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odometer off by 25%, spedometer correct
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David_Doan
Jedi Warrior
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Which way is it off? What tranny and diff do you have?
Speedo is correct all the time are just at cruising speed?
Speedo is correct all the time are just at cruising speed?
Odometer shows I go an extra 25% further (for example if I travel 100 miles, the odometer will show 125 miles).
Speedo is correct all the time.
Don't know what differential I have. Tranny has a spitfire overdrive (speedo is still correct if OD is on or off).
For the odometer I have also compared distance traveled in a modern car and it confirms it is off. Which confirms what the GPS in my phone is showning.
Can anyone give details on how the speedo and odometer work to help in figuring out why one would be correct and one will be off?
David_Doan
Jedi Warrior
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If the speedo is functioning normally, they should both be off by the same amount. It sounds like since your odo is wrong but the speedo right, that whoever did the spitfire tranny conversion calibrated the speedo without "fixing" the odo.
The odometer is setup for "x" turns equals a mile. Stock speedo for my bugeye is 1472 turns per mile. If you turn the speedo at 1472 RPM the speedo should say about 60 mph. A 2 speed cordless drill usually has speeds around 700 and 1400. You can use that to get a ballpark of where your speedo is.
Who ever changed the tranny might have tried to "calibrate" the speedo to compensate for the tranny or diff change.
Your speedo should read 25% fast. Maybe it is just dragging due to being old and needing lube.
The odometer is setup for "x" turns equals a mile. Stock speedo for my bugeye is 1472 turns per mile. If you turn the speedo at 1472 RPM the speedo should say about 60 mph. A 2 speed cordless drill usually has speeds around 700 and 1400. You can use that to get a ballpark of where your speedo is.
Who ever changed the tranny might have tried to "calibrate" the speedo to compensate for the tranny or diff change.
Your speedo should read 25% fast. Maybe it is just dragging due to being old and needing lube.
mikephillips
Yoda
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Actually your forgetting that the driveshaft will turn faster than the wheels due to the rear end gearing so that 1475 will equate to that times whatever the rear ratio is.
The way the odometer and trip meter work is by a pair of gears driving small shafts with and offset pin on the other end. The gears are driven by the cable input where there is a spiral cut which turns the gears which then moves the offset pin in a circle which then moves the tip and odometer. Only way I can see the speedo being correct and the other two being "fast" would be if the spiral/driven gear and shaft weren't correct for the application. That could drive the two faster, although it would be faster across all speeds, while the magnet in cup for the speedo would remain correct as neither drives the other.
The way the odometer and trip meter work is by a pair of gears driving small shafts with and offset pin on the other end. The gears are driven by the cable input where there is a spiral cut which turns the gears which then moves the offset pin in a circle which then moves the tip and odometer. Only way I can see the speedo being correct and the other two being "fast" would be if the spiral/driven gear and shaft weren't correct for the application. That could drive the two faster, although it would be faster across all speeds, while the magnet in cup for the speedo would remain correct as neither drives the other.
David_Doan
Jedi Warrior
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What I meant was, connect a drill directly to the back of the speedo (a square screwdriver bit works well) and if it is turning 1475 rpm, the speedo should show 60 mph. This will independently check the calibration of the speedo itself.