MGA Forum

Has anyone done a calibration of their Speedo after installing the new Mazda gearboxes? I was just wondering what the RPM is, using the Mazda boxes? I've done several Ford T9 calibrations in the past, and the most are in the range of 975 RPM, using standard MGA wheels and tyres.
The reason for asking about the Mazda conversion, is that I am putting one in my next MGA restoration, and don't want to wait until engine and gearbox are in the car, then get the Speedo converted...there are huge delays at most guage restoration specialists.

Gary,
the best solution for your speedo correction is the small, individual maufactured box from Declan Burns in Düsseldorf, Germany.
These box is attached of the rear of your speedo.
Try Declan for questions..

Declan Burns
Liedberger Weg 6A
40547 Düsseldorf
Germany
Tel. +49 211 371529
Mobile: +49 1522 1352912

declan_burns@web.de

Gary,

Speedometer Cables USA also has miniature gearboxes, They need your speedometer cable, and will fit a miniature gearbox of your choice into the cable near the transmission end so that there is minimal cable windup.

An alternative to messing with mechanical drive cables would be to fit a GPS speedometer in place of (or in addition to) the mechanical speedometer. GPS speedometers are available on the Internet for around US$80 and include the odometer. Some GPS speedometers will allow an initial odometer setting other than zero so that car mileage can be accurately maintained.

Another option could be an electronic speedometer using a sender attached to the transmission's speedometer drive. I chose this option when I installed HiGear's Ford T-9 5-speed kit in my 61 MGA. I put it in a black housing on the transmission tunnel that has black carpeting where its visible to the drive, but not to the casual observer at car shows. Works OK, but really needs a GPS reading for accurate calibration. I'm using the mechanical tach for calibration, but it isn't all that accurate. The electronic speedo I bought also loses its calibration frequently which is a pain. It was also more costly than a GPS speedometer that wasn't available then. I modified an Oldsmobile sender for the T-9's flanged speedometer drive. Overall it works, but I think I'll be switching to a GPS speedometer rather than a miniature gearbox in the speedometer cable.

I need to measure the MGA's speedometer housing to find a GPS speedometer that will fit within it, and that has a pointer and odometer display that will look reasonably like the original. the GPS speedometer also requires a GPS satellite receiver. I'll have to find an unobscured location for the receiver.

Bill

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100

I didn’t even know those existed.., learn something new everyday around here!

Gary, any recalibrating without a road test of your actual car with changes such as radial tyres etc. may result in inaccuracies with the final recalibration. Also, there may be a way around having to get the speedo recalibrated which would avoid time, delays and expense in recalibrating your speedo, like I found.

My methods in the past have been:

1. The speedo should have a number on the dial corresponding to how many times the inner cable turns for one mile. My MGA MkII has 1376 on it, so it is calibrated to turn 1376 times for each mile indicated. With my concours MG TD many years ago after changing the diff gearset to 4.3, I disconnected the speedo cable from behind the dashboard and put some tape on the inner cable so it was easy to count the turns it made for a measured distance while pushing the car along a straight and level road over a measured distance. Marking the road with chalk, also a mark on the rear tyre at that spot, then while pushing the car 100 feet along the road I counted the number of turns the inner cable made. I then calculated the exact number of turns the inner cable would do over the exact one mile distance. The ratio of the actual inner cable turns vs. the number on the speedo dial was what was needed to be corrected so the speedo then read correctly. This could be done with a correction gearset as mentioned above, or in the case of my MG TD where the specialist who recalibrated my speedo actually had a gearset that corrected the reading without need for an additional gear unit. So both the odometer and speedometer was then corrected for the new diff ratio and radial tyres, which worked well.

2. See 5th and 6th column in my table below. This is what I did with my MGA MkII, which has 195/65R15 radial tyres and 4.3 diff (another story with the car originally having a 4.1 diff ratio), after installing the Vitesse Mazda gearbox a couple of years ago. After fitting the new gearbox I drove at the various speed limits using a small GPS speedo stuck on the windscreen base with blu tack, noting the corresponding indicated mph readings at each kph speed (column 5 below). On the spreadsheet I then created, I noticed a correlation where each kph speed limit value divided by two corresponded almost exactly with the indicated mph reading. For instance at 80km/h, the indicated speed on the speedol was 41mph. For all kph speed limits from 40kph to 110kph, each actual mph reading was found to be conservative by aprroximately 1 mph.

So now when driving in a 80kph zone, I drive at 40mph indicated (6th column in table below), which is actually approx 1mph under the 80kph speed limit. Same with 100kph, the speed is then 50mph indicated. Advantage here is no recalibration was needed at all, it all works well without much mental arithmetic nor kph to mph conversions when driving along. Note that subsequent odometer readings also need correction by doubling the distances to obtain the km distances travelled.

