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MG MGB Technical - Speedo RPM Vs. MPH

Hi everyone. After getting my speedo to actually work w/out jumping, it now seems to be off. I tested it by driving 10 miles on hte highway using the mile markers. Using the speddo at 3200 RPM which should be 60 MPH,speedo readds around 70 - 73 MPH, it took 9 mimutes and 4 seconds. Return trip using speedo at 60 mPH, RPM's at 2600, it took me 11minutes, 3 seconds. The actual mileage recorded on the trip odo. = 10.3 miles. The speedo is a 1280.

Any ideas??

Ken
70 B
Ken Harris

Ken. Back in the days, the standard for mechancial speedometers was an accuracy of 10% according to the material I have read. Add to this the aging of the gauge and different tires and you have error. This can be corrected by an instrument rebuilder and Nossinger (sp?) has told me they can do it.

The stock MGB did 17.9 mph per 1,000 rpm in fourth gear. Thus, 60 mph is about 3,350 rpm.

The odometer is a direct, mechanical connection to the transmission. Hence, it is often more accurate than the speedometer which is only indirectly connected to the transmission. The fact that your odometer has a 3% error is perfectly within acceptable limits to most of us. The speedometer issue is up to you. I drive off the tach myself.

Les
Les Bengtson

I think there was a 1080 and a 1280. One was for the overdrive. I wonder if you have an overdrive speedo mix up? Not sure
Frank Baker

I know that both my tacho and speedo are off, tach reads low and speedo high, hey it makes the car seem relaxed on amotorway
Stan Best

thanks everyone. I have usually driven from the tach as well, and will continue to do so. Les, the odo i don't really care about too much, i was just showing it's reading. Now when my wife complains of my speed to high i can tell her that that is give or take 10%.

Ken
70 B
Ken Harris

Ken

We gave Diane's 70 speedo to APT in Bloomington MN (www.gaugeguys.com) for rebuilding. We first measured the TPM of the speedometer cable while pushing the car 52.8 ft, and multiplying the result by 100. After the work, GPS indicates a consistent error of less than 5% over the entire range. Much nicer then watching the needle jump constantly, or having people roar past her because she thinks she is driving the posted limit. Cost was around $225.

dave

Dave Braun

Frank,

For '68-76 models there were 1280 and 1000 tpm speedos. A common misunderstanding was that overdrive had something to do with it. Actually, the overdrive and non-overdrive models always took the same speedo. The 1280 speedos were used for both o/d and non-o/d cars from '68 until the rubber bumper cars. From '74-1/2 on, they ran 1000 tpo speedos on both o/d and non-o/d cars. It's a longer story than necessary to repeat here, but my '73 GT, originally with a factory o/d ran correctly with a 1280 speedo. Later it was necessary to switch it to a '76 o/d box, and with that change I had to switch to a 1000 tpm speedo. Overdrive was not the issue; the year was.

The problem with driving off the tach is that we're presuming the tach is accurate. Is there any reason to presume a 30 year-old electric tach is functioning any more accurately than the speedo? With my cars, in 4th out of o/d, I figure that if I get something between 50 and 55 mph at 3000 rpm, both instruments must be fairly close. If it comes up at 54 mph, I consider myself very lucky! Of course that still doesn't explain why my odo doesn't read 10 miles for ten marked miles on the interstate. But as Les observes, tires play into the equation too.

FWIW,
Allen
Allen Bachelder

O/drive cars: 22mph per 1000 rpm with o/drive engaged. This out of BL service manual I've had for years.

This seems to be close to what I get with my '74. As mentioned tho', instrument error due age can affect the readings. I'm happy enough with what it reads at this speed and I'm using a 1000 tpm speedo (changed from a 1280 originally installed in car without the o/drive).
Simon Austin

FWIW a Canadian 67 to 74 should have a 1280tpm for both OD and non-OD, and a 74 on a 1000tpm for both. The change point was chassis number 360300/360301.

It is the year of the OD and gearbox that is the issue, since it is the gearing of the speedo drive on the OD/gearbox that determines which speedo is required. Stick a later version in an earlier car or vice-versa and to maintain the speedo rerading you will have to fit the correct speedo for the gearbox/OD. Although Germany insisted on accurate speedos and so there were different tpms for that market according to what tyres were fitted, that didn't apply to other markets.

Also KPH and MPH speedos have different numbers on the faceplate, but 800 on a KPH is the same as 1280 on an MPH, and 620 on a KPH is the same as 1000 on an MPH.

"Is there any reason to presume a 30 year-old electric tach is functioning any more accurately than the speedo?" Relatively easy to check with a 3rd-party instrument. If they agree, then probably it *is* correct. I used the tach to recalibrate my speedo.
Paul Hunt 2

I checked my speedo with a GPS, it was 10% optimistic. The tach was checked on Dave Pymms garage kit strobe, at idle it reads 800 RPM but the car is actaully idling at 700 (figures from memory). I was a bit disapointed by this as it was rebuilt with a German sourced voltage board.
Stan Best

After a 5-speed conversion on my 67 BGT, the speedo was way off. With my buddy in the passenger seat, we took a drive on the highway, while his wife drove their new Honda Accord in front of us. She was her cell phone and he on his, and we checked my speed against hers in increments of 5 miles per hour and made a table from 30 all the way up to 75 mph. Assuming her speedo was accurate, I now know exactly how fast I'm going, even though my speedo is wrong.

Bernie
Bernie Lowe

As Stan notes, the inexpensive GPS units are a great resource for testing the accuracy of the speedometers. As for the inaccuracy of the tach, there used to be a very good article on testing and adjusting the Smith's tach on the web. I believe it was posted by someone who drives Sunbeams. Do not know if it is still available, but will do some looking to see if I have it book marked. May be a while. Just back from five days in California as a last Father-Daughter camping trip before Theresa's wedding using wedding cars tonbridge next Saturday.

Les
Les Bengtson

I think this is the article Les is refering to:
http://www.classictiger.com/techtips/motach.html

Iain
I D Cameron

That is it. Thanks Iain. Les
Les Bengtson

I agree a GPS unit is a good thing to use to really know what your speed actually is.

Also this site (which I have posted a lot since it is so damn useful) will show you what difference tyre size will make to the reading: http://www.miata.net/garage/tirecalc.html

I always use 165/80R14 as the standard size. I think the very early Roadsters used 155/80R14 though.
Simon Jansen

This thread was discussed between 13/06/2006 and 20/06/2006

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