urgent - diff ratios help

[wonkey eyed barmaid]

hi folks,

 

just about to order a replacement diff for my live axle classic - but I have a dilema....

 

I know the standard ratio is 3;89 - which I am keen to keep as Im happy with cars performance, and dont really want to mess with it.

 

However I have been offered a good 3;73 diff - and im swaying over whether to go for it or not...

 

i know the grad's use 4;11 which gives quicker acceleration, but lower top speed. along with (presumably) more fuel consumption, higher motorway revs etc. so I dont fancy this option as its an (almost) daily use car.

 

But can anyone confirm the differences the 3;73 is likely to make? are they going to be noticeable? (3;89 to 3;73 is less of a jump than 3;89 to 4;11)

 

has anyone tried this ratio...

 

Im told that the diff has 41/11 teeth, so is a finer set of cogs and may prove to be stronger than the 3;89

and Im presuming that the 3;73 will reduce acceleration, increase top speed, give better motorway cruising and better mpg...

 

would I be correct - or will the differences be negligable.

 

help - quick - so i can order today in time for sat

[DanB]

Switching from a 3.89 to a 3.73 is only going to give a 4% difference - so if you're doing 4000rpm at 80mph with the 3.89, with the 3.73 you'll be doing about 3750rpm instead. It may give a higher top speed although the seven's aerodynamics usually come into play before you run out of gears and as a result the top speed may actually be lower - it depends on the gearing the car currently runs.

 

The lower diff should make the car more economical (albeit not by much). Acceleration shouldn't really be affected significantly assuming you can use the 'box and use the appropriate gear to keep the engine on song.

 

Dan

[wonkey eyed barmaid]

cheers dan,

 

thats pretty much what i thought. road and race transmissions have confirmed as much to...

 

3;73 diff now ordered. i'll let you know how i get on.

 

will likely buy a 3;89 diff as well in the very near future. just to keep as a spare in case i dont like the 3;73, or incase the 3;73 goes BANG

 

if the 3;73 dont go bang, i can always put the 3;89 in when i start doing track days in the new year

 

thanks.

[Mole]

One of my favourite topics - speedo accuracy . I think changing to a different ratio diff will change the relationship between your speedo reading and your actual speed. The change may be negligible and if you have an inaccurate speedo (as so many people seem to) then it might actually be an improvement ❗ Nb the relationship between actual and recorded mileage will also change.

 

SV 52 CAT - the Mole is flying

[wonkey eyed barmaid]

hmmm.

 

interesting point mole - didnt really consider that. (funnily enough, speed was the first issue i though of )

 

so what difference will it make. lets say for arguments sake that my speedo was accurate before. so no, when my speedo says 40mph (with the new 3;73 diff). will my actual speed be + or - ?

 

and recorded mileage - with the new 3;73 diff instead of 3;89, will it be artificialy high or low?

 

im sure any changes will be very small - because just changing the tyre profile, and altering the rolling radius would have these same effects..

[Mole]

There are previous threads on the subject which should answer your question (search on "speedo" or "speedometer"). You are right about wheel size as well. A lot of people seem to fit motorbike speedos which work off a sensor on one of the wheels and are calibrated for wheel size - this takes away all the confusion of diff ratios etc in measuring your actual speed. Also (as on a previous thread) it is possible to use a handheld GPS unit to get quite an accurate measurement of speed, but only over level ground - this would give you an idea of the accuracy of your indicated speed.

 

SV 52 CAT - the Mole is flying