Soooooooo, i got a new set of rims for my car! got some 166's for my baby!

I originally had the 123's with 245/40/18, all the way around,

Now I have 275/30/19 (rear) and 245/35/19 (front) on the 166's.

Question is, I know that the hight did change and it is causing a change in my spedo readout. I was wondering if there is a way to change it to read the right number when driving.

I have the ELM327 OBDII reader with "DashCommand" which shows me for example 25mph. Then when looking up at the spedo it says 27mph. How can I get the spedo to give me the correct reading? Im assuming the one from the OBDII is correct.

Thanks.

 

Good question....interested to see what folks say because if you added a taller tire, the spedo would generally read slower than actual speed, not faster. Would like to know this for when I change out my 18s to 19s or 20s.

 

Well, considering that 19" 166 are OEM I highly doubt that the speedo would have that much variance. The same size wheels and tires come on non-M cars (style 172 on LCI M-sport 550's). From my experience the speedo always seem to read a few mph fast, regardless of wheels, I have had stock 17, 18, 19, and 20's on my old car and it always read fast. Which is odd because as the wheels get bigger it should read slow since the bigger wheels and tires would turn less to achieve the same speed.

 

The speedo is off by 3 with 18s and 2 with 19s. All e60s are like this, possibly to account for all the different wheel combos the car could come with.

 

Good info..I will adjust my driving accordingly..I dont need much encouragement to drive faster.

 

The larger the circumfrence,, the center of the wheel will spin more revolutions in a full 360 degree turn.. A smaller circumference the center of the wheel will have less revolutions in a 360 degree spin... So if u go from 18 inch wheels to 20 inch.. the 20 inch has a longer circumference and spins the center faster ,, will increase the speedometer...

 

According to an on-line speedometer adjustment calculator, the two tire sizes you noted above are near identical and no spedo changes should be necessary.

A reading on the 'old' tire at 100 MPH should be 99.14 MPH on the new one. Less than 1%, within any margin of error.

Speedometer Calibration / Tire Size Change Calculator

PittCaleb

 

If you choose Menu 08.00 V-EFF on the diagnostic menu which, when opened, is displayed on the screen between the speedo and tacho, the exact speed is shown. Menu 08-01 shows the speed displayed on the speedo. The issues regarding wheel size still apply however

 

First of all, it's not simply the wheel diameter that has an effect. You also have to consider the tire profile. You can plus size your wheel, but if you reduce the tire profile, you won't necessarily increase the overall circumference.

And to simplify this: A larger circumference will travel farther in one revolution than one with a smaller circumference. Thus, if these two setups were both traveling at the same point on the speedo (which measures wheel revolutions, not true speed), the one with the larger circumference will be traveling faster across the ground. Bottom line, if you put larger circumference wheels/tires on, your speedo will be under-reporting your actual speed.

 

True. i didnt explain it correctly.. i meant to say..larger tire circumference will spin the center more revolutions,,,, than a smaller tire circumference.. So a larger tire circumference will speed up the speedometer,,,, yes not the diameter.. i meant to say 18 inch tire and 20 inch tire.... in terms of overall circumference.... because the longer circumference has more lenght of travel in a full 360 degree turn..than a smaller circumference... thus more revolutions and speedo reads higher..