225/65x17 = 28.515 diameter, or 89.582 circumference.
245/65x17 = 29.539 diameter, or 92.800 circumference.

Divide a mile, by the circumference to find the number of revolutions per mile. The 225 goes around 707.28 times,the 245 goes around 682.75 times.

The difference here is easily why you get a speedo reading of 60 when you are really going 50.

thanks but remember i bough the vehicle with those tires already on and the factory sticker on driver door calls for 235 not 245 my laptop with wxp that had inpa on die on me other wise i could off check it that way by reading the actual abs sensor rreadings

ok here is an update i hook my reliable scanner which reads live data when i set the cruise control at 45 mph per the scanner the speedo reads 52 mph which i believe is the correct reading 45.mph at 1735+-on rpm now the question is how do correct the speedo

I have been trying to figure out this same problem since I bought my car. I have pa soft software and do not know how to reprogram the speedo. Allot of people do reprogram theirs on other bmw, but so far I have had no luck.

The speedo has a built in error so that when it says you are doing 45, you do not get a ticket for really doing 50. You are doing 40 instead. The speed you are doing is not as important than the idea that you are really doing less than the speedo say, and you are doing it for the reason I gave.

Using the On Board Computer, reset the speed while you have the cruise control set. The initial recalculation is the actual speed as calculated by the rotation of the tires, not the speedometer. The speed in the OBC is an average over time, but if the Cruise is activated then the initial speed displayed at the moment of Recalculate is the actual speed. It's still an average over time, but the time is short so the display is actual. You don't need to set the Cruise, but if the Cruise is set then the displayed speed will be constant until the speed changes and the Cruise tries to keep this from happening.

The speedo has no correction that I am aware of. You can cure some of the error by using larger tires. I've been on BMW forums for 10 years, and the only thing I know of to change the speedo is different tires.

Everybody has the same problem as you, to some degree or another. Back in the olden days of cable-driven speedos, there was a calibration process that involved a dyno. If there is a similar calibration for the electronic speedos that are used today, I do not know about them.

The speedometers on all the BMWs I have had read 2-3 MPH slower that the actual speed. I assume that is a BMW thing.

Maybe I am thinking the old, old days but we could buy a speedo gear to correct for dif changes that we also sometimes used when we went to much taller or shorter tires.

speedo recalibration?
 

I suffer a similar problem, insomuch as my speedometer is out by around 9 km/h (it reads 120 km/h when in fact I'm doing 111 km/h). I know this to be true, as I've driven past several 'speed check' mobile trailers, also compared my speedo with the GPS reading on my radar/laser detector, as well as paced my vehicle with a friend's car that has a 'certified' accurate speedo (ex-police car).
So my question is: how do you recalibrate the speedo in our cars so they read more accurately?
And - yes, I have correct tyre sizes, all wheels are stock, so rolling diameters have not been changed.
I also own a VW Golf TDi that suffers the same affliction - speedo out by more than 10 km/h but I'm led to believe you can recalibrate the speedo by means of the OBD2 port and a tool. Can you do this with the E53 too?

I found out how far it is off and I just add that amount to the speed I want to travel or forget about it and get less tickets.