Speedometer W/Overdrive

Hi all. I have a 1974 MGB roadster that I just finished restoring, mostly. The car did not come with an overdrive originally. The guy I bought it from put one in from a newer, 76 or so. It works just fine, but I have questions about the speedo being accurate. It seems to be around 8mph slower or so. So my trip meter is also off. Should I have a different speedo, or is it that any speedo, correct fitment of course, will be ok and that mine is just now working properly? I see speedometers on ebay that say they are from cars with O/D, but mostly newer.

So, do I need an overdrive speedo and will one from the newer years fit? and do I need the angle drive and is that a problem as to the accuracy with mine not having one.

Thanks

Russ

This is probably the most common question of the forum. The speedo has to match the transmission: dip stick trans needs a 1280tpm speedo (little number on face)....the side fill trans needs a 1000tpm speedo...you can get one that matches your trans and your dash....just need to know which one to look for.

Russell
What O/D do you have the Blue Label? If so you would need the 75-76 speedo that has 1000 as the last numbers on the bottom. If the black label the original speedo would be correct. You might just need to replace the O/D cable sounds like something might be wrong with that.

If you get a long enough cable.....no need for the angle drive.

I believe the later ODs (side fill) were geared for a 1000 tpm speedo. Your '74 has a 1280 tpm speedo.
I dont know the diameter of the 1000 speedo so cant comment on how it would fit the dash.
RS

75-76 with 1000 tps on the bottom of the face will work or you could send your speedo to Nisonger and they will change the gears to read 1000 tps while cleaning the unit.

http://www.nisonger.com/

Handy dandy little chart from the library here illustrates the range of tpm's used and which ones you'd be looking for:
http://www.mgexperience.net/article/speedometers.html

Well, there you go, lots of information like I hoped. I have the dipstick trans w/overdrive and the speedo is the 1000. Hmmm, will have to look around in the parts bin to see if I have the 1280. The cable length is no problem without the angle gear, so will hope I have the 1280. If not, I saw quite a few on Ebay of the O/D type.

Thanks guys!

Russ

O/D and standard are the same....don't pay more for an"O/D" type....just get a 1280 speedo (80mm).

Took a '72 OD tranny, put a side fill case on it, changed the speedo gears from 1280 to 1000 and now have a.......Black and Blue Label OD.

thanks for the good advice balloonfoot!

Russ

You can also have West Valley Instruments 818-758-9500 in Van Nuys do it. They may be less expensive and faster than Nisonger.

Throw away the angle drive and just use the od cable which is more than long enough. The angle drives are notorious for eating up cables as well as speedo gears. RAY

Well, I never had the angle drive, but wanted to make sure it wasn't a gear reduction or opposite. I did find a speedometer with the 1280 in my parts bin. I took it apart just to make sure it was all there, cleaned the glass and face some too. Now I have to get a friend to run with me to see if the speed is correct now. I bought a speedo cable a while back but when I got it it had the big end for a different speedo. I spliced the smaller screw cap to it and it worked great.

We got a cold snap here in the south finally and found the heater fan doesn't get power to it. It works with a direct wire tho, so I have to start tracing wires again. I sure appreciate all the help and advice!

Thanks

Russ