Tech Forum : Speedo after Diff change

MayanArch	Posted Tuesday, Aug 14th 5:53pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 1331 Florida, USA Baur E21 2.7 1982, Baur E30 318i 1985, 323i 1978	Do I need to get the speedometer and RPM guages updated after upgrading up to a 3:91 LSD? Shouldnt the RPM and Speedo guages be off?

sunbeam***	Posted Tuesday, Aug 14th 11:23pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 307 wirral UK 1982 323i	You will have altered the gearing after changing the diff but this will only affect the speedo not the rev counter as this works off the engine which will always do the same rpm regardless of drive ratio. I'm sure someone will know a formula for calculating actual speed based on gearing but i would just pace your car against another car with known true speedo to give you an idea of how much your speedo is out. I dont think a re-calibrated speedo will be cheap?

Jason_323i	Posted Wednesday, Aug 15th 2:16pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 455 UK, Scotland 323i 2.7 tripple webber track car. Maserati Biturbo	Hi Each speedo has a correction factor there is a sticky on Jorens fourm, bmwe21.net which matches the correction factor to the diff ratio. Also in the US there is acompany called holywood cables i think who can recalibrate the speedo. Jason

paul dyer	Posted Wednesday, Aug 15th 11:48pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 34 manchester england 1977 rhd 320i/4	your speedo should read a little high!A speedo from a late 320i(1800cc)should be spot on (one of those nasty 85mph jobs)or if you can find one a speedo from a english(mph not kmh)late model 316(1800cc)120mph max also has a 3.91 diff