3.9:1 Diff

Post by **myminor1969** » Fri Mar 25, 2005 11:19 am

Has anyone used a 3.9:1 diff on a 1098cc. I used to have a 4.55:1 then went to a 4.22:1 it made some difference, but I find on long journeys the car is happiest cruising at 55mph but is somewhat noisey. I wouldn't mind a 5 speed box, this seems to be an expensive option though. Thanks for any opinion. Alex

Post by **rayofleamington** » Fri Mar 25, 2005 2:44 pm

I asked this a couple of years ago and there were a few ppl who'd used a 3.9 diff and were happy with the lower revs at high speed. The downside is loosing a bit of acceleration and needing to change down on hills but if you do a lot of long journeys it would be a nice option.
The downside is that a 3.9 is very hard to find.

Post by **maxwoods** » Fri Mar 25, 2005 5:58 pm

Myminor,

I fitted a 3.9 diff to my 1098 earlier this year - the most noticeable effect is the loss of acceleration and the need to change down a little more often.
Conversation does prove a little easier tho'
The other effect is that of speedo readout -the speedo measures at the gearbox and assumes a 4.22 diff - if you change the diff then you change the final drive ratio, and hence the speedo will underread if going from a 4.22 to a 3.9. I am not a scientist but I believe that most speedos overread by about 10 percent anyway, and the change from a 4.22 to a 3.9 diff represents an approximate change of 10 percent. So what your speedo reads should not be far off (but do not quote me!).

A good source is any MG midget website. the diff is appropriate to the 1275 model, not the later 1500 model.
Expect to pay £60-100.

The part bolts straight into the standard moggie axle and onto the std moggie prop.

Max

Post by **Cam** » Fri Mar 25, 2005 6:00 pm

most speedos overread by about 10 percent anyway

Are you sure?

If you get the 5-speed gearbox and keep the 4.22 diff then you get the best of both worlds - higher cruising speed and decent acceleration, but of course it's hassle and can be expensive.

Post by **myminor1969** » Sat Mar 26, 2005 4:16 pm

Thanks all for the info. The reason on thinking on changing the diff was for cost reasons and it would be easier. Unless someones got a 5 speed complete with fitting kit for £100.

. alex[/code]

Post by **les** » Sat Mar 26, 2005 10:00 pm

If the 'cost' relates to fuel saved I should think the initial outlay would take forever to recover in saved fuel.

Post by **rayofleamington** » Sun Mar 27, 2005 2:01 pm

If the 'cost' relates to fuel saved I should think the initial outlay would take forever to recover in saved fuel.

I think he meant the cost benefit when comparing it to a ~£300 to £600 Ford 5 speed conversion

Post by **les** » Sun Mar 27, 2005 7:36 pm

Yes, looks like it Ray, doh!