window.status='Speedometer: speedometer readings when travelling at 60mph / 100km/h';" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><TD bgColor=#eeeeee><TABLE width=280><TBODY><TR><TD width=120>Actual Speed:</TD><TD align=right width=80>60 mph</TD><TD align=right>100 km/h</TD></TR></TBODY></TABLE></TD><TD><TABLE width=280><TBODY><TR><TD width=100>Speedometer1:</TD><TD align=right width=80>58.5 mph</TD><TD align=right>97.6 km/h</TD></TR></TBODY></TABLE></TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'"><TD bgColor=#eeeeee>Speedometer Difference: -</TD><TD>Speedometer Difference: 2.417% too slow</TD></TR><TR onmouseover="style.backgroundColor='#D3D3D3'" style="BACKGROUND-COLOR: #ffffff" onmouseout="style.backgroundColor='#FFFFFF'"><TD bgColor=#eeeeee>Diameter Difference: -</TD><TD>Diameter Difference: 2.36%</TD></TR></TBODY></TABLE>

if I go with this size will my tires start rubbing inside fender and do I need any spacers?

 

is there any website or maybe some formula to calculate how much stress will be on tie rod ends (outer ) and ball joints if you go with bigger rims and tires.
Beside that ,will be braking distance afected by this too?

I did similar things on my SUV infiniti (long time ago) running bigger size and did not have any problems but it could be possible that Benz is more sensitive to any changes tire wise.

Yes ,somebody said:... you can run with 5 km/h offset ,but I am trying to be more accurate in order to get right speed on speedometer and make transsmission to shift at right time.

 

I'm just guessing but I doubt that road speed is a factor on transmission shift points. I would think the "speed" variable would be engine rpm not mph.

As far as speedometer error, my E430 is a bit less than +1 mph but our BMW is off by a consistant +2.5mph. My understanding is that this is intentionally built in and only affects the indicated speed. Not the odometer, tach, etc.

Interestingly, when new, our BMW speedo was off by +2.5mph, but the computer "avg mph" was dead nuts accurate. But when the computer display was replaced due to dead pixels, the new computer display mirrored the speedo and was +2.5mph.

 

Hey, there's not a single car out there with correct speedometer reading.

225/45R16 will not solve your problem at all. It will give another 5% offset in the wrong direction as teh wheels gets smaller.


but are your 20inch rims only 7,5 inches wide? I would think a 20inch rims would be wider...

 

on a 35et offset 8"wide rim and you will be set.A little more accurate,I don't know why you need to be so dead on?Tire pressure alone can lead you to be off a few percent.I run 32 front and 38 rear. At 75mph who notices how accurate your speedometer is?You can always compare it to the onboard digital speed reading in the diagnostics,value #22.
Any et from 31 to 41 depending on rim width and tire size will not put undue stress on the car.E55 ran 31et front and 35 et rear with 275 18's and 245 18's.
235's on 17's all around gives you plenty of room and a fair but firm performance ride.
a 225/30/20 would ruin your ride and look silly to boot

http://www.1010tires.com/tech.asp
more accurate calculator
tirerack.com a site sponsor has great rim and tire choices
measure twice or more, install once
Are you going to drop it?If not you can go wider and lower on et and still have no problem with load forces on bearings and suspension,widen your track and help track control if you ever decide to take a few laps.

 

235/45R17 is a stock dimension with the avantgarde. Rim is 8inches wide. Not sure about the et though.. But there are plenty of room. The avantgarde is lowered about 28mm compared to classic and elegance and the fender is rolled I think.

 

so much for advises. I got good deal on Michelin pilot 225/55/16
and will take those for now before I make final decision about bigger rims .

this is the best forum so far.

