Question on RPM
Thread Starter
Intermediate
Joined: May 2018
Posts: 48
Likes: 3
Question on RPM
What should my RPM's be when I'm running 55-60 mph ? It's a '75 L-48
Right now it's doing somewhere in the 40 to 43 range. This seems somewhat high to me but maybe totally normal. I get feeling sometimes like I'm riding my bike and i need to catch another gear.
Right now it's doing somewhere in the 40 to 43 range. This seems somewhat high to me but maybe totally normal. I get feeling sometimes like I'm riding my bike and i need to catch another gear.
Thread Starter
Intermediate
Joined: May 2018
Posts: 48
Likes: 3
Its a 4-speed. At idle it is sitting at 1100 Which is a bit high but that makes me think the gauge is working. All I know about the engine is what I've been told by the PO. All the smog crap is gone along with the cat, it has SS headers and he said it had a slightly bigger cam installed. It will really get up and go in the higher RPM range.
Last edited by tomboyy; Aug 31, 2018 at 08:02 AM.
Thread Starter
Intermediate
Joined: May 2018
Posts: 48
Likes: 3
That I don't know...I'm sorry I can't tell ya'll more but that's all I know.
What should the RPM"s be running 55-60 ?
And what if the gears had been changed...what would the reason for that be ? Except for the engine (matching numbers but worked on) and exhaust everything is original.
What should the RPM"s be running 55-60 ?
And what if the gears had been changed...what would the reason for that be ? Except for the engine (matching numbers but worked on) and exhaust everything is original.
Last edited by tomboyy; Aug 31, 2018 at 08:15 AM.
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
The rpms are dependent on the rear end gears. If it has 3.08 gears the rpms would be a lot less than if it had say 4.11 gears.
The tach boards on the electronic tach which yours should be have a tendency to go bad. Even though it is working they can show incorrect rpms
The tach boards on the electronic tach which yours should be have a tendency to go bad. Even though it is working they can show incorrect rpms
Last edited by Gordonm; Aug 31, 2018 at 08:32 AM.
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
1st get your a dwell tachometer meter.. 30-40 bucks on amazon and test that your car tach matches your new meter.
2nd. test your speedometer with your cell phone gps to conform that is correct.
then if all is correct and you are still running 4000 rpm at 60 mph then you have at or near a 4.56 rear end ratio.. 4.56 is basically not streetable.
a standard 3.08 rear gear the car comes with will get you about 2200 rpm at 60 mph.
if you are idling at 1100 then you probably have a pretty hot cam and a big street setup..
strange that the previous owner would not know all this.
this is all assuming you are using 4th gear?
2nd. test your speedometer with your cell phone gps to conform that is correct.
then if all is correct and you are still running 4000 rpm at 60 mph then you have at or near a 4.56 rear end ratio.. 4.56 is basically not streetable.
a standard 3.08 rear gear the car comes with will get you about 2200 rpm at 60 mph.
if you are idling at 1100 then you probably have a pretty hot cam and a big street setup..
strange that the previous owner would not know all this.
this is all assuming you are using 4th gear?
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Race Director
Joined: Mar 2017
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From: Virginia
Get a digital timing light to confirm your tach. You'll want one anyway, and this is the likely problem.
To check your rear end, put a tape mark on the driveshaft while the car is secure on jackstands. Have a helper count the tape marks as you turn a rear wheel one turn. 3.08 would be three Mark's and a fraction more. 4.11 four Mark's and a fraction more, and so on.
To check your rear end, put a tape mark on the driveshaft while the car is secure on jackstands. Have a helper count the tape marks as you turn a rear wheel one turn. 3.08 would be three Mark's and a fraction more. 4.11 four Mark's and a fraction more, and so on.
Thread Starter
Intermediate
Joined: May 2018
Posts: 48
Likes: 3
Ok... I went out and bought a digital timing light, $100 later this is what I got.
gauge says 1200
timing light says 650 at idle
when I push it to 4000 ( gauge reading )
My timing light reads 2200
Checked the reading on my Silverado and they matched. So the timing light is right on.
The faster the RPM, the more it's off. So when I'm cruising at 60 the RPM's are really sitting at 22 - 23 not the 40 - 42 that the gauge is reading.
Thanks for the help guys.
Tom
gauge says 1200
timing light says 650 at idle
when I push it to 4000 ( gauge reading )
My timing light reads 2200
Checked the reading on my Silverado and they matched. So the timing light is right on.
The faster the RPM, the more it's off. So when I'm cruising at 60 the RPM's are really sitting at 22 - 23 not the 40 - 42 that the gauge is reading.
Thanks for the help guys.
Tom
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Ok... I went out and bought a digital timing light, $100 later this is what I got.
gauge says 1200
timing light says 650 at idle
when I push it to 4000 ( gauge reading )
My timing light reads 2200
Checked the reading on my Silverado and they matched. So the timing light is right on.
The faster the RPM, the more it's off. So when I'm cruising at 60 the RPM's are really sitting at 22 - 23 not the 40 - 42 that the gauge is reading.
Thanks for the help guys.
Tom
gauge says 1200
timing light says 650 at idle
when I push it to 4000 ( gauge reading )
My timing light reads 2200
Checked the reading on my Silverado and they matched. So the timing light is right on.
The faster the RPM, the more it's off. So when I'm cruising at 60 the RPM's are really sitting at 22 - 23 not the 40 - 42 that the gauge is reading.
Thanks for the help guys.
Tom
