What gauges are we lookin at anyway??? :S

I just got an email from a fellow who installed a late O/D box in a 67 MGB and converted to negative ground. He now has the on going speedo/tach problem...what to do. Comes up every month or so like clockwork.

I DO NOT believe in getting a speedo 'recalibrated' by some guy who fixes old Hudson gauges. I do it the way the factory would have.

Example: If I am using a 68-73 1280TPM trans....I get a 68-73 1280TPM speedo from England. They used the same 100mm gauge with matching face as the 63-67 cars. Super easy to get.

If you are using a 1000TPM trans, it gets a bit more specific. No MGB used a 100mm early face style 1000tpm gauge. However, other cars did. The BJ8 Healey had a 1000TPM 140mph gauge (64-67). That is the gauge that is in my 67 roadster (see picture). It is not an exact match.....the center of the needle is different. For an exact match you need a 75-76 UK Midget speedo (see picture). Why modify, why not just buy the correct one?

Same goes for the neg ground issue. Early MkII MGBs used the same RVI type tach as the positive ground 65-67 cars....but with neg ground. No need to change any wiring. The later large gauge UK MGB cars used the RVC type tach and is easy to wire...add 1 wire. The UK Spridget used the same setup but with a 6250 redline rather than 6000 like an MGB...see picture.

I suspect the reason most people don't do as you suggest is that they simply were not aware of those options. Great advice (as usual)!

That'll get the odometer right. The correct way is to change the gearing in the speedo drive in the gearbox.

I doubt if the factory ever considered fitting a late model OD to an early car.

You're right, it does come up often and the same incorrect answers are given.

Lloyd, what about an early car?

If I recall properly, I heard the non-OD 3-synchro boxes are 1040tpm, and the OD 3-synchro boxes are 1000 (or 1020 maybe?).

I have a '63, which has a 1040 Jaeger.

So, with such a small discrepancy...is it worth it to even modify mine?

Everybody....please with the emails, OK?

I just offered this up as the easy/correct way to fix your late trans/early car problem. I don't care how you choose to do it. Really...don't have no horse in this race.

The older I get, the more annoying I find certain tasks on an MGB. Removing/replacing the gauges is one of them. I hate that job....so I don't want to do it. I say do it right and forget it.

Someday, when I restore my 67 (IF I RESTORE IT) I'll make everything pretty. For now, I just want to know how fast the engine is going and how fast the car is going.

My 2 cents.

The speedo is calibrated via the hairspring and magnet etc. The odo is calibrated by the gears in the head as you state. They all work based on the tpm on the face of the gauge. What you are saying is this: If your MGB uses a 1280TPM gauge and you swapped in a 1000TPM gauge, the speed would not be effected...only the odo would be wrong. That is simply not true...as dozens of people on this forum have discovered. Whatever.....as I stated, I have no horse in this race.

I think most of us enjoy the polite banter...... (tu)

Herb...you missed my point....which is the early posi ground cars used the RVI type tach and a 100mm gauge face. 68-71 UK MGBs used the same RVI type 100mm tach but with negative ground. Therefore....the easy switch is to use the UK tach when you switch your early car to negative ground....just stick it in, no wiring changes or "internal fixes" needed.

EDIT: I say the same "type" ie: RVI....I don't mean the exact same specific tach. Get a tach from a negative ground car and you are good to go. If the tach is an RVI type...then you won't need to modify the car's wiring.

Wow. When I installed a 4 synchro/Od trans from a '72 into my '67, I used the internals from the '72 speedo inside the housing of my '67. An easy job, if you have a matching TPM speedo. RAY

Hi Lloyd,

It's not quite as simple as you say.
The +ve tach used a loop of the ign wire, through a gizmo (current transformer) external to the case, whilst the -ve ones had in internal gizmo, that had two connectors at the back. To fit one of these you would need to cut the ign wire, fit bullet connectors, the right way or it won't work, then connect to the tach sensing terminals.

Herb

Herb...you are correct...early has external loop....negative ground version has internal loop with wires sticking out the back.....snip the loop, check which way is the correct way (by trial and error...only 2 options) and fit up. No need to string an extra sensing wire to coil as you would with a RVC tach....no taking of anything apart...well within the capabilities of the MGB owner.

I learn something new on this site nearly every day. Thanks Lloyd and everyone else.

...and that's what it's all about!