Lotus Elan

Speedometer/odometer correction for retrofitted gearboxes

PostPost by: msd1107 » Wed Feb 20, 2008 9:34 pm

A current thread (http://www.lotuselan.net/forums/viewtopic.php?t=15954) is looking at various possible 5-speed transmission conversions.

These transmissions may come from cars with different tire sizes, differential ratios, or speedometer input turns/mile specification that lead to incorrect speedometer/odometer readings after everything is done and hooked up.

Many transmissions have a selection of drive speedometer gears (usually a press fit on the output shaft in the transmission housing that requires at a minimum a partial disassembly to change) and of driven speedometer gears (usually accessable at the side of the transmission where the speedometer cable screws on).

Enclosed is a downloadable spreadsheet that allows you to see if there are going to be any problems with a transmission, or to see what speedometer gear changes are desirable if tires, diff ratios, or speedometer models are changed. This is useful even for a standard car if you have changed tire sizes or diff ratios.

Input the tire size data, the diff ratio, and the speedometer turns/mile figure (usually the zzz figure from the small xxxx/yy zzz small numbers on the face of the speedometer). The spreadsheet then gives you the closest speedometer driven gear for a varity of drive gears, and the percentage error.

Try to get under 1%. If you can't, or the best gear is not available, there are people who sell conversion boxes, miniature gear sets that will get the correction quite close. Remember, your tires change by as much as 3% from a new full tread to completely worn.

Sorry for no user instructions like on other spreadsheets, maybe I will update it.

1968 36/7988
20080220 DiffSpeedo.xls
(113.5 KiB) Downloaded 450 times
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PostPost by: TomR » Wed Feb 20, 2008 9:54 pm

For my T5 conversion the difference was too great unless I got a tach with different gearing, so I finally decided to use an electronic sender and bought a Smiths Cobra speedo/tach set. These look much like the originals but let me cooperate with the tranny and the engine control.

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