RPM on the highway

[squidley63]

I want to get an idea on what rpm that others are seeing on the highway without overdrive as I'm pretty sure my tach is off or something as the rpm seems to be lower than indicated at anything above idle. I have a small block in it with msd6al ignition with vdo tach and have been stuck between their tech departments on if I have an issue. Msd thinks I need a tach adapter, but vdo says my tach will work fine with an mad box. Neither will tell me what the symptoms are of needing an adapter and not having one.

My rpms indicated at 55 mph is almost 3500 rpm with the stock 3 speed and 4.11 gears with 235 75 15 tires. Calculators are telling me I should see about that rpm at 65 mph. I know our trucks arent very aerodynamic but I think it shouldnt cause this much difference from calculated.

I have checked the simple stuff like slipping clutch and any bad connections between the tach and the mad box and the dip switch setting on the back of the tach. Any info others could give me would be greatly appreciated.

[64tantruck]

I'm running about 2800 rpm at 55-60mph I'm running a 3:73 gear with TH-350 tranny. and 16 inch wheels on my C-20 I'm running a 350 engine with a Holley 750 on it for a carb.

glenn

[1Bad62Pro/Street]

Formula To Determine Rpm Final Drive Gear Ratio Tire Size And Mph

Gear Math

MPH = (RPM X Tire Diameter) ÷ (Gear Ratio X 336)
Example:
MPH = (3,000 X 26) ÷ (4.10 X 336)
MPH = 78,000 ÷ 1,377.6
MPH = 56.6

RPM = MPH X Gear Ratio X 336 ÷ Tire Diameter
Example:
RPM = 70 X 4.10 X 336 ÷ 26
RPM = 3,709

Gear Ratio = (RPM X Tire Diameter) ÷ (MPH X 336)
Example:
Gear Ratio = (3,000 X 26) ÷ (70 X 336)
Gear Ratio = 78,000 ÷ 23,520
Gear Ratio = 3.31:1

Tire Diameter =MPH X Gear Ratio X 336 ÷ RPM
Example:
Tire Diameter = 65 X 3.31 X 336 ÷ 2,700
Tire Diameter = 26.8 inches

Effective Gear Ratio = (Old Tire Diameter ÷ New Tire Diameter) X Gear Ratio
Example:
Effective Gear Ratio = (26 ÷ 28) X 3.55
Effective Gear Ratio = 3.29:1

http://www.carcraft.com/techarticles...e_and_mph.html

[BADAZ chevy guy]

Originally Posted by 1Bad62Pro/Street

Formula To Determine Rpm Final Drive Gear Ratio Tire Size And Mph

Gear Math

MPH = (RPM X Tire Diameter) ÷ (Gear Ratio X 336)
Example:
MPH = (3,000 X 26) ÷ (4.10 X 336)
MPH = 78,000 ÷ 1,377.6
MPH = 56.6

RPM = MPH X Gear Ratio X 336 ÷ Tire Diameter
Example:
RPM = 70 X 4.10 X 336 ÷ 26
RPM = 3,709

Gear Ratio = (RPM X Tire Diameter) ÷ (MPH X 336)
Example:
Gear Ratio = (3,000 X 26) ÷ (70 X 336)
Gear Ratio = 78,000 ÷ 23,520
Gear Ratio = 3.31:1

Tire Diameter =MPH X Gear Ratio X 336 ÷ RPM
Example:
Tire Diameter = 65 X 3.31 X 336 ÷ 2,700
Tire Diameter = 26.8 inches

Effective Gear Ratio = (Old Tire Diameter ÷ New Tire Diameter) X Gear Ratio
Example:
Effective Gear Ratio = (26 ÷ 28) X 3.55
Effective Gear Ratio = 3.29:1

http://www.carcraft.com/techarticles...e_and_mph.html

I couldn't have said it better my self.
Now my brain hurts. lol

[fleetsidelarry]

squidley, how did you verify your mph?

[kieth]

Originally Posted by squidley63

I want to get an idea on what rpm that others are seeing on the highway without overdrive as I'm pretty sure my tach is off or something as the rpm seems to be lower than indicated at anything above idle. I have a small block in it with msd6al ignition with vdo tach and have been stuck between their tech departments on if I have an issue. Msd thinks I need a tach adapter, but vdo says my tach will work fine with an mad box. Neither will tell me what the symptoms are of needing an adapter and not having one.

My rpms indicated at 55 mph is almost 3500 rpm with the stock 3 speed and 4.11 gears with 235 75 15 tires. Calculators are telling me I should see about that rpm at 65 mph. I know our trucks arent very aerodynamic but I think it shouldnt cause this much difference from calculated.

I have checked the simple stuff like slipping clutch and any bad connections between the tach and the mad box and the dip switch setting on the back of the tach. Any info others could give me would be greatly appreciated.

measure the diameter (height from ground to top of tire or the rear)

take a look here......you will be able to see your speed @ different rpms...

TORQUETOGROUND_Master_r.pdf

[cArNaGe]

This is easier...... And it's fairly accurate. I'm @ 3000 rpms on level highway doing 60. TH350 with 3.73 gears and 275/50/15 tires.

https://itunes.apple.com/us/app/spee...557871911?mt=8
or
https://play.google.com/store/apps/d....android.tacho
or
use a GPS if you have one. My 10 year old cheap GPS has a speedometer in it.

[siggyfreud]

My guess would be that your RPM is right, and your speed is wrong. Have you verified your speed?

[squidley63]

Speed verified with GPS on phone. I did finally get in touch with someone at vdo that had me verify the potentiometer setting on the back of the tach(it should be centered). Mine was off bad and while I was there I pushed the dip switches back and forth to ensure they were fully switched. Lo and behold my rpm indicated dropped almost to what they should be according to the gear calculators.

I'm amazed at how seeing the number on the tach has influenced how I drove the truck. Just seeing the lower number has made a world of difference in how it feels to rundown the highway. The mind does some strange things to us sometimes i guess.