Hi All, Have had a gearbox swap to a modern Tremac T5 and wanted to find out how far out the speedo reading now is. The garage is supposed to of sent the speedo of for alteration at the same time but to be honest I don't believe them. Also it bounces around so much I could be doing anything from 55 to 65 MPH so was wondering how far out a new speedo would be with that gearbox. Have check out the tinternet and they talk about the ring gear but not a clue.

Thanks for help.

 

Sorry forgot to mention it is a 1966 3.8 with locking rear diff and had the later synchro gearbox.

 

You should be doing 40MPH at 1948 RPM in 4th gear as a quick check.

Jumping speedo will most likely be a kink/tight bend in the speedo cable, or damage to the speedo cable, I think the speedo connects to the rear of the Tremec as opposed to the side on the original gearbox, they may have badly routed the cable, damaged it or have a tight bend in it where they should have fitted a longer cable, difficult to tell without seeing how they have routed it.

There are some numbers just below the trip meter that should be SN6317/10 then 1100, can you confirm this is correct on your car.

Tremec T5 box comes with many different drive and driven gears for the speedo, this should have been correctly specified for the Jaguar Mk2, you can change this for different diameter tyres to get accurate speedo readings, but should have been supplied correctly for a standard tyre on a 3.8 Mk2 when it was installed.

I think from my maths the ratio of prop revolutions to speedo cable revs should be approx 2.66 --- 2922 prop revs to 1100 speedo revs for 1 mile in 4th gear on original synchro box.


This is the speedo drive gear for the Mk2 which fits on the output shaft, the correct one should be fitted prior to installation, and there is no need to calibrate the speedo to suit the gearbox, the correct speedo drive gears result in the correct speedo reading.

 

Thanks for this TilleyJon I will check your points tomorrow.
They sent the speedo of to Speedycables they claimed to be calibrated for the new gearbox but the quality of other work done lends me to think they were full of it.
The gearbox was purchased from Elite racing transmissions to go into the Mk2 so I would like to think that it has the correct speedo drive.
I think I will have to sort the speedo bounce first as it is impossible to know with any accuracy how fast your traveling.
Thanks

 

Elite racing transmissions would have supplied the correct gearing for the speedo, check the cable first off and let me know how you get on.

 

Most phones these days have GPS so easy to check your speedo against that.

 

Hi TilleyJon, The cable looks good but have ordered a new one and will hopefully replace it this weekend. The numbers on the Speedo read SN 6317/10 1100, what do they mean ?
Will get the bouncing sorted out first then check the accuracy.

Thanks

 

The SN 6317/10 is the part number and the 1100 is the drive ratio, in this case 1100 turns per mile of the speedo cable.

This is the correct gauge , they can come in different tpm, so it was a quick check that the speedo is correct.

The TPM also obviously equates to the revs per minute that would read exactly 60MPH on the speedo face. If you connected a drill to the speedo cable you can check the speedo and the cable if you know what speed the drill is rotating at, you can still check the cable for speedo bounce this way even if you don't know the speed of the drill.

Let us know how the new cable works.

 

Hi Tilleyjon,
Thought I would let you know that when we went to change the speedo cable there was a 90 Degree adaptor on the gearbox that had a 1/2in (ish) male thread and the cables came with what looked like a 3/4in female thread so where unable to change it for new. Once we slid out the inner core though cleaned it up and re-greased it the speedo seems fine. Fingers crossed, going to test the accuracy with a sat-nav although seems pretty good. Did notice though when driving back on A-roads that I am getting a lot of hot air coming out of the footwells regardless of where the heater controls are set, the heater is switched off but it feels like it is on all the time, absolutely roasting in this weather at the moment. Any ideas what it could be ? We refitted the head and heater assembly about 6 weeks ago and wonder if we have missed something.
Thanks

 

Glad speedo judder has been fixed.

Re Heater, have you connected the cable to the valve the right way round and did you test the valve prior to refitting it ? Check if the return pipe gets hot too, you need to check that as the engine warms up to see if the flow and return are about the same temp, they should not be if the valve is off and flow to the heater should be nothing/minimal (engine end of the heater feed pipe should be a lot hotter than by the valve), you will get some heat transfer but there should be a distinct difference if the valve is shut.

 

I'll check it over today. The amount of hot air that comes in is speed related hence why I noticed it on the A3 more than pottering down B roads and you can still operate the heater to blow two speeds onto the windscreen its just like a whole has suddenly appeared from the engine bay to the footwell blowers.