Technical Tips

Laycock Overdrive

In the 60s and 70s "second time" sports car buyers purchased a car with the optional Laycock de Normanville overdrive. Anyone who owns an older sports car must sometime consider retro-fitting an overdrive.

For Ford engined cars, I believe TVR used a stand alone unit mounted behind the transmission. For Triumph engined cars the easiest way is to get a transmission complete with overdrive because a different output shaft is used to mate to the overdrive unit. You also need different transmission mounts, overdrive switch, relay, cut-out switches etc. Nothing very difficult here.

Your overdrive should have the correct speedo driving gear. My first overdrive had the wrong one and the speedometer read 40 mph at 65 mph. To solve this I got a programmable speedometer that I could calibrate against mile markers beside the road, and later against those "YOUR SPEED IS:" radar signs when I found one. (You MUST be the only car moving in your direction in front of the sign - EVERY other car has a much higher radar profile and will washout your signal. That's right-some of those tickets are not yours!)

There are no instructions on how to operate the overdrive, i.e. use the clutch or not when switching in or out of O.D. People do say it should not be run in reverse. This is like saying "Do not open hatch while submarine is submerged". Any distance in reverse is too much, even six inches. The horrible banging noises will tell you that you are going to be walking REAL SOON NOW. It happened to me. My car was equipped with column switch and wiring just like a Tr6. How can it happen with cut out switches? Did they fail? No. The column switch just happened to be on, which would not be a problem because the cut out switches were open since I was not in second, third or fourth gear. The torque of operating the angled handle of the switch tends to loosen the retaining nut, allowing the switch to rotate until it contacts the mount for the turn signal switch, grounding one of the contacts and bypassing the cut out switches. This contact normally is grounded ONLY when one of the cut out switches is enabled. Interestingly, the illustration in the Bentley Manual seems to show a rubber cover over the switch contacts. This cover is not shown in the parts manuals nor is it found on actual switches, Not even NOS switches.
The overdrive switch, bezel and associated parts from the Bentley book. Notice the mythical rubber cover.

I wonder how many Triumph owners had to replace overdrives because of this little bug? Triumph must have lost an amazing amount on waranty repairs, assuming they covered the overdrive. My car was wired exactly as shown in the Tr6 manual. The contact that grounds is connected to the black wire.

Tr6 overdrive wiring 1972-1974 USA models. From the Robert Bentley Tr6 shop manual.
You really need this book.

Key To Overdrive Wiring
afrom fuse box
bfrom fuse box
72Overdrive relay
73Overdrive column switch
74Overdrive gearbox switch - 2nd gear ON
75Overdrive gearbox switch - 3rd/4th gear ON
76Overdrive soleniod

Prevent Sudden Overdrive Death Syndrome

After this disaster I wrapped the switch in several layers of tape and since I've seen tape wear through, rewired the car so that shorts to that contact would not override the cut out switches. It is not clear why Triumph choose to put the cut out switches in the column switch side of the circuit instead of the the solenoid side. The only guess I can make is that the current flow through the solenoid is too high for long life of the switch. One certainly would not want the switch contacts to fuse in the ON position. You don't need the second gear switch as it's normal effect was surprise upshifts when leaving toll booths.

Revised overdrive wiring.

Another interesting fact is that Triumph used a left hand switch for left hand drive cars where you shift with the right hand, and a right hand switch for RHD cars where you shift with the left hand. I'm not making this up. When installing overdrive, use a right hand switch in a LHD car because it's ergonomically correct and the left hand switch blocks the wiper switches when overdrive is not engaged.

Shifter Mounted Overdrive Switch

The Spitfire, GT6 and Stag used a shift knob mounted slide switch. As the Stag used the same transmission as the Tr6, it should not be too difficult to use this knob on a TVR. The knob is black or dark grey plasic. This is probably less reliable than the column mount because the wires flex during shifting.
The Stag overdrive switch. Image from Triumph Stag owners manual.

Experiences With Overdrive

I have a Laycock overdrive and a 3.7 rear in my 2500M. I find that OD 2nd and 3rd are useless and annoying. My car doesn't like to run below about 1900 rpm in 4 o.d. so below 50mph I switch it out. These are the speeds in gears in my car with the 3.7 rear and 195/70-14 Goodyear Eagle GT II tires:
Speeds in Gears with 3.7 Rear
Gear/RPM100020002500300035004000 500055006000
1 6.5 13 16 20 23 26 33 36 39
2 9.2 18 23 28 32 37 46 51 55
3 14 28 35 42 49 56 70 77 84
3od 17.7 35 44 53 62 70 89 97 106
4 19.8 40 50 59 69 80 99 109 119
4od 24.6 49 62 74 86 98 123 135 148

If you have the normal 3.45 rear end, you will be going about 7% faster in each gear.


I haven't listed the color code because you have it memorized.
OK, this is more like a novella than a tip.

Last updated on February 15, 2005.