Just a quick question for all the wheel experts out there. I have heard by getting a bigger diameter rim/tires , it messes with the speedometer. I have 20's and I have been told that if I am going 60 I am actually going 63.5. Does this also apply for the trip/odometer? Do I need to add or take away from these two? I use the trip meter often to calculate mpg & I am concerned that I am not going to be getting an accurate reading because I have 20's.

 

If you search for a tire size calculator, there ar eplenty that will give you a percentage that the speedo is off. When I would calculate MPG, I would take percentage away (smaller tires/wheels) or add that percentage to (larger) the total mileage and then figure the mpgs. So yes, it does affect the mileage count.


displayed 60mph
actual 63 mph

.05 off

go 300 miles on odometer
.05 is 15 miles
actual miles traveled where 285, use this for mpg calculation. not completely accurate, but more close.

 

All the trip computer info is a function tire circumference/diameter, not wheel diameter. If you keep the outside diameter of the tires the same, then the speedo, odo, and mpg will be very close to accurate. That is one of the reasons that 225/45 is the most popular size for 17's, and 225/40 is the most popular size for 18's.

Here is the tire & wheel calculator to show you what the resultant changes will be. Just plug in the stock #'s on the left side, and your #'s on the right, then look at the % change of the speedo given down below.

http://www.rims-n-tires.com/rt_specs.jsp

Keep in mind that the stock wheels are 16"x6.5" with +45 offset, and the tires are 205/55x16".