To solve the problem of cruising at 3k RPM, I am installing a 3.07 R&P in my '67 OTS. I now need to address the unwanted side effect of the speedometer not reading correctly. Can the speedo be re-calibrated? If so, by whom? If not, what do I need to do? Learn to do real quick math in my head? Post-Its on the dash?
Herb
Firstly you should note that the Speedometer in your car is actually two instruments: one that measures your speed, and another that measures how far you have travelled (the odometer). Though they both use the spinning speedometer cable as their input, the mechanism they use to measure speed and distance are independent. With this in mind, there are several options to adjust for a change in rear axle ratio:
Replace the speedometer instrument with one designed for the new ratio - Jaguar fitted a wide range of speedos for the different rear end ratio options.
Recalibate your current speedometer. Though not impossible to do this yourself, it is probably best left to one of the companies that specialise in these instruments - the archives contain recommendations. Note that both the speedometer and odometer aspects of the instrument needs recalibration.
Add a miniature gearbox in the speedo cable to compensate for the change in ratio. This allows you to keep your current instrument and recalibrates the speedometer and odometer together. I believe @John_Walker has done this successfully and speaks highly of this option.
Ignore the speedometer and use a stand-alone GPS-based speed/distance measuring instrument, or just use an app on a smartphone
Good luck!
Surely the speedometer drive just needs a new drive gear of the correct ratio to match the differential gearing.
Are there not matching drive gears available?
No. In their infinite wisdom, the Smith’s system was designed to be calibrated at the speedometer head, not the transmission drive gear. There are some gearing variations in the right angle drives, but at this point, it’s hard to say where you could source one with the correct gearing. Of course, nobody has taken up the manufacture of new odo gears for the speedometers, either. So you need to find one of the handful of shops that still has supplies on hand.
Please don’t call me Shirley.
This is the thread where @John_Walker discusses the in-line ratio adapter. I’m planning to use the same solution and the same shop for my 3.07 conversion. The Denver shop just rebuilt the original speedo in my '64…fast turnaround, very fair price, and excellent results. I was going to live with the error. but decided to go for the adapter as the interior is currently removed and I don’t want to do that job again.
Six years later it’s still going strong. No issues.
In answer to the question posed in that other thread, Smiths used BSC fittings, which had a pitch of 26 regardless of major diameter. BSB may also work, but when similarly sized metric fittings are used, the result is a loose fit.
John, It appears from the photo in your string that you positioned the reduction gearbox roughly over the top of the transmission? Do you happen to have specific measurements and perhaps a part number or ratio for the reduction box itself? Or would the shop in Denver know the specifics for what I need? Thanks, Eric
In my Gear-Head days, I upped the gear ratio in my muscle car from 3.42 to 4.11. I asked the local police department where they had the speedometers calibrated in their squad cars.
Took my car to that garage; he had a 1-mile loop that he drove that gave him variance between the actual mile and distance recorded on the odometer. He then built a small gear set he inserted in the speedometer cable loop. I had an accurately reading speedo a day later. Cost ~$75 in 1984.
In this day and age, cars may come with speedos accurate enough to support a police officer’s claim in court but I’ll bet they need to be calibrated/certified nonetheless.
Correct, that’s where it sits. I have no measurements; I laid out the cable and eyeballed it. I supplied the speedometer cable and marked where I wanted the box spliced in. It’s a large area under there; you’ve got leeway. I told them the previous differential gear and the new one, they took it from there, selecting the reduction factor. They do this every day, they are the experts, I left it to them. They supplied the following according to the Invoice:
877F 1.1555 CB Adapter, quantity 1
J6 tip, quantity 1
JB53 tip, quantity 1
B23 ferrule, quantity 1
B26 ferrule, quantity 1
D2, nut, quantity 1.
It arrived fully assembled by Deluxe Speedometer.
Note that 1.1555 is the reduction ratio. 3.54 / 1.1555 = 3.063; close enough to the 3.058 gear set in my differential that it doesn’t matter.
Thanks, that’s very helpful.
Just had this done in here in Scottsdale, at Dick’s speedo! They had a machine similar to a dyno which they drove the car on to determine the correct ratio. then installed the correct item in my car. They put it under the Brake Master Cylinder.
$300 total.
Speedograph Richfields in Nottingham, UK can either modify/calibrate your existing speedo or provide the little gearboxes.
I have been very happy with their calibrations.
Will a speedometer from a 3.8 E-Type (3.07 gear ratio) work in my '67 4.2 with the gears changed to 3.07? Does the speedometer look the same? Are the electrical connections and cable the same?
Herb
The only difference may be the color of the light filters. I’m not sure when they changed from blue to green. You can just knock out the filters and use appropriately colored LED’s, which will give you more light, or swap the filters if you drill them out carefully.
Another option is to swap mechanisms. The mechanical portion of the gauge was identical except for calibration until they started making electronic speedometers, probably around '86. For my five speed conversion, I used the innards of an XJ12 speedometer in the stock E-Type case. Nobody would ever guess that it was swapped.
Redline marking is different on the 3.8 (5500) vs the 4.2 (5000), but something you can just remember.
But who really cranks these things up to 5k, anyway? 4 to 4500 seems plenty to me.
Or fit a 123 distributor as they come with a rev limiter feature. 
How does the tachometer factor in here?
It factors in because I didn’t clearly read the OPs question. Mea culpa. 