Help charting peak values when the X-axis values are not the same

Not sure I fully understand your requirement but this may help.
http://www.andypope.info/charts/intersection.htm

It uses a user defined function, which required VBA, to calculate the points where 1 line crosses another.

And this addin will made future exports of images easy
http://www.andypope.info/vba/gex.htm

It's not peak values you want, it's max values at a given speed.
This would probably be easier manipulating data rather than trying to get data from charts.
You must already have a data series for each gear. It's a question of either lining up the data so that the speeds all line up and choosing the max value from each row, or doing the equivalent in vba.
(I know you say "the MPH values associated with each torque or acceleration value are not the same from gear to gear", but you have managed to convert them somehow.)

Show us some data.