Quick question about the 3.7 / 3.9 diffs.
I know the car will get off the line a little quicker with the 3.9 and it will offset the extra weight of the lpg tank 
What is the difference in mpg between the 2 diffs in the real world ?
Anybody run both on there cars who could tell me ?
If we just forget about Auto boxes for the minute as its a bit more important to get the ratios right so that the Auto box locks up at the correct Torque and Speed. Asfor the Manual Ive run Manual V6's with both 3.7 and 3.9 Diffs, both were ex Plod so the speedo will be fairly accurate compared to standard which generally read around 3 mph higher at Motorway speeds.
Manual with the 3.7 is the standard set up so on a 70 mph run your doing around 2600 rpm in 5th, which equates to about the same as an Auto with a 3.9 diff. So expect normal MPG figures of the car.
Manual with the 3.9 fitted, does feel a bit more lively off the mark, but not by much, mpg wise when pottering at slow speeds theres not much in it, however when sat at the same 70 mph run your doing 3000 rpm in 5th, so simple maths is going to tell you your going to be down on mpg on the same constant run, in fact I would also say it doesn't feel right you think you need to change up another gear if your used to driving V6's.
Bearing in mind, your planning to change from an Auto to Manual in order to achieve better mpg economy, I would say its important to get the right diff ratio (3.7) in there.
As said on the other post if you feel like experimenting to get a bit more mpg over the norm, why not fit a 3.45 Diff from a Diesel, admittedly you may loose a tad on acceleration but you still got 5 gears to play with as a work around and would expect greater mpg improvements on a run, haven't calculated it but reckon your be doing around 2200 rpm at 70 in Top, I reckon the Torque of a 3.2 would be good enough to cope with a 3.45 but have never dont it myself. Would be interesting to see the results of this though.
Chris.
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Topic: Auto to manual conversion list (Read 2264 times)