gear ratio rpms

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Dec 13, 1999 | 08:03 PM

charles_1

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gear ratio rpms

Does anyone know what rpm Ill be at with 29 inch tires, C6 tranny, and 411 gears. How about 370? Thanks in advance.

Dec 13, 1999 | 09:16 PM

StockMan

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From: Inthepast

gear ratio rpms

Charles:

The rpm will be "0" unless your stepping on the gas pedal in this equation. [img]/dcforum/Images/happy.gif[/img] In other words, I think you are missing part of the equation. I suspect you are trying to increase your top end; or perhaps you're trying to find out how fast your engine is spinning at highway speed. It would help to know what you're trying to determine.

Stock Man
1967 F-250 FE 390 4wd
1966 F-250 I6 240 2wd LWB Flare Side
<a href="http://www.hometown.aol.com/tbeeee">http://www.hometown.aol.com/tbeeee</a>

Dec 13, 1999 | 10:42 PM

BigBrownTruck

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gear ratio rpms

Engine RPM(@60mph w/ 1:1 transmision)=20168 X gear ratio/tire diameter

So.....4.11 X 20168 = 82890.42/29=2858RPM @ 60mph
or
3.70 X 20168 = 74621.6/29=2573RPM @ 60mph

Assumming that you have no overdrive gearing in your transmission.

Dec 14, 1999 | 08:50 AM

jowilker

Fleet Owner

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From: Creedmoor, North Carolina

gear ratio rpms

Biggie, I'm just an ole country boy that didn't learn all that fancy math. How did you come up with them numbers, and what if the speedo is off?

John

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

Dec 14, 1999 | 04:04 PM

charles_1

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gear ratio rpms

Sorry, Stockman I meant to put in 60 mph. I am soon going to change my gears and cant decide how low to go. I thought that the numbers would help me decide. Also thanks to big brown truck for the reply. This may be a dumb question but is 3rd gear in a C6 a 1:1 ratio. Thanks again for the replies.

Dec 14, 1999 | 04:27 PM

StockMan

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From: Inthepast

gear ratio rpms

I was just being a "wise-acre" No apology necessary. I pretty much assumed you were trying to make a gearing decision. 60 mph is kind of a standard speed that is assumed so that you can compare the results across gear changes. I'm not familiar with the actual formulas used but I know there are a lot of gear formulas posted up on the net. Some actually have calculaters built in that spit out the results. 3rd gear or drive on the C6 is 1:1 final ratio AFAIK. Unless the tranny is some sort of overdrive then the final gear is 1:1. This is true without any exception that I'm aware of.

Stock Man
1967 F-250 FE 390 4wd
1966 F-250 I6 240 2wd LWB Flare Side
<a href="http://www.hometown.aol.com/tbeeee">http://www.hometown.aol.com/tbeeee</a>

Dec 14, 1999 | 04:30 PM

jowilker

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Posts: 24,552

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From: Creedmoor, North Carolina

gear ratio rpms

Yes Sir 3rd = 1 to 1. All non over or under drive trannys are 1 to 1 in high gear.

Which engine are you running? I have 2.75 behind a fresh 352.

John

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

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Dec 14, 1999 | 06:52 PM

charles_1

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gear ratio rpms

Thanks again for the replies and Im running a 390 but at 60 mph and right on 2000 rpms Im having to downshift to 2nd just to get rolling fast enough to pass anyone. I know Ill be giving up some gas milage but I guess Ill live with it. Thanks again.

Dec 15, 1999 | 09:43 PM

#10

Trail_Rider

Fleet Mechanic

Joined: Nov 1999

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From: Lower 48

gear ratio rpms

Sounds like a truck I used to have. My rear was a 3:08 turned 2200 @ approx 60 (speedo was off).

Trail Rider

Dec 16, 1999 | 12:18 AM

#11

BigBrownTruck

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gear ratio rpms

I got my formula from a sheet of paper I have from Auburn Gear. It has a matrix with ratio's from 2.56 to 6.5 and tire sizes from 26" diameter to 36". It also lists the formula that I spelled out. If you know that your tach is good, and you know your tire diameter and your rear end ratio, you'll find out whether your speedo is good. I found out I had a bad tach (hand out from a buddy-Sun) because the numbers didn't add up. When I got a new tach the numbers did add up. Of coarse you could also check your speedo on a known mile (we have some marked freeway miles in WA state) with a stopwatch. 60MPH =1mile per minute. I currently have an inaccurate speedo on my 69 F100 and I suspect that if I return the truck to factory tire diameter, it will correct itself. Anybody know what a 69 F100 OEM tire diameter would be? I got 31" tires on it now and it's about 10mph off. 50 on speedo is really 60.