Some time ago, i created spreadsheet in MS Excel, which can help to see, how changing diff or trans ratio, or tires/wheels size affect to vmax and rmp's in particular trans gears.
How it work:
-In first sheet called "factory", in yellow fields you write your tires data. next, transmission ratios and factory diff ratio.
-In second sheet called "after", write tires data, if you want to calculate only changes after diff/trans ratio change, rewrite unchanged, like in first sheet. In next field write trans and diff ratio after change. Easier - if you want calculate only diff change, rewrite unchanged all data except diff ratio. All fields all filled by sample data - it calculate differences beetween w124 e320 with 5-speed 722.5 trans and 4-speed 722.3 trans. 722.5 has extra 5-gear and changed diff ratio.
-next, in "factory speeds" you can see, what speed you can obtain in specified gear and engine rpms. All is generated from data given in "factory" sheet.
-in "after speeds" you can see data calculated with changed diif/trans ratios obtained from "after" sheet.
-next sheets is graphs, also with imposed data before/after.
I tested this spreadsheet many times and all datas are coincide with reality. One important thing - generated speeds are real car speeds - if we comapre it with speedometer, they may arise very small differences. Also, with automatic transmissions some little differences with rmp's are normal thing - this is due to the construction of torque converter without lock-up mechanism.
Currently, i working on adding possibility to calculate speedometer K index. Function work propertly with mechanical speedos scaled by kmh and mph, but i have problem with electrical speedos used in .034 and .036.
If anyone finds any errors, or have questions - please write
File in attachment. I must zip it, because forum don't allow .xls files.