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MG MGB Technical - Speedometer question

I would like to know if anyone could tell me if the speedo I have is the correct one for my car? I have looked some but not found any info on my particular one. Car is a '70 MGBGT with Overdrive transmission.
Speedo I have is part SN 5227/06 1280
Thanks
Mike
MK Mike

As far as I know, the 1280TPM is correct for all 1972 cars, whether O/D or not. If the 5227/06 is correct, I don't know.
Derek Nicholson

Mike

If you search the archive for "speedometer 1280" you will find a thread with basically the same title. It states that all 68-74 Bs had the 1280 speedo.


A question to all: any ideas on what would cause a speedo to read slow? Just bought a 72 GT and drove it home at a reasonable (for California Freeways) indicated 65 to 70 mph. After we got home my wife, who was following in our 4Runner (which has a speedo that is spot on), asked me what I was doing running at 75 to 80. Can anyone give me a couple of "calibration points" of rpm vs correct road speed?


Larry
72 BGT (new to me)
69 C in restoration
Larry Hallanger

Mike,

the BL parts catalogue up to 1976 (AKM 0039, February 1977, first edition) does not list the speedo no. you quoted. The most close to it's matching number is SN5227/16 for car no. 187211 to 258000 when fitted with automatic box.

May be it came from a midget, looking identical to the one used on the B or does it show up to 140 MpH, than it was taken from a C.

Ralph

Ralph

Larry - in top gear, for every 1000 rpm your engine turns, you are travelling at 18 mph, unless you have overdrive engaged, then you are travelling at 22 mph. So, if your tach is accurate, which it probably is, then you can figure your speed in top gear.
I have found that if I am revving 2500 rpm in 3rd, I am travelling very close to 30 mph, probably spot on 50 km/h, which I believe is slightly over 30 mph. 3000 rpm in second seems to give 25 mph, and 3000 in first seems to give 15 mph.

Hope that helps,

Erick
Erick Vesterback

My speedo was checked to be within 1 mph all the way up to 70. My tach, on the other hand (using the same /18/22) reads 200 turns high. This is perhaps due to my non-original electronic ignition. --- 1970 roadster.


Dan Robinson

My speedo was checked to be within 1 mph all the way up to 70 on a rolling road. My tach, on the other hand (using the same /18/22) reads 200 turns high. This is perhaps due to my non-original electronic ignition. --- 1970 roadster.


Dan Robinson

After searching some more info in the archives and elsewhere it seems that just the part number of the speedo is not all that matters. As tire size, rear end ratio play a factor. I guess I want to match a speedo as close to accurate as possible with my larger tire size, 185/70 14 and OD box. This is the setup that came on the car so I am hoping that it's close to accurate. Will test it out when I get it back on the road. Thank You to all
Mike
MK Mike

Mike - your tire size is about perfect for having the stock rear ratios and stock spedometer work properly together, as far as I know.
My experience with the 2 MGBs I have owned is that the spedometer tends to read alittle lower - ie in both of mine, travelling at 55 mph the spedometer reads 50. This is why I have figured out what my speeds are based on the tach readings. I think it is because the magnets inside the spedometer become alittle weak with age, and don't pull the pointer as far around the dial as a result. If your speedo is in good shape, it should match well with your tires.
Erick Vesterback

Erick

Since I didn't know I was "speeding" I didn't carefully note the rpm but, as I remember at an indicated 65 it was just about, or very slightly over, 3000.

There are mileage posts on I-8 headed east. Guess I will have to make a calibration run or two when I get it on the road regularly.

Thanks

Larry
Larry Hallanger

Larry,

I am having the same problems I caused. Installed a 1976 OD in my 1972.

Searched the internet and got addresses and phone numbers for shops in Califormia and NY that will do the calibration (including gear changes in the speedo) if required.

Instructins were to measure from a starting point maked with tape to 59.5 feet. Disconnect the speedo cable and count the turns from the start tape to the measured 59.5 feet tape. Have someone count the revolutions, they will calibrate using these numbers.

James
James Huggins

PS

Sorry Larry, that was for Mike.

James
James Huggins

Erick, James thanks for the great info. I sort of figured the way the PO had it set up with the tires and rear end ratio and that speedo might be close. I will be replacing the tires with the same size and see what I get...If it ever gets warm here again Thanks
Mike
MK Mike

Hey Mike, I can't tell you which speedo to use but here is what I did to determine if I could use one. If you do the measurements and calculations, you can determine which speedo calibration you will need for your setup.

I am showing how to measure the acquire the calibration numbers needed off of 20 rotations on the rear axle.

Given:

Speedo calibration # = 1020 (Number at the bottom of the speedo, yours is 1280)
Mile = 5280
Rear end ratio = 3.909:1
unknown = speedo drive ratio
unknown = number of speedo drive rotations per mile (needs to match Speedo Calibration #)

Measurements:

For 20 tire rotations * 3.909 driveshaft rotations per tire rotations gives the driveshaft rotates 78.18 times or 78 + 64 degrees

So turning the driveshaft input to the transmission 78 times + 64 degrees, It was recorded that the speedo drive rotated 22 times + 280 degrees or 22.78 times.

Tire roll distance is 5’ 10.8” for 1 rotation, or 5.9 feet for my 205/50-15 tires.


Calculations:

Calculate tire rotations per mile:
5280 feet per mile / 5.9 feet per tire rotation
= 895 tire rotations per mile

Calculate the number of times the driveshaft turns in one mile:
895 tire rotations per mile * 3.909 driveshaft rotations per tire rotation
= 3498.55 driveshaft rotations per mile

Calculate the speedo drive ratio:
22.78 speedo drive rotations / 78.18 driveshaft rotations
= a ratio of 0.291 speedo drive rotations / driveshaft rotations

Calculate number of speedo drive rotations per mile:
0.291 speedo drive rotations per driveshaft rotations * 3498.55 driveshaft rotations per mile
= 1018.1 speedo drive rotations per mile (What you are looking for Mike is 1280)


Results:

Shooting for speedo calibration of 1020 and got 1018.1.

< .002 accuracy.

Going through this process will at least show you what speedo you need.

Hope this helps!

Gregg Hanks
http://homepage.mac.com/racerx69/PhotoAlbum1.html
73 MGB SC
63 MGB
69 MGC GT
70 E-Type 2+2
G Hanks

Hi Larry -
In overdrive, since there are 22 mph per 1000 rpm, 3000 rpm gives 66 mph, so your 65 indicated is great. With my first MGB, I was frustrated by the fact that the car would turn 4000rpm to go around 60 mph, but then it turned out my speedo was reading low, and I was actually speeding by a fair amount. Luckily never encountered a speed trap before I figured out what was wrong - wondered why there seemed to be so many slow drivers at the time!

Erick
Erick Vesterback

Eric

The problem is that the chase car, 2006 4Runner, speedo has been (I thought) very close to true speed. Difference is about 15%.

It is going to take some sort of "ground truth" verification. I guess a timed run throught the mileage marker posts on I-8 is in my future.

Thanks

Larry
Larry Hallanger

Hi MK Mike
Moss catalogue says for cars 1968-1973 the speedo no is SN6144/23, I believe the 1280 no is for an overdrive box

Colin
Colin Lanning

1280tpm was for both OD and non-OD and used from 67 Mk2 production for the remainder of chrome bumper production. After that it was 1000tpm again OD and non-OD.

Before 67 there were different speedos for OD and non-OD, at variously 1020, 1040 and 1060.
Paul Hunt 2

This thread was discussed between 05/03/2007 and 11/03/2007

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