From what I've read, the 2.88 LSD diff from an XJ40 will fit my '97 XJ6, however, I am wondering how to calculate and figure the change in RPM's and if it will be a smooth crossover drive ability wise. Has anyone done this?

I'd like the benefit of the LSD plus the lower gears will help fuel mileage no? Or do I have that backwards and the lower 2.88 will spin faster than the stock 3.58 I have now?

 

I found this site:
Differential Gear Ratio Calculator | West Coast Differentials

I did some calculations based on my 225/60R16 tires @ 26.63" diameter

2.88 gears = 1,908 RPM @ 75 MPH
vs
3.58 gears = 2,371 RPM @ 75 MPH

463 RPM less on the 2.88 it would seem, and therefore some level of fuel savings.
I am just concerned about the acceleration, performance, etc.

 

I went the other way, from 3.58 to 3.77. Cost about 1 mpg on the highway.

Depending upon where you live and the terrain you drive in, going to 2.88 might cause it to be downshifting into 3 more, where the 3.58 might stay in overdrive, which will cost you both fuel and performance.

 

I think you will find that the 2.88 diff is from the v12 jags,xj/xjs,
I am just building one into a ha4 axle,but I will run a 4.2 v8.

 

My Manual XJR X300 averages 18 - 20 mpg on my daily drive circuit, all conditions averaged.
My auto 3.2 averages 21-22 on same trip, again over a long cycle (eg a whole tank of gas, 300+ miles)

the XJR accelerates 80-120 km/h in 3rd gear in about 3.5 to 4 seconds
the 3.2 does 100-120 in about 3.5 to 4 secs.

Power and ratios seem to have minimal effects on economy of hauling an 1800kg car around. Having said this, the XJR has great propensity to drink a whole lot more fuel if the available power is used, and if so the performance is vastly better than the 3.2

 

I believe the EARLY XJ40 (3.6 Liter engine) had the 2.88 Diff.
I think you will have trouble because that diff has the U joint flange type coupler as opposed to the JURID flange.

As long as the diff/propshaft couplers are compatible it should fit.

The 4 pinion end mount holes in the XJ40 diffs have a different thread pitch (M10X1.25 vs M10X1.5) than the X300 but that is really the only difference.

bob gauff

 

Our '88 XJ40 had a 2.88 diff with the Jurid flexible coupling at the input. What I don't recall is whether it was an LS or open diff.

The first thing I noticed when I purchased the '93 XJ40 with 3.58 diff was that accelleration off the line was MUCH better (part of that was the increased engine displacement, HP and torque, but I think most of it was the rear end). I think going back to a 2.88 would be highly disappointing unless you really don't mind slow starts.

The second thing I noticed was that the '93's engine ran nearly 500 rpm higher at all speeds, which is consistent with SD96XJ6L's calculations.

Cheers,

Don

 

Yes the off the line aspect has made this 2.88 dead, an LSD however, is another story.