Final drive percentages

[Zoobeef]

I run a 4.17 final drive which is shorter than the 3.95 standard.
I was told by the guy that wrote my mapping software to reduce the tyre circumference by 5.3% when I entered it in the software to compensate so the software was accurate.
I've just been looking at it out of interest and as far as I can work out that final drive reduction is actually a 5.57% change not a 5.3% change. Am I doing my maths correctly?

Not sure it makes a massive difference in the software calculations as it only takes another 5mm off the tyre circumference. When I got it power tested on a proper MAHA rolling road it registered 3bhp higher than the software which is pretty close anyway given the software powerruns are just calculated on the road. But if I'm correct then that change may make it even more accurate.

[Mignon]

Zoobeef said:

I run a 4.17 final drive which is shorter than the 3.95 standard.
I was told by the guy that wrote my mapping software to reduce the tyre circumference by 5.3% when I entered it in the software to compensate so the software was accurate.
I've just been looking at it out of interest and as far as I can work out that final drive reduction is actually a 5.57% change not a 5.3% change. Am I doing my maths correctly?

No.

[GreenV8S]

You are calculating the ratio upside down. For every 1m that the car would have moved with the original diff, it will travel 0.947m ish with the new one. That's a reduction of about 5.3%.

[Zoobeef]

Righto. So difference divided by the new ratio x 100.
0.22 ÷ 4.17 × 100 = 5.28

I did have that way of working but didn't know which way I was suppose to look at the sum.