RPM at 80mph?
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edhchoe

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RPM at 80mph?
« on: September 30, 2006, 02:49:24 PM »
I cannot remember the exact RPM but last night, it was at about 3700 in 5th gear.  Is that normal?  That is pretty high.  Do you think one of the previous owners swapped the rear diff out?
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asubimmer

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RPM at 80mph?
« Reply #1 on: September 30, 2006, 03:07:40 PM »
thats about right, 318is's have a 4.10
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dude8383

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RPM at 80mph?
« Reply #2 on: September 30, 2006, 04:13:06 PM »
yeh, like 3550 @ 80 mph
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FL318is

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I'll check.
« Reply #3 on: September 30, 2006, 08:26:37 PM »
Since I like running the ponies, I will give it a look see on the way to church.
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Alpine003

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RPM at 80mph?
« Reply #4 on: September 30, 2006, 10:11:45 PM »
A bit higher for me since I'm running on my rims from my 2002(195-50-15 size).
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gjuuu

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RPM at 80mph?
« Reply #5 on: September 30, 2006, 10:32:05 PM »
Quote from: edhchoe
I cannot remember the exact RPM but last night, it was at about 3700 in 5th gear.  Is that normal?  That is pretty high.  Do you think one of the previous owners swapped the rear diff out?


That seems about right for a 4.1:1 diff - (depending on wheel overall diameter).
If you are really interested there is a fantastic rev calculator HERE
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D. Clay

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RPM at 80mph?
« Reply #6 on: September 30, 2006, 11:21:03 PM »
Quote from: gjuuu
If you are really interested there is a fantastic rev calculator HERE
Nice info, especially the 240 Getrag ratios. I have a Gear program that was written for Hewland type trans with interchangable ratios. It includes inventories and also prints Hewland type charts. A friend and I wrote in 1995. It's 16 bit Windows but runs as legacy ware. Also have another one that prints a chart with the complete inventory.  PM me if interested. We no longer sell it but I'll give you a copy if you need it. It also includes corner weight, lap times,  fuel consumption, etc.
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MustPayDaddy

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RPM at 80mph?
« Reply #7 on: October 29, 2006, 05:49:39 AM »
Quote from: Alpine003;12496
A bit higher for me since I'm running on my rims from my 2002(195-50-15 size).


since youve got 15's, which are an inch bigger than stock, shouldnt you be revving lower? always thought the bigger the wheels, the fewer revolutions it takes to travel a certain distance..
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RED E30

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RPM at 80mph?
« Reply #8 on: October 29, 2006, 06:30:42 AM »
Whilst his wheels are of larger diameter his tyres are of a lower profile, in total the the rolling diameter is smaller than the stock setup.  Stock tyres for a 14 inch wheel are in the 195/60/14 region yet for the 15's he is using 195/50/15
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MustPayDaddy

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RPM at 80mph?
« Reply #9 on: October 30, 2006, 04:28:13 AM »
ahhhh.. yea, that 50 mustve slipped by me. iirc the tire placard on our doors say 15" rims require 205/55/15 tires to match the stock size of 195/60/14
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Alpine003

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RPM at 80mph?
« Reply #10 on: October 30, 2006, 10:16:11 AM »
Yes that is correct. I've since replace my tires to 205-55-15's and I'm hating it.

Really thinking about stepping down to at least 205-50-15's. It revs higher but it has the same effect of increasing your FD ratio. Some of you think 4.10 is high then you should try lower profile tires.

There is also too much sidewall flex for my tastes. Time to play with the tire pressures but if that doesn't do it, then 50 series it is.
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GuySchalnat

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RPM at 80mph?
« Reply #11 on: October 30, 2006, 09:06:16 PM »
When I drove my autocross tires on the street (205/55R14), the car didn't really want to go 80.  It was like 4000 rpm.  Now, that may have been the slicks, but it may simply have been too high for the m42.  The m44 in the Z3 cruses at 4000 rpm for 80 mph just fine, so maybe it was the tires, but I kept finding myself not wanting to push the gas petal down that far for general driving.

-Guy
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D. Clay

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And...
« Reply #12 on: October 30, 2006, 09:47:30 PM »
A tire with a given nominal size may vary in overall diameter by .25"-.75" depending on brand and wear. The actual variable is the square of the distance from the axle center to the ground times pi. This distance varies with inflation pressure and centrifigal force. A dragster getting "up on the tire" is an extreme example. This is not of major significance as at about 80 mph it varies by 2-3 mph. But at 200 mph it's the difference between the pole and starting 28th or banging on the rev limiter.
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