I want to lower the gearing on my 3.2 auto.

Why? It has 18"wheels with 245 40 18 tyres which may or may not be oversized, not sure. But it also has an LPG tank in the boot which adds a little weight. So it feels a fraction sluggish. Only a fraction mind, it's a seat of the pants thing so I may well be imagining that bit or I have just got used to it....

But more than that, as said before, as stock it's geared for top speed which I'll never see round here, too much traffic, and it's a free tune up so to speak. Better acceleration with out altering or stressing the engine tune as stock.

As some know I have already swapped the stock dif thinking it was lower geared, but it's the same as the stock one. This was intended to be an experiment to decide if the gearing was suitable for a rebuilt LSD to that gearing, but as it's the same we're back to square one.

So, anyone know what gearing resides in a plod spec auto dif...? Needs to be lower than 4.2. And should give 3.500 rpm at 80 mph as opposed to 3000 rpm at 81 or whatever it is as standard.

I have suspected for some time that there are two different ratios of plod spec (e.g. LSD) diff.

I am pretty sure my mini facelfit 3.0 estate auto have a 3.9:1 ratio diff which I know was LSD (as this is what was used for the oil change guide!)

Ah yes, memory fails again. 4.2 was the target spec. Not the stock spec of 3.9.

So if that is a 4.2 installed currently it's not low enough. Or it's brought the current gearing back to standard with those wheels...?

So yes, need to evaluate what's currently fitted correctly.

Continental SC3 in 235/45/17 has a WRPM figure of 821.

At 245/40/18 it's 811. Not really a significant difference.


At 80MPH with the 235/45/17 Wheel RPM is 821*(80/60) = 1095

Multiply by diff ratio 1095*3.90 = 4269 at the propshaft

Multiply by gearbox ratio 4269*.72 = 3074 at the engine.

With the new tyre size 3074 * (811/821) = 3036 RPM

With the 4.22 diff 3036 * (4.22/3.90) = 3285 RPM

So, bearing in mind that the speedo will be over-reading, if you are seeing 3,000 RPM at an indicated 80 you might well have the 4.22. I know mine shows almost 85 at 3,000 RPM.


EDIT: Hmm. That might not be right, actually. Need to have a look on the way home.
Either way, at a true 80MPH you should be doing 3285.

Kevin Wood wrote on 03. Aug 2011 at 11:56:

Oh ok. So "my seat of the pants" is quite accurate. And given I haven't really understood most of those calculations, what does that tell us... 

I'll set my thingy nav on the way home and see what rpm we have at a gps indicated 80mph in top and third gear. See if we arrive at the same figure. 

Having found a nice roundabout out Brackley way last weekend I can see a fitting end coming up for those Linglongs my wheels came with.

Anyone got any Falkens they want to kill. 

Just got home. True 80mph on nav (flicking between 79 and 80 over a good 10 miles)with cruise set shows 3250 rpm. Press sport mode or engage 3rd gives 4550.

Ok thanks Kev. 

So that proves that, which is good. But it also proves plod spec is lower geared/higher rpm at 80.

Keeping things simple for a second  an idicated 80 in an auto plod spec car gives 3500. An indicated 80 in mine gives lass than that. Needs another 250 rpm at 80. As I understand it.