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3.9 diff makes a diff!
Posted: Tue Jul 01, 2008 4:40 am
by Bazzalucas
Just finished installing a Midget 3.9 diff in my '58 2-door with a 1275 engine. My, what a difference! An easy swap, too- took about an hour. I filled the diff through the breather hole. It's now a relaxed drive at highway speeds.
Does anyone have a general list of the indicated and actual speeds? (My original diff was 4.55:1) Thanks!
Posted: Tue Jul 01, 2008 8:28 am
by d_harris
Indicated speeds vs actual speeds can vary depending on your setup. You don't mention what gearbox you have for example.
Easiest way to do it is get your hands on a satnav which displays speed and get a friend to read off at certain speeds and make notes.....
Posted: Tue Jul 01, 2008 12:53 pm
by Bazzalucas
Thanks, Dan. I have a Midget gearbox as well and the original "white" face speedo. I just thought that since this has been done quite a lot that someone would have a general idea of indicated versus actual speeds.
Posted: Tue Jul 01, 2008 8:02 pm
by Mogwai
Ive used this program (from the link below) to check my speedo
all you do is input tyre size gear ratios etc & it calculates your speed at different engine rpms & gears
http://www.locost7.info/gearcalc.php
Posted: Wed Jul 02, 2008 12:25 pm
by rayofleamington
based on changing from 4.55 to 3.9 the factor is 1.16
so 30mph would be indicated at 25.7
40 --> 34.3
50 --> 42.9
60 --> 51.4
70 --> 60
Posted: Wed Jul 02, 2008 2:09 pm
by Bazzalucas
Ahh, many thanks, guys. I don't have a tachometer or any friends with GPS, so the speed calc, while interesting, was problematic. I really just needed the rough numbers- thanks again.