Differential calc

[XfireZ51]

My math teacher sister in law says engineers can't solve anything correctly unless the formula is already set up for them.
We are splitting hairs and at the end of the day the 4.10 is likely the most practical solution. I'm just going to continue the disagreement because it's fun.

Proportionality is the most accurate method of solving this:

Effective gear:Actual Gear::Effective gear:Actual Gear

Paul,

You can't accuse me of the Bugger Factor because I was not "looking" for a particular answer. In fact, I had originally thought of 3.90 gears.
The .156 is the error of the indicated speed not the actual. That's why your proof does not work.

WVZR-1,
The error is 15.6% whether you arrive at it by (74/64)-1 or (74-64)/64. The divisor is the indicated speed because that's what I'm trying to solve for.
I use proportionality because I want to maintain the same relationship (since I'm keeping the wheels and speedo gear) I currently have between the 3.45 gear and what it acts like (2.91) when I swap gears.

ZeeAreOne,

That was my suspicion also. Car had 4.10s and at some point 3.45s were installed but the speedo gear never swapped. Of course that under reports the mileage. HMMMMM! No matter.