Trimatic overdrive

[Sandaro]

I run 2.78 in my cars and get 2500rpm at 100km/h. Dad has 3.36 and gets 3000rpm at 100km/h. (all approx.). These are with approx standard diameter tyres. With 3.55 you'd be getting 3200rpm at 100km/h and 3.08 2750rpm at 100 km/h.


The Laycock overdrive is 0.78 so 78% of the above revs depending on which rear axle you have.

[Puzzigully]

I did the "old, lift one wheel off the ground, turn the tailshaft" trick.

Here's what i found....just over 1.5 turns of the tailshaft gave a complete revolution of the lifted wheel.

This means (using the doubling required) that the diff ratio is most likely (if not certainly) a 3.08:1
Given i have a 235/60/15 rear tyre, this seems quite odd as to the rpm I'm seeing at 100 km/h....the fuel economy seems to support the rpm ;)

online calculators don't agree with what I'm seeing :(

My math says:
3000 rpm at engine =
3000 rpm tailshaft revolutions =
974 wheel revolutions/min = pi x diameter = 3.1415 x 663mm
2083mm/min =
2.083 m/min =
125 km/h?????


I've got something wrong????

[Sandaro]

The only mistakes I can see is your calculations of 2083mm/min should be 2028m/min, and then 2.028km per minute which is 121.7km/h, so not a huge difference.


But the other thing is unless you have a recalibrated speedo it is going to be under measuring your speed by about 7km/h at those speeds due to your larger diameter tyres (the speedo still thinks you've got standard 14 inch on there). So you're seeing 100km/h on the speedo but really doing 107km/h. Somewhere between inaccurate speedos and inaccurate tachos would explain the other 14km/h.


If you really wanted to be certain on the ratio redo your test but turn the wheel 10 times and count the tailshaft revolutions (will be more definite as to 3.08 v 3.36)

[Puzzigully]

Thanks Sandro,

I've checked the speedo vs GPS speed and its pretty accurate...

I'll give the tailshaft a few more turns to confirm ratio though.

[axistr]

You need to calculate for any slip difference from the torque converter. At 100kph there might be 300-500rpm difference between the engine rpm and trimatic transmission output shaft. In later transmissions like the 4l60E they lock up the torque converter to give it a so called overdrive. In a lot of autos its not actually an overdrive ratio only a direct drive speed (engine and transmission speed the same) Its the converter lockup that allows lower engine rpm for better economy.

Its always better to jack up the vehicle with both wheels off the ground, use a marker on both wheels and floor, pinion and housing then rotate the pinion till both wheels have done exactly one complete turn and count the pinion turns. Excessive back lash in the hemisphere gears may give you an inaccurate reading doing one wheel only and double the readings.

[Puzzigully]

Just rechecked - yep i've got a 3.08:1 diff, now to check the tacho as the speedo is correct when checked via gps

[Beachy]

If you drive at 120kph read the tachometer that is your diff ratio eg 3.08 will be 3080rpm this works for manuals with standard wheels and tyres this was shown tome back in the 80’s works every time in

[axistr]

Unfortunately not accurate with an auto without a converter lock up. It will change due to differences in stall converters.

[Dr Terry]

In later transmissions like the 4l60E they lock up the torque converter to give it a so called overdrive. In a lot of autos its not actually an overdrive ratio only a direct drive speed (engine and transmission speed the same)

4L60 & 4L60E both have a 4th gear overdrive (0.7:1 ratio), with 3rd as 1:1. They also have a lockup converter. AFAIK all GM 4, 5 & 6-speed autos are set-up this way.

Dr Terry

[axistr]

Sorry, stand corrected I stuffed up on my wording on that one. They will only have an overdrive ratio once the converter is locked. But if you put an overdrive on a Trimatic your probably not going to get a true overdrive without a converter lock and I wouldn't think It would be possible on a Trimatic. Much better off going a later transmission and if your not using a PCM to control it you will need an aftermarket controller module which can get a bit expensive.