Highway rpm - Team Camaro Tech
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post #1 of 34 (permalink) Old Jan 27th, 20, 08:31 PM Thread Starter
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Highway rpm

Hello Eveyone,

I just purchased a 69 Camaro a few months ago. Im running the original L34 396, 4speed M20 wide ratio and 3:31 rear gear ratio.

At 70mph the car is running about 3800rpm. Is this normal? I would think with 3:31 it would lower.

Is anyone running a similar set up and can tell me what your car is running at?

Thanks again,

Nate
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post #2 of 34 (permalink) Old Jan 27th, 20, 08:43 PM
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Al
 
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Re: Highway rpm

Here's an online calculator. Need to know your tire diameter but 3800 rpm is high.
https://crawlpedia.com/rpm_gear_calculator.htm

Al

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Last edited by allanjs; Jan 28th, 20 at 11:20 AM.
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post #3 of 34 (permalink) Old Jan 27th, 20, 09:08 PM
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Re: Highway rpm

3800 rpm with 26 tire would indicate. 4.11 rear ended ratio.

Maybe be speedo is off. Maybe tach is off. Maybe you have 4.11s?
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post #4 of 34 (permalink) Old Jan 27th, 20, 09:49 PM
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Definitely a wrong number someplace,
20.5 would be a pretty short tire. 70mph, 3.31 gear, 20.5",tire = 3,799 rpm

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421" Dart SHP block
TKO 600 RR
Dutchman 9" w/3.70
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Last edited by ilikeike; Jan 28th, 20 at 07:54 AM.
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post #5 of 34 (permalink) Old Jan 28th, 20, 03:47 AM
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Re: Highway rpm

3.31s and a 26 tire would be 3000 rpm at 70 mph
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post #6 of 34 (permalink) Old Jan 28th, 20, 04:27 AM
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Re: Highway rpm

As others have said, something is off. My car with approx. 27" tall tires and 3.31 gears turns 2350rpm at 55. Sure sounds like 4.10 gears. Is the speedo gear the correct one for rear end ratio? When you think you are going 70... maybe you are actually going much faster. Much, much faster.
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post #7 of 34 (permalink) Old Jan 28th, 20, 05:59 AM
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Re: Highway rpm

Reads more of 4.56:1 gears.
You can do the old trick of jacking up rear axle, chalk mark driveshaft, and grab a tire valve stem and rotate the tire one revolution and count the chalk marks as they go by.
Also, you could do the GPS trick, and/or, do the Mile-A-Minute test -- 60 MPH=one mile traveled.
Or visit a speedo shop and check speedo calibration.

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post #8 of 34 (permalink) Old Jan 28th, 20, 09:09 AM
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Re: Highway rpm

OP

as mentioned you need to confirm the rear gear ratio (spin tire 1 revolution and count DS rotation. i.e. DS turns "about" 3 1/4-3 1/2 turns =3:31)

Then tire height. Typically searching tire brand site gives every tire size 411

There are online calculators that you enter tire size, gear ratio and it will show what RPM vs mph you get

RPM calculator
https://spicerparts.com/calculators/...rpm-calculator

and this will show what "driven" gear to use to correct speedo. These are cheap and easy to install. Just unscrew speedo cable and remove bullet and swap the gear. Each increase/decrease in tooth count changes speedo by about 5mph +/-

Speedometer Driven Gear Calculator
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post #9 of 34 (permalink) Old Jan 28th, 20, 12:05 PM
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Re: Highway rpm

You need to shift into 4th lol. Seriously though, I have an 69 L34 also. It doesn't make a difference for your rpms. With an M21, 26.57" tires and 3.31s I'm at 2,930 RPMs at 70. It's purely math and easy to calculate. It doesn't really matter which 4 spd you have either. They're all 1:1 in 4th. It's only when you get an overdrive that things change. ~3,800 would be 4.11s with a 26" tire.

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post #10 of 34 (permalink) Old Jan 28th, 20, 03:50 PM
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Re: Highway rpm

Nate,

I am in a similar situation. I have a 1968 Camaro SS 396ci, Muncie 4 speed, 12 posi. with 3.55s. I run 3000 rpm at 60 mph. I even opened up my diff and counted the teeth on both pinon and ring gear and did the math. I can start out in 3rd gear easily. I think it was set up for the 8th mile. I'm considering a change too.

Randy

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post #11 of 34 (permalink) Old Jan 28th, 20, 04:12 PM
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Re: Highway rpm

Quote:
Originally Posted by Randy Brutout View Post
Nate,

I am in a similar situation. I have a 1968 Camaro SS 396ci, Muncie 4 speed, 12 posi. with 3.55s. I run 3000 rpm at 60 mph. I even opened up my diff and counted the teeth on both pinon and ring gear and did the math. I can start out in 3rd gear easily. I think it was set up for the 8th mile. I'm considering a change too.

Randy
so your tire would be about 23.8" tall then.. what tire do you run ?

1968 Butternut Coupe,
421" Dart SHP block
TKO 600 RR
Dutchman 9" w/3.70
Speedtech torque arm,Ridetech HQ coilovers
Global West arms
Budnik wheels

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post #12 of 34 (permalink) Old Jan 28th, 20, 04:45 PM
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Re: Highway rpm

70 mph @ 3170 rpm
3:55 gears
Tires are 255/45-18's
Diameter inches 27.04"
Circum. inches 84.93"
Revolutions per mile 745.99

'68 Camaro 454, 4sp
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post #13 of 34 (permalink) Old Jan 28th, 20, 05:54 PM
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Re: Highway rpm

60 mph @ 3000 rpm
3:73 gears
Approx 26" dia

68 302 M20 - 85 IROC Z
The older I get the faster I was.
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post #14 of 34 (permalink) Old Jan 28th, 20, 07:01 PM
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Re: Highway rpm

First thing is you need to start with 1 accurate measurement.
You can often use your phone GPS to get an instantaneous reading that is usually within 1 mph.

One of the easiest tests is to follow a friend with a new car that has cruise control & preferably a digital dash.
New cars with stock sized tires are typically accurate.

Find a level road & put on the cruise at say 60mph. If you can do this for 5 miles or more... all the better. You can then compare your odometer reading at the same time. They may not match that closely. The leader can signal the start & end points to give you point of comparison. Highway mile markers can also be a good indication.

The odometer number will help verify the speedometer gearing correction needed.
The speed reading may need further adjusted by calibrating the gauge also. By comparing the speed readings at a few different speeds, you can get a better idea of what causes the error.

If you are lucky the odometer & speed will be off by the same percentage & it will be a repeatable error, such as 4 off at 40 & 6 off at 60 & the mileage will be off .5 at 5 miles... then you know you need to alter the speedometer gearing by 10%.

The more data points the better & also don't assume your factory or economy brand tachometer is dead on either.

For example my Firebird is off by the following amounts.
Tach at a steady state is rather accurate, but it hits the 6700 rev limiter at 6000 displayed.

On a 100 mile trip the speed was as follows:
car gps
75 70
65 61
50 47

On a 68 mile trip using mile markers the car registered 74 miles.
They all correspond to being off by 6%...

My Nova I had was way off due to gearing & tire size changes. 4.56's with a 28" tire. I had to correct it by almost 20%.
Once corrected for miles, the speed was still off a consistent 3 mph across the board, so I had to alter the clock spring adjustment in the dash to get it more accurate.
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post #15 of 34 (permalink) Old Jan 29th, 20, 09:06 AM
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Re: Highway rpm

I use the Speed Box app on my phone as a GPS MPH tool
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