Something doesn't add up!
 

My tire and gear calculations aren't coming out right.
235/50/18 tire
3.31 gear
.62 overdirive
70 mph
should add up to a 1771 engine speed

but in reality i have right at 1900. So, what are the possibilities here?

235/50/18 tire
3.55 gear
.62 overdrive
70mph
adds up to a 1899 engine speed

or

235/50/18 tire
3.31 gear
.68 overdrive
70mph
adds up to a 1942 engine speed

the door jam sticker says "cg" on the axle ratio, which stands for a 3.31 gear ratio, if I'm not mistaken....what I'm leading up to is the question: are we sure the overdrive ratio is .62? I ask because I'm figuring tire and gear size for my upcoming gear swap, and I don't want any surprises. I'm pretty sure I'll go to a 4.30

Either the door jam sticker is wrong, or the od ratio isn't a .62.

thanks for your help!

RE: Something doesn't add up!
 

auto or 5spd?

RE: Something doesn't add up!
 

According the the spreadsheets I have for the Mustang gear calculators, the manual has a
final drive ratio of .0675 and the auto is 0.71.

RE: Something doesn't add up!
 

.675? that makes more sense...from a calculator standpoint. Someone on here told me the final drive was a .62
manual by the way

RE: Something doesn't add up!
 

Here are the 2 spreadsheets I play with. Just save them to your desktop and rename them from
.txt to .xls. Once opened, you can select the different worksheets using the tabs on the bottom.

These spreadsheets were created by other forum members so don't thank me.

RE: Something doesn't add up!
 

Well, I'm gonna thank you anyway!

RE: Something doesn't add up!
 

Another possibility is that you have the wrong number for the 235/50R18 tires. I've seen a few "online calculators" list different revolutions per mile for the same tire size. Compare that to what the website for the tire company says, and they probably show a different number of revs/mile than the calculator websites. I say trust what the tire company says moreso than the online calculators.

Not sure if that helps you or not, but it could be a possible cause for your numbers being off.