• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Bizarre speedometer problem

Share this page

The_architect

The_architect

Jedi Trainee
Offline
I recently sent my Bugeye speedometer overhauled by an outfit that has a very good reputation for doing this kind of work. I had a couple of nice lengthy conversations with the lady who was going to rebuild it. She asked what size tires I was going to use, what the rear-end ratio was, how many turns the speedometer cable made when the car was pushed 50 feet and so forth, and she said she would fix it up.

Well, I got it back, put it in and went for a drive...and I thought, hmmmm. I'm only going about 45 and the speedo's a little over 70...I've driven 4 miles and the odometer is reading about 6.4. For every mile I go it's reading 1.6 miles. I have to give my Dad (79 years old and still a genius) credit for noticing that 1.6 is roughly the number you use to convert miles to kilometers.

Now, this unit was not installed in the car when I got it--it was just in a box of stuff that came with the car, so I have no idea of its condition or what its internals were...all I know is that it is a Bugeye dial and it says "miles per hour" on there.

If I understand one of the manuals about Smiths speedometers that is available on the web, the number of teeth on the gears of a KPH odometer are is different than what you would find on an MPH speedometer--this is not a matter of calibration, but the whole gearset of the KPH speedometer is different. So,you're not going to turn a screw inside there little too far to the left or right and suddenly get a 60% error.

Can anyone verify this for me? I'm suspecting internals from somebody else's gauge might have been accidentally put into mine but who knows, maybe my internals <span style="font-style: italic">were</span> from a KPH speedometer. Either way wouldn't this have been obvious when the unit was being calibrated?
 
Brit speedos are set up differently than most others that use 1,000 RPM as a standard 60MPH.

If they asked your gear ratio, tyre size, and turns of cable on 50 feet, that told them what the cable should be turning and they should have set it up that way.

I have heard a lot of interesting comments on various rebuilders and results that were not stellar....hence my comments on using error correction gearboxes in the cable to fix it IF speedo, odo and trip are all off the same percentage.


To do odos and trip, takes new gears (which are sometimes made of unobtainium).

Do you have any speed correcting gearboxes in the line that may have been added by a PO to fix an issue?
Shouldn't be, if you counted the turns and reported correctly.
Turns in 50 feet should have told the speedo folks exactly what was in the mechanicals.



BOTH of your readings are hosed, so you're going to need to start over.
 
I recently went through the same process you did on getting the speedo rebuilt. What number did you give the rebuilder for the # turns per 50 ft?
Scott in CA
 
Your readings are both off by 60%.
1 mile X 60% is 1.6 mile.

45 MPH X 60% is 72 MPH

That means both readings, speedo and odo, are correct (or close) to each other.
But 60% is WAY off.

No matter what your tyre size or rear end ratio, the cable turns in a given distance should have allowed the rebuiler to set it just right.

Do a recount.
 
Thanks for your replies. No, there aren't any extra gizmos, unless they are inside the transmission. If I recall correctly, the number I gave was 15 more or less. What was yours, Scott?
 
I'm not sure the number of turns per 50 ft, etc, but it was calibrated to 1350 turns per mile. This is the standard way that speedos are calibrated. I have a 3.9 diff and running 155/80-13 tires.Doing the math with 15 turns/50 ft. and 5280 ft./mile yields 1584 turns/mile. Looking up the history of Sprites, 1472 was used with the 4.22 diffs, 1376 was used with the 3.9 and when the 1500 was used, it went to 1000.
Hope this helps.
Scott in CA
 
Yes, it helps. I have the 4.22 diff, and am running 175/70-13 tires, so I presume the 1472 would be closer to what I am looking for.
 
The difference in what your speedo should be calibrated to vs what you told the speedo company would not account for the wide inaccurancy that you have. Something else is going on.
I would suggest you call up the speedo company and talk to them.
Good Luck,
Scott in CA.
 
Back
Top