speedometer drive

[another old car]

Hey folks<BR>I am swapping rear end gears , going from a 4:11 to a better ratio of 3:50<BR>nows heres the question - the drive gear is 15Teeth (one on tail shaft ) - driven gear is 42Teeth ,<BR>what change do I need to make to get speedometer reading right again <BR>

[ply33]

With the original rear end and the speedometer gears you mention, the speedometer cable turned 1.467857 times for every time the axle turned. (4.11*15/42).<P>To get about that same relationship with your new 3.5 rear end and same speedometer drive gear you would need a 36 tooth driven gear (3.5*15/36 = 1.458333). This would be a difference of about 1 percent.<P>I was told many years ago by a speedometer repair specialist that all American cars were set up for 1000 turns per mile on the cable. If that is true, then you either have <ul>[*]A 14.8" effective wheel radius. (From center of wheel to road.) I'd hazzard a guess that this works out to a 17 or 18" rim diameter. This sounds a bit large for a 1941 vehicle.[*]Your speedometer was not reading correctly before the rear end swap.[*]You have counted the speedometer gear teeth incorrectly.[*]The repair person told me wrong (and my car which fits his rule is an exception).

I'd double check your teeth count before spending money on replacement gears.<p>[ 03-18-2002: Message edited by: ply33 ]

[another old car]

To ply33<BR>thanks for advice about the gears - yes you are correct about now needing a 36 tooth driven gear , stopped by trans shop on way home , the guy sold me a 36 tooth gear , but didn't go into detail , but ratio needed was 2.4<BR>thanks again - always good advice here on this forum