Higher ratio overdrive

[tim hunt]

Does anyone have experience of a 28% overdrive on a TR2-4A with standard diff (3.7). I calculate that it will bring o/d 3rd a little closer to direct top than is the case with the normal 22% overdrive but am mainly interested in reducing revs a bit for better economy during motorway/trunk road cruising in o/d top.

 

Any input would be appreciated.

 

Tim

[Tom Fremont]

Hi Tim,

 

You may want to check the overall ratios for that setup as you could wind up with this sequence:

 

1st, 2nd, 2nd O/D, 3rd, 4th, 3rd O/D, 4th O/D.

 

Doesn't work for me, but apparently others are fine with it.

 

Cheers,

Tom

[tim hunt]

Thanks Tom but as for gear ratios these will be 2.01, 1.57,1.33, 1.03, 1.00 and 0.78 to two significant figures for 2nd, o/d 2nd, 3rd, o/d 3rd, 4th and o/d 4th respectively so although quite close o/d 3rd is still a lower gear than direct 4th, contrary to what you suggest. You would have to go to a 32% overdrive before o/d 3rd is higher than direct top.

 

Tim

[Paul Heritage2]

I’ve got 28% o/d (with Revington logic device) on a TR4 with 3.7 diff, and it basically makes the lower o/d gears the same as the standard gear above it. If I’m in 3rd+o/d and change up to 4th there’s no noticeable difference in revs, but for me it’s to reduce the overall revs when cruising at 70+

[tim hunt]

Thanks very much for your response Paul. As a matter of fact I had the chance to share another member's TR4A on Club Triumph's Coast to Coast Run through the night for Drive it Day. We did some 800 miles door to door and I soon realised that this car had a 28% o/d. The sat nav confirmed that the tachometer was pretty well spot on since it showed 60mph at an indicated 3000 rpm and 40mph at 2000. I noted that 3000rpm in o/d top corresponded to 75mph. As you say, the higher ratio o/d brings o/d 3rd very close to direct top, on making this change I noted only about a 100rpm drop. This is of no concern to me since my main interest, as yours, is reduced engine revolutions at 70 (or 70+ autobahns). As a consequence of my direct experience I will be making the change on my car (quite convenient since the gearbox and o/d are currently at ORS for overhaul!). I didn't test it out on the car but according to figures I have seen with a 3.7 axle, 28% o/d and Dunlop SP 165x15 tyres then overdrive second will give 12.5mph per 1000 rpm and direct third 14.9mph so at say 40mph changing from o/d 2nd to direct third should result in a 515rpm reduction, still quite significant although with a 22% overdrive the reduction would be higher at 650rpm.

 

Thanks again,

 

Tim

[TR 2100]

Seems to me a 3.45 CWP may be a better bet, although you

would want to have a bit more ooomph to make it more drivable.

 

AlanR

[mike ellis]

If the car was doing an indicated 75 at 3000rpm I would say that it had a standard overdrive. The o/d top ratio is 0.82 for 22% o/d not 0.78 as quoted.

 

A 22% o/d means that at any rpm the road speed is 22% greater when the OD is engaged.

 

Mike

 

Just had a recalculate and 75mph would be between the two as 22% should give 73mph and 28% should give 77mph at 3000rpm.

Edited April 23, 2018 by mike ellis

[ChrisR-4A]

My standard TR4A 165-80 tyres and 3.7 diff with A type OD shows 69/70 at 3000 rpm in OD top on TomTom. But It is reading 75 on the Speedo

 

However I wonder how accurate the rev counter is after 50+ years?

 

Chris

[Paul Heritage2]

My speedo is on about 80mph when doing 3000 revs and is reasonably accurate after recent refurbishment, so 77mph would be believable on similar size tyres.

[Charlie D]

Chris,

 

I bought a digital laser tachometer a while back. Less than £10 (Less than £2 if you buy direct from China !) 100s advertised on eBay

 

You simply stick a bit of reflective tape onto what ever you want to measure and point the hand held device at it.

 

I bought one for checking the speed of my drill, but would be perfect for checking the rev counter.

 

Plus it’s another toy to play with…

 

Charlie.