Speedo calibration

Hi,
Planning to install a 78 OD gearbox in my 72B, what is the best way to match my 1280 tpm speedo with the 1000 tpm drive in the gearbox.
Also if left as is, how will my speeo read ?
Thanks,
Barry

ratio is 1000 divided by 1280. Or about 78% of what it should read.
Hot rod shops and instrument calibration places will sell you a"box of gears". Here they cost about $60. I've got one for the toyota gearbox I'm using behind the V8. This is a matchbox sized box of nylon gears that changes the turn ratio. It goes between the end of the speedo cable and the gearbox and is a quick 5 minute fit, hand tighten deal. Unless your threads are very dirty (probably) in which case some wd40, a rag and a large pair of pliers will be needed to get the knurled threaded bit (not really a nut) undone.
You can just tell the shop the above ratio, or better yet drive behind a well calibrated car, or use a GPS, a distance and read the odometer to get an exact calibration. The furthur , the better. A distance of 5 to 10km will be heaps, long as you know exactly.
After 40 odd years the speedo may not be as accurate as it once was and this is your chance to fix it.

Barry
Just look on e bay for a 75-76 speedo. Or ask in the trader section. they were 1000. The kind that works with the later blue label o/d. Also buy an o/d cable so the angle drive is not needed.
Ken

x2 for Ken....just do what the factory did and switch speedos....forget all that 'calibration shop' stuff....too hard.

You can't use a later speedo, as earlier mentioned, the RBB blue label O/D cars use a larger speedo/tach, same size as the early metal dash cars. Investigate what Peter Sherman suggests or replace the drive and driven gears with the proper set. Be prepared to reset the bearing pre-load on your output assemby since the drive gear is part of this setup. You need to set the preload the same as your existing setup or you may get exagerated bearing noise or wear. It is not too difficult to do but the various distance pieces are no longer readily available so it may take a little ingenuity to achieve proper results.
Don't let me put you off on the task, just be forewarned.

Brad

Brad
sorry your wrong!!! 75-76 did not use the larger speedo please check it out 77-80 changed the dash style. I have one so that's how I know. Now you may come across a late 76 with the 77-80 dash but that production date should have been titled as a 77.
Ken

Ken's right.........USA models used both 80mm 1000tpm speedos and 100mm 1000tpm speedos..........you just need to buy the correct year and you are good to go.

OOPS! Yes, Ken, you are correct. I must have been experiencing a "Brain Fart" and got stuck on the 1978 year mentioned?

Thanks for bringing it to my attention.


Brad

Another alternative is to send your speedo to Nissonger for recalibration. That will keep the original speedo (and mileage).

Thank you all, I'm going to get a 75-76 1000 tpm speedo to speed up my project.
Cheers,
Barry

You guys are aware that the speedos do go out of calibration? Just spinning magnet after all and they are all very old now. And of course wheel size tyre size has an effect. Speeding fines are severe over here. Be a bit expensive if you get it wrong. Drive 10 km exactly , read the odometer ie 7.85miles, tell the rod shop this. screw on $50-60 ratio box. Done.
As opposed to spending quite allot on a second hand speedo of uncertain exactitude, that will take ages to get to you and you've got to get up behind the dash to fit.
decisions decisions