Help With Rear Gears

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MMAX
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Joined: Wed Jan 02, 2008 8:24 pm
Location: Alberta, Canada

Help With Rear Gears

#1 Post by MMAX » Sun Jul 26, 2009 9:44 pm

I'm currently running 4.57 gears in a Ford 9" in my 68 Camaro. Last year I was running 32 X 14 slicks and in the 1/4 mile I would go through the traps at 6300rpm. I switched to 31 X 13 slicks this year and ran in an 1/8 mile race today. As I went through the traps in 3rd gear, my shift light was on telling me I was at 6300rpm. I wouldn't think 1" less tire diameter would make that much difference. If I was to run 1/4 mile, what gear ratio would I need to keep from red lining my engine?

sc racing
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Re: Help With Rear Gears

#2 Post by sc racing » Mon Jul 27, 2009 8:21 pm

You cant go by the size of the tire on the sidewall (32" or 31" ) you have to measure the actual rollout and divide that by 3.14 this will give you the real diameter of each tire. What mph were you going in the 1/8th and in the 1/4 .I dont see how you could run the same rpm at half track with the same gear as you ran in the 1/4 something else must have changed?? Did you cahnge anything else? Also your 32 might grow a lot and your 31 might not.

MMAX
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Joined: Wed Jan 02, 2008 8:24 pm
Location: Alberta, Canada

Re: Help With Rear Gears

#3 Post by MMAX » Mon Jul 27, 2009 10:36 pm

The only thing I changed were the tires.

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wikd69
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Re: Help With Rear Gears

#4 Post by wikd69 » Tue Jul 28, 2009 2:18 am

Yeah, I found out the hard way that 31x18.50 doesn't mean 31x18.50. It's the actual runout on the tires / slicks that matter, not what the vendor labels them... Simplest way I found to check / measure runout is to use a piece of string and run it around the center of the mounted tire circumference, mark where it meets the end and then measure. That'll give a pretty good idea of the runout and will let you more precisely calculate mph, gears, shift points, etc. etc.

Hope this help :mrgreen:
1969 ProStreet Camaro RS Best 9.75@139 1.46 60'
Blown 427 BBC, TH400 w/Brake, Back-Half Ladderbar
Narrowed 12-Bolt, 4.10 Gears, Spool, Moser 33 Spline

http://www.marsh-racing.com/harrys_camaro-1.htm

pro70z28
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Re: Help With Rear Gears

#5 Post by pro70z28 » Tue Jul 28, 2009 6:34 pm

My wife's cloth sewing tape measure works great for measuring roll out.
{As long as I get it back in her sewing basket before she knows it's gone} :thumb:

I've since bought one of those 100' cloth tape measures from the box store.
{Safer} truce:
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wikd69
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Re: Help With Rear Gears

#6 Post by wikd69 » Tue Jul 28, 2009 6:46 pm

pro70z28 wrote:My wife's cloth sewing tape measure works great for measuring roll out.
{As long as I get it back in her sewing basket before she knows it's gone} :thumb:

I've since bought one of those 100' cloth tape measures from the box store.
{Safer} truce:
Hey, that's a great idea !!! I'll pick one of those up :)

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