I has 2.87 gear on rear here is pic

 

Our STR's have a LOW 2.87 gear ratio.......Now if we could do a 3.55 or 3.73 that would be OUT FLIPPING STANDING!

 

I was gonna post the ebay link to that
http://cgi.ebay.com/ebaymotors/Jagua...Q5fAccessories

 

Maybe Motive Gear or another aftermarket company might have stock or can make a gear for them

 

http://www.motivegear.com/images/art/chart/chart.pdf

The only one that shows 2.87 is DANA......Who makes our rear difs? Ford?

 

According to what I have been reading DANA Type 44 use a 3 bolt output shaft and the rest use a 5 bolt so if we look at the picture it looks like a DANA these where used in the XJ though in the 80's for Jaguar.

 

So then we could order the gear through lets say Motive Gear for a taller gear in the rear end.....That is if the ring dia. is the same size.....
Then the next step would be is re-calibrate the speedometer/odometer to read correctly....

 

have no idea, I will be going to mustang plus this weekend to see what they say

 

What would be the approximate cost a dealer would charge to install a Quaife unit? Would using a dealer or Eurotoys be a better choice?
Does the ECU have to be reprogrammed on an XKR (05) if a Quaife unit is installed.
Since there appear to be a few Jag. mechanics here, maybe an installation cost estimate is possible.

 

The rear ratio is the primary inhibitor of performance to the XKR/XK8. The newest ones (XKR) are 3.31 but all the early ones are 3.03. The Nissan GTR is so quick because it's running 3.70 rear gear.
Has anyone found a way to lower the rear differential ratio down to 3.73 or lower? Nothing would more quickly improve the XKR performance.

 

if it is indeed a dana 44 c4 corvettes came with that diff and if i'm not mistaken the auto's ran a 2.92 and that sticks ran a 3.07

http://www.vetteweb.com/tech/vemp_09.../photo_05.html

 

This is like trying to find out the answer if there is life on other planets!

 

LOL - VERY frustrating, isn't it?
Such a simple question for almost any other brand.

 

That it is......I been looking and calling all around and no luck anywhere....

 

You are all looking in all the wrong places, if you are looking for a diff to fit the S-Typr R it is the same as fitted to th XJ sedans. Cars fitted with supercharged engines have the 2.87, v8 normally aspirated have 3.07 & Europe only have a V6 which is 3.31 ratio. You can purchase this thru any Jag dealer or possibly Coventry West could build you one. The problem is LSD you will have to buy a Quafe thru one of the 3 suppliers who have exclusive rights to it. The problem you will have is the TCM as I have spoken to someone that did this & the transmission goes into the limp mode with the message incorrect gear ratio. I atempted this on my own XJR in 2004 & gave up after speaking to a friend who works for ZF. If you want more power you should look into a twin screw supercharger, cylinder head porting & water methanol injection.

 

Do I read you correctly that you trying a Quafe unit and the computer shut you down? Since mine is an 05 XKR - essentially an 04 carry over - it reads like it literally is impossible due to the computer. Very bad news indeed if so.

Since the new XKR comes with 3.31 rear this might work in earlier models? It sounds like you buy it, have it put in and then find out if the TCM will allow it.

I guess since mine has factory 20 inch wheels I could go smaller wheel/tire combination - that sucks - but then the TCM could read that as wrong gears too.

What actually is reading the speed of the car? How does the computer know how fast the car is going? OLD cars just had a cable to the transmission. Is it reading tire rotation? Disc rotation? Where, exactly, is the sensor that is reading the speed the car is going (ie the rate the wheel/tire is turning? Sometimes a sensor can be tricked in various ways. I could live with a speedo not registering correctly.)

(BTW - What is "ZF?")

Thank you (VERY MUCH) for taking time to reply. Really. I'm in the wilderness on this and it appears not the only one.

 

Let me narrow the relevant question...
If I put on smaller diameter wheel/tires, would the speedometer misread accordingly? Would that "upset" the TCM to going into the limp mode?

 

Not sure. The Mercedes Box used in the 4.0 SC cars will check if the box itself indeed in the right gear. It does this via checking the input shaft speed and the output shaft speed. It has an input shaft speed sensor, but (and there is the issue) not an output shaft sensor. It will relay on the ABS speed sensor of the rear wheels to check it. If it is not right, you go into limp mode. So when you change the rear end ratio on these cars, the TCM will not work with that.

I am not sure if this is also the case for the S-type/XJ 4.2 cars with the ZF box, as that one does have a rear output sensor to check if the box is in the right gear.

 

Thanks for the analysis. I suppose one way is to buy a cheap 18 inch tire/wheel combo and see what happens.

Different topic - it's been suggested your the guy to talk to about a Whipple set up. Paramount said they'd build one for me for $7000 upfront and they'd then need about 2 months. Price included the blower. (05 XKR coupe)

Did you build/fab a Whipple setup yourself? Did the ECU accept it or require some remapping? Fit under the hood? (It's TIGHT under there!)

 

Changing tires will not make the difference; it will be the output shaft speed ratio to the speed of the wheels. Putting on smaller wheels does not change that ratio, only the rear end ratio will create a difference here.

Best would be to contact ZF themselves, they should know where the trigger is (in %). Chances are this could be around 10%/15%, leaving not much room for a major ratio change. Another route is to search for Audi/BMW tuners, they might also know more about teh ZF box, and if it is possible to alter the software.

Please PM me for the off topic part, but in short I did make everything myself.

Everything fits under the hood, no major changes there, and surprisingly the ECU offers a tremendous range and fuel supply. So far that hasn't been the border...