Originally Posted by jonbays
With a manual XR6 you should have a 3.45:1 diff and 225/50R16 tyres so at the moment fourth gear should be giving you about 33.1kmh/1000rpm. In top 46kmh/1000rpm. Its already fairly low and without serious mods to allow the engine to rev right to the 5900rpm cut out big gains won't be had going lower. Lots more wheelspin in first maybe and slower times or faster times taking off in second. A 3.73:1 in a standard AU Auto drops third from 36.8 kmh/1000 to 32.1kmh/1000rpm which is good it gets it to an XR6 spec. In an XR6 I don't think its really going to be worthwhile but it will drop fourth to 30.6kmh/1000rpm and top to 42.5kmh/1000rpm. Your speedo will read slow but a change to a black speedo drive gear will fix that Part no.XE17271F, $10 from Ford.
How much faster well at a guess you will drop the 1/4 times by 0.4-0.2 seconds not much but if thats your game!
Money wise wreckers prices are pretty negotiable and it can be $250-$500 odd for the centre. maybe the same agin to fit.
I have been tossing around the option of changing diff gears to 3.73 in my auto T3, so i gather from what you have stated that it would drop the speedo reading by 5kph? A 100kph reading would be 95kph. Can this be corrected easily?
Any idea on what the engine rpm would change to at 100kph?
Just thinking about fuel consumption. I had a price for new set of 3.7's installed for around 1400 bucks, i'm not sure if the gains would warrent the overall cost.