I realise my method relied on having the MkII speedo calibration (1376TPM). The Vitesse website has a Vitesse speedo correction calculator, which was found quite accurate for my tyres, diff and speedo combination, confirming my GPS to indicated readings. The indicated mph speeds at the various kph speed limits are shown in the 4th and 5th columns in my spreadsheet below. If you cannot find a simple correlation between speed limits and indicated mph readings, perhaps inconspicuous marks around the speedo dial for the different speed limits could then be made, or temporarily attaching a GPS speedo, to help avoid speeding fines.

My suggestion to Gary would be to wait until the car is completed then see what the actual speedo reading differences actually are, which may save time and money if a simple solution is realised like I found. And drive the car in the meantime with a small GPS speedo available on ebay is attached with Blu tack.

Hi Gary,
I use a digital G.P.S. speedo because it will give you a reading in either M.P.H. or K.P.H. so it's more accurate when you have a M.P.H. speedo in the car & K.P.H. speed signs. G.P.S. speedos do have limitations they do not work in tunnels (which Sydney has many & they always have speed cameras activated) or country roads with overhanging cuttings or tree branches, also they are hard to read in direct sunlight. Where it would be handy for you is you could get a reading from your cars speedo and at the same time the actual distance covered from the G.P.S. speedo, give these readings to a gauge repairer to get the car speedo recalibrated. Attached is a link to the one I purchased from JAYCAR. https://www.jaycar.com.au/lcd-gps-speedometer/p/LA9025?pos=1&queryId=219151939ea0998c7bae07ea0973f905

Gary,

Although the speedometer and odometer are both are driven by the same cable, recalibrating the speedometer will not also recalibrate the odometer unless the odometer drive gears are changed so that the odometer correctly measures the distance traveled. A miniature gearbox in the speedometer cable or at the transmission speedometer drive corrects the turns per mile or km input to the speedometer and odometer, thus a miniature gearbox will correct both the speedometer and the odometer. Simply recalibrating the speedometer reading will not do both.

The electronic speedometer that I installed in my MGA is hard to read in direct sunlight, but this is due to the display, not the accuracy of the unit. Not many tunnels here, but there are occasional bridges. If the satellite signal is interrupted, there is always the tachometer and the engine exhaust note that if monitored, will give an indication of speed. I wouldn't expect racing through tunnels except perhaps at Monte Carlo where there wouldn't be speeding tickets for a racing through tunnels.

Several of the GPS speedometers have dials with pointers. I haven't looked into them that deeply yet, so I don't know if the pointers are mechanical or electronic, probably electronic like the dials in modern cars. Electronic displays tend to be more difficult to read with direct sunlight upon them, but most newer vehicle displays are shrouded in the dash. They're not convertible roadsters with flat dash panels. So if you want something that's visible under most driving conditions, the mechanical pointer original instruments likely would be preferable with either a corrective miniature gearbox, or GPS calibration markings. I like the look of the original instruments. So I've kept the original speedometer in the dash with the speedometer cable disconnected. I've fashioned a magnetic cell phone mount that fits below the rearview mirror in order to use a GPS speedometer app on my cell phone. I converted my MGA to negative earth and installed a USB outlet under the dash so I can connect a charging cable to my phone for long drives.

If the speedometer is incorrect, the tachometer can always be used as a general indicator of speed. You can either use the rear gear ratio and tire turns per mile or km to calculate transmission gear rpm for a given speed, or use GPS to determine rpm for a given speed. I have found that the mechanical tach on my MGA reads low, but it's consistently low so I mentally compensate for the error.

Bill

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100

Hi gear kit here. I simply used the John twist video attached here, did the math and gave my Speedo to West Valley instruments. They put the proper gears in it and it’s perfect.
William is correct that the odometer and speedometer both need to be addressed as there is no mechanical correlation between the two. It’s magnetic.

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"Time flies like an arrow......Fruit flies like a banana"

Hi Gary,
I did an observation run today and used a rally trip meter/speedo app on a smart phone. This would suit your requirements better as it would not only give you the speed error but also a milage error which would be of more use in calculating the new speedometer calibration, also some are free.

Thanks for all the suggestions.
Using the standard MGA specs, 4.3 diff , tyres 165/70/15, Mazda gearbox, the calibration is 837 TPM for a MPH Speedo (543 TPM for a Kilometer Speedo)
All my previous calibrations using the T9 gearbox were 975 TPM.

Lest anyone become complacent, the Ford T9 transmission was used in numerous vehicles with various tire sizes and differential ratios. To compensate for these varied applications, Ford fitted the T9 with 6, 7 and 8 tooth worm gears and 22, 23, 24 and 25 tooth speedometer drive gears. Thus, depending on the original application from which the T9 was taken, 12 different speedometer drive gear ratios are possible. Depending on your vehicle, it may be possible to correct some or all speedometer error by changing the speedometer drive gear, or if you wish to dive more deeply into the transmission, changing the worm gear on the transmission output shaft. I suspect the same may be true with Mazda transmissions.

