Pedal ratios - The 1947 - Present Chevrolet & GMC Truck Message Board Network

C15_Sierra · 11-12-2011, 06:17 PM

My truck has manual brakes, and was a 3spd manual truck from the factory. When the automatic was swapped in, the PO removed the clutch pedal, but kept the small brake pedal in, instead of swapping a bigger auto pedal in. Now I know why.

This week, I snagged the correct brake pedal from an '86 K20 that my buddy was junking, went to install it today before I put in the new steering column. When I got the old pedal off, I noticed there were different pedal ratios. I was curious if there were different pedal ratios for power/manual brakes, now I know what they are.

The manual pedal has a ratio of ~5.82:1, the auto pedal has a ratio of 4.57:1. This gives me about 21.5% less force on the MC when I press the pedal. Figured I might as well install the pedal now, work with the decreased pedal ratio until I get around to installing the vacuum booster setup I have

BUT

When I went to install it, there wasn't enough travel in rod to line up with the pin on the auto pedal, and there's no hole drilled for manual brakes like in the manual pedal. So looks like I'm gonna have to do the booster conversion now

I'm was just posting 'cause I couldn't find any info on it before, now I know and sharing so that others in a similar situation can know

Obligatory pics...auto pedal on top, manual on bottom

Quick calcs...

J Knight · 11-12-2011, 06:24 PM

Thanks for sharing, before I thought the only difference was the pedal width.

C15_Sierra · 11-12-2011, 06:36 PM

As you can see, the rest of the holes lined up for power brakes (and presumably the hydroboost one is the one closest to the pedal).

So you're right in thinking the only difference is width, when comparing manual/auto brake pedals with the same type of assist.

But since auto trannies probably weren't available with manual brakes, they didn't drill for manual brakes on the auto pedal.

I'm just highlighting the different pedal ratios for manual assist vs. vacuum assist.

**Corts60** · 11-12-2011, 08:01 PM

Great info man! Thanks for posting!

*mrhino* · 11-13-2011, 08:53 AM

Yep I had to find out about pedal ratio's the hard way as well on my last project. took me two days to figure out why I couldn't bleed the rear brakes on my power rear disc brake conversion. Thanks for sharing.

11-12-2011, 06:17 PM	#1
C15_Sierra Registered User Join Date: Apr 2011 Location: Hartford CT Posts: 223	Pedal ratios My truck has manual brakes, and was a 3spd manual truck from the factory. When the automatic was swapped in, the PO removed the clutch pedal, but kept the small brake pedal in, instead of swapping a bigger auto pedal in. Now I know why. This week, I snagged the correct brake pedal from an '86 K20 that my buddy was junking, went to install it today before I put in the new steering column. When I got the old pedal off, I noticed there were different pedal ratios. I was curious if there were different pedal ratios for power/manual brakes, now I know what they are. The manual pedal has a ratio of ~5.82:1, the auto pedal has a ratio of 4.57:1. This gives me about 21.5% less force on the MC when I press the pedal. Figured I might as well install the pedal now, work with the decreased pedal ratio until I get around to installing the vacuum booster setup I have BUT When I went to install it, there wasn't enough travel in rod to line up with the pin on the auto pedal, and there's no hole drilled for manual brakes like in the manual pedal. So looks like I'm gonna have to do the booster conversion now I'm was just posting 'cause I couldn't find any info on it before, now I know and sharing so that others in a similar situation can know Obligatory pics...auto pedal on top, manual on bottom Quick calcs... __________________ -Steve 1980 GMC C15 Sierra LWB Dark Carmine Red/Carmine Int Was: 250/3spd (1980-88), then a 305/TH350 (1988-2011) Now: 5.3/4L60E, 2.73 12-bolt open rear

11-12-2011, 06:24 PM	#2
J Knight Registered User Join Date: Jun 2011 Location: Baytown TX Posts: 2,219	Re: Pedal ratios Thanks for sharing, before I thought the only difference was the pedal width. __________________ John 82 Chevy Silverado 350ci/TH350

11-12-2011, 06:36 PM	#3
C15_Sierra Registered User Join Date: Apr 2011 Location: Hartford CT Posts: 223	Re: Pedal ratios As you can see, the rest of the holes lined up for power brakes (and presumably the hydroboost one is the one closest to the pedal). So you're right in thinking the only difference is width, when comparing manual/auto brake pedals with the same type of assist. But since auto trannies probably weren't available with manual brakes, they didn't drill for manual brakes on the auto pedal. I'm just highlighting the different pedal ratios for manual assist vs. vacuum assist. __________________ -Steve 1980 GMC C15 Sierra LWB Dark Carmine Red/Carmine Int Was: 250/3spd (1980-88), then a 305/TH350 (1988-2011) Now: 5.3/4L60E, 2.73 12-bolt open rear

11-12-2011, 08:01 PM	#4
Corts60 Just here to tinker Join Date: Jun 2008 Location: Holland, MI Posts: 3,697	Re: Pedal ratios Great info man! Thanks for posting! __________________ 78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/showthread.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 73 Chevy C20(Daily)http://67-72chevytrucks.com/vboard/s...d.php?t=852767 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(sold)

11-13-2011, 08:53 AM	#5
mrhino Account Suspended Join Date: Aug 2007 Location: USA Posts: 1,223	Re: Pedal ratios Yep I had to find out about pedal ratio's the hard way as well on my last project. took me two days to figure out why I couldn't bleed the rear brakes on my power rear disc brake conversion. Thanks for sharing.

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