How much speedo will be off with N/A 4 cilynder speedo and SVO motor and tranny

[Slow5.0]

First off all i'm born in Bosnia / Europe now live in USA ,my english sux so don't critize me

Here is a deal

My buddy bought 89 Mustang hatch, its ex n/a 4 cilynder car now with SVO motor,tranny ,8.8 rear end and everything except for Instrument dash, he still have stock n/a 4 cilynder dash (speedo,tash ,ect.) so i was thinkin maybe its off but don't know how much, i know we could drive side by side and find out whats difference but i wanna know for tach too
He have some high gears ,he thinks 4.10s but way speedo and tach works i think he have even higher than 4.50s because hes top speed in 5th is around 105 the ways speedo looks, and gears are too short, he shifts in 4th on 75mph on 7000rpms

 

[Bearded Forever]

It's definately off because of the rear gears most likely. We'd have to know which gears are in the rear to know how much it is off.

 

[Slow5.0]

It's definately off because of the rear gears most likely. We'd have to know which gears are in the rear to know how much it is off.

Thats what we tryin figure out,he thinks 4.10s because previous owner told him that, but i thinks its even higher than that