Further, as Barney advises, Smiths/Jaeger speedometers have the turns per mile printed on the dial face. For example, the speedometer in my 61 MGA has 1440 printed on the dial face, e.g., 1440 turns per mile, or at 1440 speedometer cable rpm, the speedometer will (should) read 60 mph. That is, using John Twist's method, 14.4 speedometer cable turns (14 turns plus 144 degrees) over 52.8 ft (52 ft 9.6 in) should yield a correct speedometer reading.

It being possible that the speedometer may have been recalibrated, the speedometer's turns per mile should be verified. This can be done by fitting a square drive into a drill motor having a known rotation speed, and using it to turn the input in the back of the speedometer. If for example the drill motor turns at 900 rpm and the speedometer dial reads 1440, the speedometer should read 37.5 MPH at 900 rpm. (900 / 1440 x 60) Use a reversible drill motor and be sure to turn the speedometer input in the right direction.

I suspect that metric speedometers would also have a turns per km printed on the dial face, probably based on 100 KPH. The method and the math would be the same, just in metric units.

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100

The Miata transmission can be fitted with a speed sensor in place of the manual Spedo take off.(photo attached) This is now finally available at Vitesse but not Moss. This avoids a mechanical spedo take off and cutting your transmission cover to run the mechanical cable to your Spedo. This also avoids the problems using a GPS speedometer or trying to jerry rig an electronic sensor (supplied by Moss) to take a reading off the drive shaft. On EBay, you can find used Miata sensors (Much cheaper) - Note - the gears are different colors depending on the number of teeth. (Vitesse shows a yellow gear on their sensor) Easy install. I picked one up last year off EBay and about to commission my MGA. Smiths has helpful information about how to calibrate with your electronic Spedo. I should know more in a couple months but calibration looks to be fairly straight forward and installation was very easy.

The speed sensor requires an electronic speedometer, yes? Is there an electronic speedometer having an MGA dial face or adaptable to the MGA mechanical speedometer dial face? Does the Miata speed sensor require a Miata speedometer? How is this setup calibrated? How does this speed sensor work, if it does, with the MGA's mechanical, cable drive speedometer?

As I posted previously, I installed an electronic speedometer in my 61 MGA. It's mounted some distance back on the transmission tunnel on black carpeting in a black housing so it's not readily apparent to the casual observer, but visible to the driver. I machined an Oldsmobile speed sensor for the T9 flange speedometer connection. In my case, the electronic speedometer is calibrated by driving 45 MPH and then pressing the calibration button on the electronic speedometer. I left the stock mechanical speedometer in the dash because I like the look, and of course, it's 100% correct when the car is at rest.

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100

Yes, you need an electronic speedometer, Smith makes one appropriate for an MGA. Moss carries them - you need to specify the diameter of the hole in your dash and if you want Kilometers or Miles for the display. You do not need the optional speed sensor Moss sells - this is a device that needs to be mounted in such a way to take a reading off the drive shaft. This type of jerry rigged sensor is tricky to install and subject to getting out of alignment. The Miata speed sensor simply replaces the mechanical drive on the transmission. One screw to make the swap and then run two wires to the electronic Speedometer.

Last year, I got the runaround from Moss and Vitesse about replacing the mechanical takeoff with a standard Miata electronic speed sensor on the transmission. However now Vitesse offers this option for their transmissions. Shame on Moss for not providing this option here in the US. Ordering a speed sensor from a Miata dealership is expensive and they rarely stock this item as these sensors rarely go bad. EBay has a whole selection of used Miata sensors for sale. Similar to the T9 transmission, these Miata sensors come in a variety of gearing. My hunch is that any of these sensors will work regardless of the number of teeth on their gears - not knowing the answer, I ordered a yellow one last year (same as Vitesse offers now) as the number of teeth are similar to the manual take off. Did not want to have the possibility of miss matched gears grinding up into bits.

For the electronic Speedometer, all it needs to know is the number of electronic pulses generated per mile or kilometer. Any gearing arrangements for the sensors are not relevant as far as the electronic speedometer is concerned. Smiths and other web sites provide calibration instructions. Hopefully I will get my MGA on the road in the next couple of months and work on the calibration.

Henry,

This sounds like a great solution for my 61 MGA. It has a Ford T9 5-speed and I have already fitted a speed sensor to the transmission speedometer output. I don't have the pulses per mile at hand, but the information is in my files. I will check with Moss for a "Smith face" speedometer that has the correct dash hole diameter.

Thank you!

Bill

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100