Engine RPM question

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Jun 24, 2002 | 07:36 PM

Chief Muktar

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Racer

Joined: Oct 2001

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From: Surrey BC

Engine RPM question

I've noticed that on my 1990 6-speed in 6th gear @ 70 MPH my engine is running just under 2000 RPM. I believe that the only gear ratio available is a 3.33 ?? It seems like the motor is spinning higher than it should. Could a previous owner have installed a different gearset in the car? The only other thing I can think of is that whoever owned the car before me installed some 235/45/17 tires. (don't even ask me why:mad ) Anybody have any ideas?

Jun 24, 2002 | 08:43 PM

HighHopes85

Drifting

Joined: Mar 2001

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From: Lafayette IN

Re: Engine RPM question (Chief Muktar)

Tire OD = 25.327 inches...
I figure 4.1X gears will put you at 1900 RPM at 70 MPH with a 0.5:1 6th gear ratio.

Jun 25, 2002 | 10:59 PM

dlmeyers

Burning Brakes

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From: Camden SC

Re: Engine RPM question (HighHopes85)

Hello,

I have a 90, zf6 with 3.33 gear set, 275-40 ZR17's. At 70mph, she's tach'in 1750rpms.

Hmmm.

dlmeyers 90 coupe zf6 3 speed shocks

Jun 26, 2002 | 12:39 AM

Chief Muktar

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Racer

Joined: Oct 2001

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From: Surrey BC

Re: Engine RPM question (HighHopes85)

Quote:

Tire OD = 25.327 inches...
I figure 4.1X gears will put you at 1900 RPM at 70 MPH with a 0.5:1 6th gear ratio.

Does this use a 275/40/17 tire or a 235/45/17 tire??

Jun 26, 2002 | 01:00 AM

HighHopes85

Drifting

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From: Lafayette IN

Re: Engine RPM question (Chief Muktar)

Tire diameter is sidewall profile times treadwidth. 50 series means sidewall is 0.5* the height of the tread width. 40 means the tire is 0.40* the treadwidth. That is in millimeters. Multiply by 2 since the diameter includes 2 sidewalls (bottom and top of tire IOW). Add the height of the wheel and you have tire diameter.

LOL. I typed too fast. 'Add' wasn't quite what it was supposed to be and the censors tore it up :D

I did the calcs with the 235. i.e....

(235*0.45) * 2 = 211.5mm
211.5mm = 8.32677 in
8.32677 in + 17 in = 25.32677 in
25.32677 in = 0.003197824 furlongs = 0.0003473533 nautical miles

[Modified by HighHopes85, 11:09 PM 6/25/2002]

Jun 26, 2002 | 05:57 PM

Dr. Evil

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From: DrunkDefender

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Re: Engine RPM question (Chief Muktar)

Make sure your tachameter is correct. All most every 90 model has problems with the factory tach reading high. Some are worse than others but many including me have factory tachs that are off by as much as 2500-3000rpm. In most cases the higher the rpm, the more rpm it will be off by.