On 28 Aug 2003 at 21:38, AOH9999@aol.com wrote:
> is there any difference between 3rd with
> overdrive and fourth without it? I'm new to the whole overdrive thing and
>cant
> figure out when to use which setup.
Well, it depends on which car model, since the gearing could be
different even if the OD's are the same from one model to another!
I can speak about the Spitfire 1500. The OD has a gear-up ratio of
.802, if I remember correctly, or approximately 4:5. The ratio of
3rd to 4th is a bit wider (don't remember the number exactly), so OD
3rd is slightly lower than non-OD 4th. However the difference isn't
enough to matter for most performance reasons. A gear by gear
upshift would go 1, 2, 3, OD 3, 4, OD 4, but the OD 3 to 4 shift
isn't worth the trouble. (Plus, it's hard to pull the shifter back
while thumbing that switch forward!) Better just to go straight up,
then hit OD when you get up to speed on a highway. If I really want
to accelerate quickly when, say, as pulling out from an uphill
entrance ramp, I may choose OD 3rd instead of 4th, then shift to OD
4th when I'm up to speed. But it really doesn't matter much,
especially considering that the upshift will have to come sooner.
Which begs the question of why OD 3rd at all. I've found it useful
in two situations. One is downshifting on a highway, such as for an
exit ramp or to pass someone or accelerate up a steep hill. Having
OD 3rd means I don't have to think about whether OD is engaged or not
- just shift as normal, down to 3rd then back to 4th, regardless of
the OD switch. (However it's not cool to downshift to 2nd while the
OD switch is ON. :-o The 4th-3rd-4th shift sequence is so convenient
that every so often I forget about the OD switch entirely, then
"discover" it when the car reminds me.)
The other use of OD 3rd is in situations where 4th would mean
marginally low rpms for the desired speed. Hitting the OD switch is
simpler than shifting and saves cycles on the clutch cylinder seals.
(Once you've rebuild your master cyliner or had it drip brake fluif
on your shoes, you tend to worry about things like that!)
Once I had to carry a large box on the passenger seat that blocked me
from shifting into 4th. So I just did the required highway running
in OD 3rd. OD 4th sure is nice, tho'.
Just my $.007 on the subject.
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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