You can easily check the tire diameter difference by using this technique:
(from here)
http://www.onallcylinders.com/2015/...ffects-of-tire-diameter-on-final-drive-ratio/
The easiest way to determine tire diameter is to call your tire dealer and ask for it—easy, but not always an option. The second way is to use the following formulas and figure things out yourself. Please note that these formulas are for metric-rated passenger and light truck tires. Slicks, large off-road, and some larger, Pro Street style tires are already rated by diameter, so you don’t have to do any calculating for them.
We will be using a P235/60R-15 tire as our example: 235 is the section width, 60 the aspect ratio, and 15 the required wheel rim diameter. Here is the basic formula:
Tire Diameter = 2 x (section width/25.4) x (aspect ratio/100) + rim diameter
When you plug in the values from our sample tire, the formula looks like this:
Tire Diameter = 2 x (235/25.4) x (60/100) + 15
Now, let’s do the calculations:
Tire Diameter = 2 x (9.25 x .60) + 15
Tire Diameter = 2 x 5.55 + 15
Tire Diameter = 26.1 inches (round down to 26)
Edit: Here's another cool site with some handy calculators under the Calculator section on the LH side of the page:
http://www.4lo.com/calc/gearratio.php