gear ratio/5 speed/diff

[mcjjordan]

hi guys been to a show today down on the south coast
had to go on motorway and wasnt nice-

i have a 1980 316 4 speed and at 70mph engine was at 4000rpm !!!
but speed was 100% accurate according to gps

whats my options?
-diff
-5 speed box
-wait and ditch the whole running gear and replace with 6 cylinder engine ie m50?

thing is cars only done 39,000 miles

[Mihajlo]

source 5speed gearbox and 3.91 diff

I know your problems.. been there, done that.

[mcjjordan]

how will that affect the speedo?

[Mihajlo]

You will travel little bit faster than speedometer will show.
But you can allways take both diff and speedometer from donor car and not have worries about speed differences.

In my case, my speedometer get more accurate when we changed diff ratio from 4.1 to 3.91 (according to GPS and speedometers by the road. )
Oh, yeah, and it also depends what size of tyres you have. I have 195/50/15

[Jeroen]

If you change the final drive (diff) then it is advisable to checnge the speedo as this has a correction factor related to the final drive. Then it will be accurate again. See here: http://www.bmwe21.net/forum/viewtopic.p ... 9&start=28

[mcjjordan]

think i may well live with it until i can do a m50 conversion

but thanks for your help