 

<table style="border-collapse: collapse;" background="images/spacer.gif" border="1" bordercolor="black" cellpadding="4" cellspacing="0"><tbody><tr bgcolor="#d3d3d3"><td>Stock Tire - 215/55R16 >Search Tires </td> <td>Tire 1 - 225/55R16 >Search Tires </td> </tr> <tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Section Width: the width of a tire; the width at its widest point from sidewall to sidewall.';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Section Width:</td><td align="right" width="80">8.46 in</td><td align="right">215 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Section Width:</td><td align="right" width="80">8.85 in</td><td align="right">225 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Diameter: height of the wheel this tire will fit on';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Rim Diameter:</td><td align="right" width="80">16 in</td><td align="right">406.4 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Rim Diameter:</td><td align="right" width="80">16 in</td><td align="right">406.4 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Rim Width Range: recommended rim widths that this tire will fit on';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="100%"><tbody><tr><td width="100">Rim Width Range:</td><td align="right" width="80">6 - 7.5 in</td></tr></tbody></table></td><td><table width="100%"><tbody><tr><td width="100">Rim Width Range:</td><td align="right" width="80">6 - 8 in</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Diameter: overall height of the tire';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Overall Diameter:</td><td align="right" width="80">25.31 in</td><td align="right">642.87 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Overall Diameter:</td><td align="right" width="80">25.74 in</td><td align="right">653.79 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Sidewall: the height of one tire sidewall';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Sidewall Height:</td><td align="right" width="80">4.65 in</td><td align="right">118.11 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Sidewall Height:</td><td align="right" width="80">4.87 in</td><td align="right">123.69 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Radius: height from the center of the tire to the outside edge';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Radius:</td><td align="right" width="80">12.65 in</td><td align="right">321.31 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Radius:</td><td align="right" width="80">12.87 in</td><td align="right">326.89 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Circumference: distance around the outside edge of the tire';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="100">Circumference:</td><td align="right" width="80">79.51 in</td><td align="right">2019.5 mm</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Circumference:</td><td align="right" width="80">80.86 in</td><td align="right">2053.8 mm</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Revs per Mile: number of times this size tire rotates when travelling one mile';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table><tbody><tr><td width="100">Revs per Mile:</td><td align="right" width="130">821.8</td></tr></tbody></table></td><td><table><tbody><tr><td width="100">Revs per Mile:</td><td align="right" width="130">808.0</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'; window.status='Speedometer: speedometer readings when travelling at 60mph / 100km/h';" onmouseout="style.backgroundColor='#FFFFFF'; window.status='';"><td bgcolor="#eeeeee"><table width="280"><tbody><tr><td width="120">Actual Speed:</td><td align="right" width="80">60 mph</td><td align="right">100 km/h</td></tr></tbody></table></td><td><table width="280"><tbody><tr><td width="100">Speedometer1:</td><td align="right" width="80">58.9 mph</td><td align="right">98.3 km/h</td></tr></tbody></table></td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'" onmouseout="style.backgroundColor='#FFFFFF'"><td bgcolor="#eeeeee">Speedometer Difference: -</td><td>Speedometer Difference: 1.707% too slow</td></tr><tr style="background-color: rgb(255, 255, 255);" onmouseover="style.backgroundColor='#D3D3D3'" onmouseout="style.backgroundColor='#FFFFFF'"><td bgcolor="#eeeeee">Diameter Difference: -</td><td>Diameter Difference: 1.68%</td></tr></tbody></table>

 

There are small differences in tire sizes between manufacturers for tires with the same size printed on sidewall.

European manufacturers are required to fit speedometers that guarantee the instrument will never display a slower speed than the car is actually going (given owner installs specified tire size). Therefore, most of them read fast.

It's comparing apples and oranges to compare your speedo with a GPS.

There is, BTW, no relationship between this and odometer display.

 

It's comparing apples and oranges to compare your speedo with a GPS.

Yep - GPS standard is +/- 0.11 mph.

 

Don't forget how much tread is left when comparisons are made. Tread depth can alter mph/kph readings a few percentage points. Stay away from the 20 inch rims. A good wheel repair shop will tell you that once you exceed 18 inch diameter in rim size you are much more prone to wheel distortion due to road hazards. The rim becomes too large to hold a true round given that the spokes on todays wheel are so far offset to the outside. Even with 18s care is needed to avoid the divits and a bent rim.

 

I will stick with 16 inch rims for now and see how it is rolling