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inaccurate odometer issue

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R

recordsj

Jedi Warrior
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I have a 1974 MG Midget.

I noticed that my odometer accuracy is about %15 off (i.e. I went 39 miles, but the odometer shows I went 46 miles). I realized this when I drove my 2000 CRV on the same drive in which it logged in 39 miles (on the same drive the MG loggs in 46 miles). I also noticed, as a 3rd check, when driving the MG past the odometer checks on the interstate that the MG was off.

Any direction and or good website on how the odometer works and possible repairs?
 
The speedometer is a magnetic device, and it's easy for it to be off quite a lot. But the odometer is direct coupled--a certain number of turns of the cable gives you one mile on the odometer. No room for uncertainty. So, the only way it could be off by this much is if you had the wrong speedometer, wrong drive gear, or something like that. 15% error is too much for it to be caused by a higher-ratio rear end, (who would do that, anyway?) or underinflated tires.

There are places like Nisonger who will rebuilt the speedometer, but if the cause is something like an incorrect drive gear, that won't help.
 
Steve has summed it up well.

Recalibrating an odometer is going to be more expensive than finding a used speedometer that is "right" for your car's gearing and tire size. Somewhere between the cost of a used speedometer and a re-calibrated one is one other option, a ratio converter box.

As its name implies, based on measured distances the speedometer repair center builds you an inline gearbox that mounts on the speedometer cable. It converts turns-per-mile "in" to the correct turns-per-mile "out" that your speedometer needs to see.

In your case, you are measuring 46 miles per "real" 39 miles, about an 18% error. Look on your gauge's face near the odometer window or at the bottom of the face and you should find a four digit number that is clearly not a serial/model number. That will be the turns/mile the gauge is calibrated for. For argument's sake let's say that number is "1000". Since your gauge is reading 18% too high, that means it is receiving 1.18 x 1000 or 1180 turns per mile. You could look for an 1180 TMP speedometer or have a place like APT (gaugeguys.com) build you a ratio converter to "slow" the cable by 18%.

For info on how the speedometers and odometers work, download Anthony Rhodes' 27 page PDF on repairing Jaeger and Smiths Speedometers.
https://triumph.daveola.com/NOTES/Speedo_Repair.pdf
 
That number (18%) off sounds suspiciously like your car should have a 3.9 differential but instead has a 4.22 and that you have the speedometer from a '78 (which had a 3.7)
You may want to look at getting a different differential gear and speedo.
BillM
 
I took a look at the guage and there is a 1000 on it as you talked about.

Also on the front is a sn-5230/13, what does this mean? Or is it a serial number?
 
The 5230/13 is a model number. It's the one you'll see people quote whenever they list a speedometer on eBay. With cross-reference tables you can use that number to tell when the gauge was built and what its intended application is.

I chose 1000 TPM in my example because it is the "generic" calibration used on most aftermarket cable driven speedometers. It also appears to be what BL finally decided on sometime in the 1970s. Prior to that you'll see numbers all over the place for the TPM numbers.

I don't know how helpful this table will be but see the link below. It lists several speedometer calibrations that were used on the Spridgets.
https://www.mgcars.org.uk/midgetspriteclub/MascNew/TechTips/SpeedsRatios.html
Focus on the row labeled "speedo available/fitted"

I don't see any that would appear to match what you probably need. Surely there must be other speedos that would address this. I'm sure someone here will have suggestions like those of Billm that will help you correct this. Don't forget the other option I mentioned (gaugeguys.com) and the ratio box.

EDIT: The link below is also interesting if you want to experiment with the numbers to figure out what you need.
https://www.bgsoflex.com/speedo1.html
 
Many who want precision install bicycle speedo's. Just getting the "right" one isn't a guarantee of accuracy, as many people have changed one or all of; tranny to a 5 speed, pumpkin, or tire size. The last of which is practically guaranteed to have been changed as the spec tires are all but no longer available.

My own is off 7% in speed, and 15% in the odometer category. A little mental math is all that's required as one drives, IF one knows the error factor, but those bike speedos can and have been set up to be right on the money.
 
I took the el cheapo route and simply use my $100 Garmin GPS unit. I "calibrated" my speedo with the GPS and put a label on the console that tells me my actual speeds.

With a Datsun gearbox and the wrong angle drive, my speedo is way off. It's easier and cheaper simply to check the speedo against a GPS and note what your actual speeds are.

Plus, it appears that I may have to partially pull the gearbox to get at the angle drive. Ugh.

Not the solution for purists, but works for me . . . :crazyeyes:
 
I hadn't even brought up the prospect of the bicycle speedos. They are a great option.

On the Mini, my Smiths speedo is in KPH, not MPH and I wanted an accurate (MPH) speedometer without changing the one in my car so I installed a Sigma bike speedo. It is dead-nuts accurate and represents a very small investment.

I am currently running a BC1600L Sigma speedometer as it has two incredibly nice features. First it has a low battery indicator. Second, it has an EEPROM which stores the odometer and calibration variables so you don't loose anything during battery changes.
 
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