70-442
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70-442
My 70 -442 stock 455 motor,stock turbo 400 w/stock converter.323 posi.235-60-14 tires.My question is what should this motor tach at at about 60mph?It seems to rev around 3000rpm.It just doesnt seem wright.Any help,Thanks.
Old(s) Fart
Joined: Mar 2007
Posts: 50,815
From: Northern VA
Your 235/60-14 tires are a Honda-sized 25.1" outside diameter. The original G70x14 tires were 26.82" outside diameter. With that tire size and your 3.23 gears, you should see about 2700-2800 RPM at 60 MPH, depending on converter slippage. Are you sure your tach and speedo are accurate? With the taller original tires, that would have been about 200 RPM lower at 60. This is completely normal. These cars were designed to run at 70 MPH all day long. Sustained 3000 RPM will not hurt the engine one bit. People who are used to new cars with 6, 8, or 10 speed transmissions always seem to freak out over this.
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Joined: Aug 2019
Posts: 2,092
Joe P covered it like a blanket. All I can add is exact numbers that as he said will vary with converter slippage.... But still backup everything he said.
Your rear gear rear tire combo turns 2725 RPM at 60 MPH with a 5% slip.
Your rear gear factory tire combo turns 2550 RPM at 60 MPH with a 5% slip.
Our factory speedometers usually read higher than they actually are going, which can get dramatically worse depending on age and wear and tear. One benefit to your short tires is that your effective rear gear ratio is closer to 3.50. Meaning your 442 is quicker than stock.
Your rear gear rear tire combo turns 2725 RPM at 60 MPH with a 5% slip.
Your rear gear factory tire combo turns 2550 RPM at 60 MPH with a 5% slip.
Our factory speedometers usually read higher than they actually are going, which can get dramatically worse depending on age and wear and tear. One benefit to your short tires is that your effective rear gear ratio is closer to 3.50. Meaning your 442 is quicker than stock.
Registered User
Joined: Feb 2013
Posts: 15,432
From: Phoenix, AZ
My car with 3.23 rear gears and 245-60-14 tires turned around 3400 RPM at 70 MPH. As said, it’s not an issue as long as your cooling system can handle it.
My coolant temp would start climbing to 210+ at that RPM with the AC on in 110 degree summer temps. Slowing down to under 3000 RPM and coolant temp dropped to 190.
edit: the above was at 70 MPH, typed 60 by mistake
My coolant temp would start climbing to 210+ at that RPM with the AC on in 110 degree summer temps. Slowing down to under 3000 RPM and coolant temp dropped to 190.
edit: the above was at 70 MPH, typed 60 by mistake
Last edited by Fun71; Jun 20, 2024 at 06:22 PM.
Vincit qui se vincit
Joined: Aug 2019
Posts: 939
From: SE USA
^Amen
My daily drivers eight speed is really lazy, around 2000 RPMs at 80mph. Hard to believe there are 10 speeds out there, I was pumped with a four-speed in high school. Crazy stuff.
My daily drivers eight speed is really lazy, around 2000 RPMs at 80mph. Hard to believe there are 10 speeds out there, I was pumped with a four-speed in high school. Crazy stuff.
Last edited by vCode442; Jun 20, 2024 at 06:51 PM.
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