Speedo gear swap - Couple Q's

bloopbloob said:

just wondering when switching from 2:73 to 3:73's, what happens if you don't swap the speedo gears? will your spedometer read higher or lower than your actual speed? by how much? Also, how difficult is it to swap each of the speedo drive gear? the driven gear? 355's only need the driven gear, but 3:73s need a drive and driven gear swap, correct?

Click to expand...

ryan218 said:

be off by 8 mph so when it says your going 35 your really going 27mph...with 4.10 when it says your going 35 your really going 25....thats the way my dad's is with his 4.10's and that's the way shanes was before he was t-boned in his stang.

Click to expand...

TRWXXA said:

Your speedo will not be off by a specific mph. It will be off by a percentage -- about 36%. So the faster you drive, the more mph it will be off.

If you go from 2.73s to 3.73s without changing you speedo's driven gear, you can expect the speedometer to read about 41 mph when you are actually doing 30 mph (an 11 mph difference). When you are actually doing 60 mph, the speedo will read about 82 mph (a 22 mph difference).

If you went to 4.10s the error would be about 50%.

'89 and earlier 5-speeds only need to have the driven gear changed to a 21-tooth one (with stock tire height). '90-'93 5-speeds need to have both the driven and drive gears changed.

Click to expand...

ryan218 said:

with ours it aint...when it says were going 65 were going 55 when it says were going 85 were going 75...when it says were going 15 were going 5...we have a radar gun

Click to expand...

bloopbloob said:

thanks!. i think i'll have to do the swap then i guess. so how hard is the speedo gear swap? is it just as easy to do both (drive and driven gear) as it is to do one? just wondering if i should do the single driven gear (for 3:55's) and be close, or get both and do it for 3:73's correctly

Click to expand...

TRWXXA said:

Well I guess your car somehow defies the laws of physics and mathmatics, or you have something wrong with your speedo.

When you change rear end gears, or the gears for the speedo, you are changing the RATIO. The ratio, or the change in ratio, can be expressed as a PERCENTAGE. The percentage of change made to the rear end gears will change the percentage of error in the speedometer by the same amount. That is basic math. No gear swap or tire height change you can make will change your speedometer's reading by 10 mph throughout it's entire range.

Perhaps you should get your radar gun calibrated along with your speedometer.


The driven gear is an easy, 5 minute swap. The driven gear requires you to take the tailshaft housing off and remove the drive gear from the output shaft.

Click to expand...

ryan218 said:

..our cars are different like that

Click to expand...