hi i dont know if anyone can help ive got a formula i use to work out speed from gear ratio, diff ratio and wheel revs.

i work it out like this 8000 rpm x 60 = 480000

wheel revolutions a minute 897, diff ratio 3.64, 5th gear .80.

so 897 x 3.64 x .80 = 2612.064

480000 / 2612.064 = 183.7 at 8k in fifth standard wheels.

but i have it on good authority that with 17s and 40mm tyres actual speed is 195 ish.

so i tried to work out wheel revolution a mile by calculating circumference of larger wheels and it works out at 165 mph

now the only thing i didnt check for myself in the above equation is the wheel revs which i was told is 897 per mile on standard wheels.

does anyone know what im doing wrong