Your new tires are 3.3% taller, roughly. So, your actual milage should be 3.3% greater than you think.
Original Fitment Proposed Fitment
width 225 mm width 245 mm
aspect ratio 50 % aspect ratio 45 %
wheel diameter 16 in wheel diameter 17 in
offset 50 mm offset 50 mm
original diam. 24.86 in proposed diam. 25.68 in
Potential Fitments: Geometry Calculations:
tire aspect wheel change in ride height 10.5 mm
width ratio diam inwards add'l width req'd 10.0 mm
245 40 17 outwards add'l width req'd 10.0 mm
255 30 18 add'l total tire height 20.9 mm
265 20 19 add'l vert clearance req'd 10.5 mm
Instructions: additional rolling diameter 65.7 mm
Change values in boxed cells change in drive ratio 3.3 %
to see effects of tire fitment
changes. Unboxed cells show Speed Calculations:
resulting geometry. Hold the speedometer reading actual speed
cursor over inputs or results to 120 kph or mph 124.0 kph or mph
view associated pop-up help. desired speed speedometer reading
Note to TRX owners: 100 kph or mph 96.7 kph or mph
TRX 390 mm = 15.35 in.