3.9 diff makes a diff!

Post by **Bazzalucas** » Tue Jul 01, 2008 4:40 am

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!

Post by **d_harris** » Tue Jul 01, 2008 8:28 am

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.....

Post by **Bazzalucas** » Tue Jul 01, 2008 12:53 pm

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.

Post by **Mogwai** » Tue Jul 01, 2008 8:02 pm

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

Post by **rayofleamington** » Wed Jul 02, 2008 12:25 pm

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

Post by **Bazzalucas** » Wed Jul 02, 2008 2:09 pm

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.