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Discussion Starter · #1 ·
i have a 78 280Z 2+2 4-speed and i was reading something on an earlyier post that said 4000 rpms= 65 in top gear
this is not the case with my Z i run 90 at 4000 and about 2500 at 65

is this because of a replaced differential? i am the third owner of the car and have no idea what modifications may have been made by the previous 2 owners

i'd just like to know about it because until now i thought this was normal

thanks to anyone who replies
 

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> i have a 78 280Z 2+2 4-speed and i was
> reading something on an earlyier post that
> said 4000 rpms= 65 in top gear
> this is not the case with my Z i run 90 at
> 4000 and about 2500 at 65

Hey! I have a '72 with the 4spd, and I'm pretty sure the rear-end is stock. I haven't paid attention to the RPM when going on the freeway, but it's by no means revving when going 65. I'll check it out next time I drive it and post the RPM. I think it's pretty low though. I suspect that the 4000 RPM @ 65 mph thing could have been a little exaggeration. :)

type to you later,
-rr
.
 

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> Hey! I have a '72 with the 4spd, and I'm
> pretty sure the rear-end is stock. I haven't
> paid attention to the RPM when going on the
> freeway, but it's by no means revving when
> going 65. I'll check it out next time I
> drive it and post the RPM. I think it's
> pretty low though. I suspect that the 4000
> RPM @ 65 mph thing could have been a little
> exaggeration. :)

> type to you later,
> -rr
> .

My '77 280z runs about 3000rpm at about 60mph.
 

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Matt,
According to the manuals your 2+2 has the same 4-spd and rear differential as the coupe.
That means that originally you had a 3.545 differential which would give you about 80 mph at 4,000. Even installing the earlier 3.364 ratio would only get you to about 85 mph at 4,000.
something isn't right here.
Phantom
 

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> Matt,
> According to the manuals your 2+2 has the
> same 4-spd and rear differential as the
> coupe.
> That means that originally you had a 3.545
> differential which would give you about 80
> mph at 4,000. Even installing the earlier
> 3.364 ratio would only get you to about 85
> mph at 4,000.
> something isn't right here.
> Phantom

If you are running larger (taller) tires than what came stock, it could account for higher speeds at lower rpm. I did this once on my Honda because I wanted to cut the rpm/speed ratio to improve milage and reduce engine wear, but for the next set of tires I went back to stock because the power suffered with the little engine and the milage did not improve. The rpm/speed ratio is linearly related to the tire height, so you can calculate where you are relative to stock tires. Cops love this one when they pull you over.
 

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> If you are running larger (taller) tires
> than what came stock, it could account for
> higher speeds at lower rpm. I did this once
> on my Honda because I wanted to cut the
> rpm/speed ratio to improve milage and reduce
> engine wear, but for the next set of tires I
> went back to stock because the power
> suffered with the little engine and the
> milage did not improve. The rpm/speed ratio
> is linearly related to the tire height, so
> you can calculate where you are relative to
> stock tires. Cops love this one when they
> pull you over.

Kane,
For him to know that he's doing 90 I would asume that his speedometer would be indicating that. The taller tire speed increase would not be reflected on the speedometer.
Phantom
 
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