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TR4/4A Diffferential gearing vs speedometer error

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nbailey

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On a previous post I asked for advice on changing my 4.1 diff gearing to 3.45. I just realized I will probably have significant speedo error. My speedometer currently is fairly accurate with the 4.1. Would anyone have any idea how much change I could expect, or if I can get my speedometer modified to correct ? Thanks for any advice.
 
When your speedo showed 50mph, you would actually be doing 60mph. The formulae is:

Actual speed = (indicated speed X 4.1)/3.45

The speedo remains unchanged...the correction is done by changing the gear at the tranny.
 
alanjohnturner said:
There are a several outfits in the UK that can recalbrate a speedo to fix this.
Click to expand...
Be sure to ask, though, if they will also recalibrate the trip meter and odometer. Those are much harder to change, because they are gear-driven rather than just relying on the strength of a magnet. Some shops will agree to recalibrate the speedo and not change the other two.

A ratio adapter is probably easiest & cheapest, if your original speedo head is in good condition (and matches its original calibration) otherwise. It's been a few years ago, but a local speedo shop offered to build me a ratio adapter for $100 with any ratio I wanted. He also pointed out that later ratio changes would only be $40.

PS, if you really have a 4.1 speedo head, I might know someone that would like to buy it. They're apparently pretty rare, and I wouldn't sell him mine.
 
Are the tranny gears hard to get for these? They are so easy to switch...if they are available.
 
John-

For the TR2-6 gearboxes at least, I don't believe you have a changeable gear within the gearbox - they actually had different speedometers for the different final axle ratios.

Randy
 
Later TR6 did use a different drive gear (instead of different speedometer heads), but the only step was pretty coarse unless you had overdrive.

Lots of folks get caught by that, because the late drive ratio was about 10% different than the early one. So late gearbox in an early car or vice versa will cause about 10% error in the speedometer.
 
Here it is, from the TRF catalog, the early 15 tooth driven gear is 147965, the later 17 tooth is UKC5933.

The driving gear (cut into the output shaft) has six starts, so the 15 tooth gear gives 6:15 or 2.5:1. The later gear gives 6:17 or about 2.83:1
 
Myself, I had the speedometers changed when I converted from 3.7 to 3.45 gears. This also allowed me to get the correction for the more modern tires that we are running on our cars. Not that these speedo are all that accurate to begin with. But I will send this caution. If you send in a speedo to be worked on, they will ask you to send them info taken from driving your car over a certain distance. Be sure that the speedo shop fully understands that you want the complete speedo converted from the 4.1 to whatever you are changing it to. And that you expect the speedometer, odometer and trip odometer to be changed. I used a shop in northern CA one time and got back a very satisfactory product. But when I sent in another unit several years later they did not touch the odometer at all. When I contacted them they said they were not capable of changing the gears in the unit. I think that the place had changed ownership. I finally sent it to Nisongers in NY who did a very good job.

Charley
 
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