Hey Tom P.....? for ya....
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Joined: Jan 2000
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From: Pt. Pleasant, WV (Home of the Mothman!)
Hey Tom P.....? for ya....
I know you have already told me this but I can't remember what you said 
After you installed the 3:73 posi what is your cruise rpm at...say...60mph?
At 55 I'm running around 1750 to 1800 give or take.At 60 I'm close to 1800rpm's.
If I can ever get my hands on that camaro I've been trying to get for 5 months now
I'm slapping that 3:73 posi under my RS.And a few other items.I just want an idea on what RPM I can expect at cruising speeds.
Thanks Tom
------------------
Kevin S
89 RS w/2.9
modded some but not heavily
After you installed the 3:73 posi what is your cruise rpm at...say...60mph?
At 55 I'm running around 1750 to 1800 give or take.At 60 I'm close to 1800rpm's.
If I can ever get my hands on that camaro I've been trying to get for 5 months now
I'm slapping that 3:73 posi under my RS.And a few other items.I just want an idea on what RPM I can expect at cruising speeds.Thanks Tom
------------------
Kevin S
89 RS w/2.9
modded some but not heavily
Supreme Member
Joined: Mar 2000
Posts: 13,414
Likes: 6
From: Central NJ, USA
Car: 1986 Firebird
Engine: 2.8 V6
Transmission: 700R4
Rear gear: 3.73/posi
Rear tires: 275/60r15 (28 inches diameter)
Technically, those above two specs give me an "effective" rear gear ratio of 3.46... which is close to the 3.42 that originally came in my car. My stock tires were with P215/60r15's.
All RPM measurements were taking with light throttle (not coasting) in overdrive (4th gear), 1.0:1 ratio.
55 MPH = 1800 RPM
65 MPH = 2000 RPM
80 MPH = 2200 PRM
In fact, here's the old message link:
https://www.thirdgen.org/messgboard/...ML/000240.html
I'd imagine you can do some kind of formula for cruising rpm's, and tire sizes... my overdrive is a 0.70 ratio gear. Actually, wait, I just checked my favorite tire/wheel/gear calculation webpage, and you can do it there!
http://www.tolan-hoechst.com/cars/tirecalc.htm
Put in my tire size (P275/60r15, 8 inch rim) as "current", and your tire size as "planned". (Everytime you move from a field, the whole page recalculates itself). Then, roll down to "effective gearing results". Change the speed to 65. Change the rear gear to 3.73. Leave the trans gear ratio at 0.7. Looks like my old "engine rpm" estimate of 2000 was close, eh? (Who the heck can read 39 rpm on a tach?) Then you'd just look over at what your RPM's would be. If you have the stock P215/65r15 tires, they'd be 2193 RPM @ 65 mph... not too much higher.
Hope that helped!
------------------
-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!
Rear tires: 275/60r15 (28 inches diameter)
Technically, those above two specs give me an "effective" rear gear ratio of 3.46... which is close to the 3.42 that originally came in my car. My stock tires were with P215/60r15's.
All RPM measurements were taking with light throttle (not coasting) in overdrive (4th gear), 1.0:1 ratio.
55 MPH = 1800 RPM
65 MPH = 2000 RPM
80 MPH = 2200 PRM
In fact, here's the old message link:
https://www.thirdgen.org/messgboard/...ML/000240.html
I'd imagine you can do some kind of formula for cruising rpm's, and tire sizes... my overdrive is a 0.70 ratio gear. Actually, wait, I just checked my favorite tire/wheel/gear calculation webpage, and you can do it there!
http://www.tolan-hoechst.com/cars/tirecalc.htm
Put in my tire size (P275/60r15, 8 inch rim) as "current", and your tire size as "planned". (Everytime you move from a field, the whole page recalculates itself). Then, roll down to "effective gearing results". Change the speed to 65. Change the rear gear to 3.73. Leave the trans gear ratio at 0.7. Looks like my old "engine rpm" estimate of 2000 was close, eh? (Who the heck can read 39 rpm on a tach?) Then you'd just look over at what your RPM's would be. If you have the stock P215/65r15 tires, they'd be 2193 RPM @ 65 mph... not too much higher.
Hope that helped!
------------------
-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!
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