What ratio is my differential?

[Rex]

My car is a 2002 SV with a U.S. spec Zetec and a 6-speed box.It also has an AP Suretrac limited slip. I've never known if it's a 3.92 or a 3.62. Is the only way to know for sure to remove the boot floor so you can see it?  Thanks to anyone who can help.

[Mark w]

if you have 13 inch wheels and a 3.62 it will be running at 4000 rpm at 70 mph give or take a smidgin .

if your speed limit is 55 then you best pro rata the 4000 

[Rex]

My wheels are 16" and my speedometer reads 5-7 miles high at 70mph.

[Mankee]

You may be able to read the markings on the metal tag attached to one of the bolts holding the back plate of the diff on. However, that doesn't mean that someone hasn't had a fiddle with the insides before you got it.

[Jonathan Kay]

As above, but use a GPS device to measure the road speed.

Or count the turn ratio of a wheel and the prop shaft. But I'm not sure how that sort of LSD affects this method...

Jonathan

[7 wonders of the world]

LSD has no effect on the final drive ratio.

[Paul Deslandes]

With the AP Suretrack lsd both rear wheels should rotate together.  Unless you want to lie underneath the car, remove the tunnel cover to expose the end of the propshaft and either make a chalk mark or apply a bit of sticky tape to the UJ and jack the back end up.  You really need a helper who can count, rotate the rear wheels 10 times making sure they are rotating equally together, and count the prop revs which should be either roughly 39 or 36.

[Jonathan Kay]

Thanks, Paul.

Jonathan

[Rex]

Well, I've used a GPS and at 70mph with th six speed I am reading 4000RPM. Does this mean I probably do have a 3.62 diff? I don't know if my wheels would be a factor here' but thet are 205 16 45.

[Jonathan Kay]

I make that 3.92.

Jonathan