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Tire size & the speedometer

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Toups

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I recently picked up a 1980 MGB w/ overdrive, and I think my tach is way off. I have noticed when going with the flow of traffic (usually about 75-80 around here) my speedometer only registers 60-65 (hit 70 once) Also, I drove a route which I know is at least 30 miles and only 25 were reported on the odometer.

My question is, is there any way to recalibrate the speedometer for the tires on the car (185/70 14) to report the correct speed/distance traveled?


TIA
 
It almost sounds like you have a non-overdrive speedo in your car and either the originaly tranny has been swapped for an overdrive without the proppergauge or the OD gauge has been swapped for a regualr one for some reason.
 
My 78B, 4 speed, reads 10 over with 185/70R14 tires according to my friend who drove behind me in my F150 after I bought the car. I just figure if I drive no faster than the speedo says I probably won't be pulled over!

Guinn
 
Hi Toups,

I would agree with Adam that something was likely swapped out at some time. Trusting the information on this website (https://www.w-p-c.com/speedotach.htm), on the face of your speedo, you will see "serial" numbers. Your 1980 overdrive car should read "SRM 6006/00", rated at 1000 TPM (turns-per-mile) Take a look at your speedo, especially at the TPM number.

On my 72 MGB, I swapped the original 4-speed with an O/D unit and bought an O/D speedo off eBay to match it according to the info on the website I referenced above. I'm happy to report that my verified MPH seems to be spot on.
 
I will be checking the speed with my GPS later this week, and will post the exact variance.

BTW, I recall it does read 1000TPM, but do not know about the model number.
 
185/70 14 would be a correct tire size. There were two transmissions built between 1968 and 1980. The 1968 to about 1974-75 was a 1280 tpm transmission that had a dip stick on the top for checking the oil level and refilling. The later rubber bumper cars had a 1000 tpm transmission that did not have a dip stick and are filled and checked by removing a bolt on the side. These units are sometimes called side fill tranny's. Overdrive transmissions had the same tpm as the non-OD tranny's of the same year. Since the dash in 1977 to 1980 used a speedometer that would only fit those years, you probility have the correct speedometer. Check under your car from the passenger (right) side and look at the transmission. If the tranny has a dip stick on the top on the right side it is the wrong tranny for the car. If there is no dip stick and only a single bold in the middle of the case, it is a 1000 tpm tranny. Wrong transmission and you need to have the speedometer re-geared. Right tranny for the car means that you need a new speedometer or need it repaired. Before you send out the speedometer you need to know the model of transmission under the car.
 
Rick
Both of your 4-speed transmissions have a dip stick on the top and are top fill transmissions. They should both be 1280 tpm. Both OD and Non-OD top fill transmissions were 1280 tpm. That assumes that they are 4 sync transmissions. The early 63-67 transmissions were very different. I don't think he has one of the early trannys' as they have a different backing plate on the motor and different starter than all of the 68 and on transmissions.
 
The speedometer can be recalibrated for the specific tire size but it sounds like you might have a more fundamental problem than calibration. In the transmission housing at the speedometer cable junction, there is a plastic gear that drives the speedometer (easily removeable). The gear is usually color coded to the application (differential ratio). Check to make sure that the right gear for the differential ratio is installed in your OD transmission.
 
Ok,

Checked speed with the GPS last night, and at 40 (GPS read 44-45) at 55 (GPS read 62-63) and at 70 (GPS read 81-82)

Sounds like a definate rebuild to me. Has anyone else encountered this, and where did you get yours rebuilt?
 
All MGB's used the same rear axle drive gear. The speedometer drive pinion on the transmission can be removed, however to change the turns per mile out put you have to change out the gear of the output shaft. That requires a transmission tear down. These transmissions are not like the GM transmission where the output gear ratio could be changed.

Toups
You know how fast you are going and what the speedometer is reading. Before you send it out make sure you know what transmission you have and what the turns per mile are on the speedometer cable. Have you checked the number of miles show on the odometer to actual miles traveled? Go out to a road and start at a know mile post. Travel ten miles and check the mileage. If the mileage is correct the problem is in the speedometer head. If the mileage is wrong, the problem is in the transmission and the ratio must be corrected. https://www.gaugeguys.com/ratiotest.htm
 
One other thing... I have noticed some jumping of the needle as well, so I think everything is pointing toward a rebuild.

I will take it apart this weekend and have a look.
 
Toups, as I understand the problem, you are indicating slower than you are actually traveling. And you are using 185/70R14 tires which are definitely larger in circumfrence than the stock 165/80R14 tires. I would do as Jim suggests and check the actual, miles recorded over a known distance. It could be just the tire sizes! I may be a coward but I try to always check the easy things first! /ubbthreads/images/graemlins/smile.gif

Guinn
 
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