work this out diff change

pcmx5 · Postby **pcmx5** » Thu Aug 02, 2007 5:45 pm

Just picked up my car from the diff place and it now has a 3.909 dif instead of a 4.1, in preparation for the 6 speed going in tomorrow.

With the 4.1 ratio it was an indicated 60 ( edit 50)klph at 2000rpm in 4th and now it is 65(edit 55)klph; how does that work, the speedo drive comes of the gearbox(it does doesn't it?), the gearbox is the same and the ratio means that the engine is turning over less at the same speed?

I was actually expecting it to read the other way,

Is this a seniors moment for me or is it strange?

Please be kind in your replies!!!

Peter

john! · Postby **john!** » Thu Aug 02, 2007 5:57 pm

Not a seniors moment, you now have a taller final drive ratio, so for a given rpm in a given gear, you should be going a bit faster.

Having said that however, when the diff was changed, was the drive gear on the gearbox end of the speedo cable changed to match the new diff ratio?

john!

pcmx5 · Postby **pcmx5** » Thu Aug 02, 2007 6:04 pm

john! wrote:Not a seniors moment, you now have a taller final drive ratio, so for a given rpm in a given gear, you should be going a bit faster.

Having said that however, when the diff was changed, was the drive gear on the gearbox end of the speedo cable changed to match the new diff ratio?

john!

Hi John,

Thanks for your response,

I understand that I will go faster at the same revs in the same gear with the same gearbox

I haven't changed anything else(yet) so don't understand how the speedo is reading faster when the engine, and therefore the 1 to 1 4th gear are spinning slower.I expected to be travelling faster but indicating the same!

It is not a big issue as the 5 speed goes for the 6 speed tomorrow I may need to get the speedo recalibrated, just a bit confusing.

Peter.

Matty · Postby **Matty** » Fri Aug 03, 2007 6:56 pm

The relationship between speedo reading and tacho reading should be unchanged.

pcmx5 · Postby **pcmx5** » Mon Aug 06, 2007 8:51 am

The 3.9 diff and six speed are in and the theyt is sensational; speedo nows reads spot on @100klmph(used to be an indicated 105 = a true 100); gear change quality is beter than expected,some slight baulking, occasionally 4th tp 3rd,which seems to have a bit of a dogleg to the right.

Performance increase by being in the right rev range more often and the increased responsiveness has to be felt to be believed.

Couldn' t be happier.

Peter