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speedo slow, TPM

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Hairyone

Hairyone

Jedi Trainee
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My Frogeye's back on the road. Found a 3.7 diff and fitted it,
nice at around 70mph. (1275 engine in it).
Speedo's well out, about 17% slow. 50 is the new 60.
My speedo has a TPM (turns per mile) of 1472, (may not be original one) the easiest fix would be to replace it with one with less TPM, but from what car?
Gearbox also not original.
Any ideas??

My bugeye
 
Three ways to go. ..

Bike Speedo, with sensor on the brake drum, route the wires thru the tunnnel.

Send the speedo out for recalibration.

Make a chart to reference, then live with it and do the math in your head as you drive.
 
It's reading low because the driveshaft is now turning 12% slower that it would with the original 4.2 rear end. If you check it, you'll find that the odometer is also reading low. The rest of the error is just...error! You might be able to fix this part of the problem by simply cleaning up and relubricating the speedometer internals (with graphite or dry moly lube), but you'll still be 12% low.

Weren't the 3.7 rears only on 1500 cars? If so, you can't just swap out the speedometer drive gears, unfortunately, because the transmissions were different. You probably could, however, put in a gear for the 3.9 rear end, and the error would only be 6%, which is lost in the noise. I'm assuming that the later cars with 1275s and ribcases use the same TPM number; if not, a speedometer shop might have an appropriate correcting gear set.

I see the gears on ebay occasionally. I'm not sure what the restrictions are on interchanging them, if any--perhaps someone more familiar can comment. However, I have a ribcase (later) transmission with the speedo gear for the 4.22, and it all seems fine.
 
If you are sure you are going to stay with that gearing, then I'd spend the cash on recalibration of the speedo.
 
forgot to mention... I switched to electronic so that I can adjust as needed with varying tire sizes and measured miles on TSD rallies. :wink:
 
Which 'box do you have in it?
Both Datsun conversion kit makers (Rivergate and Morrisservice) sell Speedo correction gears (which mount on the trans I think). You send them the tire diameter, diff ratio and speedo info.
R'gate is about $120 IIRC.
Not sure about the other conversions..T-9 is it?
 
Get or borrow a GPS unit. Drive the car at 60 with the face off the speedo. Find out exactly where 60 is reading on the speedo. Pull over and manually move the needle up to where it was reading at 60 and hold the stem with some needle nose pliers behind the needle and pull off the needle and stick it back on at 60. Then let it go. It will read above 0 when you are stopped but will be accurate in the ticket getting range. Mine was off about 22%. I have the speedo reading correct but the odometer is off, so I have to add 22% to it to find out how far I have gone and MPG, etc.
 
I haven't taken one of these speedos apart yet but I managed to get one (on an Opel) to read accurately by pulling the needle off and changing how much preload there was on the spring. Not sure if this applies or is even relevant but what I did was:
take face off, pull the little pin the keeps the needle from going beyond 0, let the needle fall back, mark it, pull needle off, put it back on a little more anti-clockwise then before, move ahead, replace pin, drive and check.
 
Thanks all, I've no idea what the gearbox is but it ain't broke (yet)so it's staying, & I love the gearing. I know there is a reverse light switch on it & the speedo drive is white. I've seen how to move the needle & that's OK, but it'll only be somewhere accurate at 60. We have some Nazi police here with Gatzo's (speed cameras), a lot of which are in 30mph limits.
Anyway; There are later speedos with 1280 TPM if I change to one of these it will be closer I think. I've also found one on fleebay with 1152.
Any thoughts?
 
You may want to try the speedo out of an MGA or early MGB, I don't know if the correction will be perfect but it should be closer. They are both the correct size and do run slower than one out of a Spriddget.
BillM
 
YES. Speedo is now about right. Against a sat-nav the speedo over reads by about 5%. That plenty good enough for me.
The solution: I picked up off fleebay a speedo with a TPM of 1152. The trip was located on the back, but inside the unit they both had the same mounting holes. So I was able to fit the "new" speedo into the original frogeye case with the original trip.
ta. ta!
This is with a 3.7 diff
 
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