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Discussion Starter #1
ok, I know when you're in 5th gear doing oh say 80mph, your tach should be at 4k rpm's.

My question is this. (i just did this to see) but when i'm doing 30mph in 5th gear, it's right on at 3k rpm's. But when I start speeding up, doing about 60, it's almost right at 2750(like i'm doing 55)

so should I trust the tach, or the speedometer?

my wheels and tires now are only CM off of the stock size wheels and tires I had on it before.
I have 205/40/17's on there now, but never really noticed a problem up untill the past few days.

any help would be awesome!
 

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Jason said:
when i'm doing 30mph in 5th gear, it's right on at 3k rpm's.
Damn! Just wrote a long reply and then lost my internet connection!

I think you have a typo in the above; did you mean to say "30mph at 1.5krpm"?

But I get your drift; rpm and mph should be proprtional, in a given gear, and yours aren't.

Tire size is irrelevant to tach/speedo proportionality, btw, because your speedo gets its data from the transmission, and doesn't know what size tires you have (and even if it did, rpm should still stay proportional to mph, for given gear and tires).

A slipping clutch could prevent proprtionality, but it doesn't sound as if this is the problem in your case (double-check by going to WOT at low rpm in 5th gear, and seeing if the rpm rises faster than the mph).

The tach reads off the disty; the speedo off the tranny. Which would I trust? I don't know. Neither?
 

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The tach isn't accurate at high RPMS to a significant degree. I've personally never tested it to see how accurate it is at midrange.

On most of them, 7500rpms looks like 8500 or so indicated.
 
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Discussion Starter #4
Thanks guys!!

and yes nick, I ment 1.5k RPMS!

I guess I'll jsut stick to the speedo.
 

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NickR said:
Tire size is irrelevant to tach/speedo proportionality, btw, because your speedo gets its data from the transmission, and doesn't know what size tires you have (and even if it did, rpm should still stay proportional to mph, for given gear and tires).
tire size will affect ur speedometer reading if it's changed from stock and not plus sized. here is a tire size calculator that will tell u ur speedo reading when ur tire diameter is changed from stock: http://www.miata.net/garage/tirecalcold.html
 
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