LONG POST ALERT....
right - I've had chance to work on this a bit. There's lots of stuff in here, so I've tried to put it into sections. I'll also be using some figures, mostly mph per 1000 revs in 4th and 5th gears.
In summary, you have the closer ratio gearbox fitted, with a 3.7 diff. You have the wrong size tyres, giving you higher revs when cruising than a standard car.
Wide-ratio gearbox.
As far as I can tell, there's only two different ratio sets for Omega B :-
ratio set A is 3.95 2.19 1.39 1.00 0.84. Because of the low 5th gear, we'll call this the close-ratio set. It was fitted to V6 models and has the higher torque capacity (R28)
ratio set B is 3.81 2.11 1.34 1.00 0.76. This we'll call the wide-ratio set. It was fitted to everything else, including 2.5 TD. It has the lower torque capacity (r25)
It is not possible to build a wide-ratio R28. The higher-capacity parts are the gears themselves, so transferring them into another box would achieve nothing.
There is absolutely nothing wrong with the lower capacity box. You're not going to break it, even if towing in the LAke District. I have one in my track car and it's been there for years. You could conceivably use a wide-ratio box in a V6.
What diff have I got ?
Ok - so here's the figures for your car.
With a 3.9 diff, you'll be getting 18.44 mph for every 1000 revs in 4th (calculated)
With a 3.7 diff, you'll be getting 19.44 mph for every 1000 revs in 4th (calculated)
you've measured 19.35, so it's a safe bet that you're currently equipped with a 3.7.
What gearbox have I got ?
Assuming that you've got a 3.7 diff.....
the close-ratio gearbox gives you 23.15 mph for every 1000 revs in 5th
the wide-ratio gearbox gives you 25.58 mph for every 1000 revs in 5th.
from your precious post, you have 3800 on the clock at 90 in 5th, so you have the close-ratio box fitted.
Tyres.
first thing to say is theat you've already got the wrong tyres on. 225/60 x 15 gives you 834 wheel revs per mile, whereas you need 817 ideally. If you corrected this, then you're up at 19.8 mph per 1000 revs (4th), which is already 100 revs less on the clock at 60 mph.
Your previous tyres ( the 245/40 x 17 ) were even further away from where you need to be at 842 wrpm. Swapping from the 245 to your current tyre has lowered your cruising revs ( by 50 revs at 60 mph )
your "ideal" tyre size of 275/30 x 19 ( which I wouldn't recommend for a number of reasons ) is a bit too far the other way, at 808 wrpm, which would give you 20 mph per 1000 revs (4th). But at least this would further lower your cruising revs by another 50.
Ok - so where to go from here.....
assuming that you correct your tyre problems, you can either change the diff to a 3.45 unit or fit the wide-ratio gearbox. Here's what you'd achieve :-
Your car with the correct tyres :-
60mph in 4th is 3022 rpm
80mph in 5th is 3385 rpm
Your car with the correct tyres and a 3.45 diff :-
60mph in 4th is 2819 rpm
80mph in 5th is 3155 rpm
your car with the correct tyres and the wide-ratio transmission :-
60mph in 4th is 3022 rpm
80mph in 5th is 3063 rpm
your car with the correct tyres, a 3.45 diff and the wide-ratio transmission :-
60mph in 4th is 2819 rpm
80mph in 5th is 2856 rpm
hope this all helps.
Author
Topic: quick query 3.45diff (Read 6456 times)