How many rpm's?
Post Fiend
Joined: Nov 2005
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From: Charles Town, W bygod Va
https://www.ford-trucks.com/calculat...php/p-mph_rpm/ I went to tirerack.com for the specs on my size tire, youll have to figure out the OD reduction
Super Moderator
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Calculators here allow entry of OD ratio along with all other variables.
Novak Conversions - Gearing & Gearing Math for Jeeps
Novak Conversions - Gearing & Gearing Math for Jeeps
Junior User
Joined: Nov 2006
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my 86 f 150 4x4 has an aod and has 31 inch tires.. the calculator says 2501 rpm.. and factor in the 18% reduction gives me right around 2 g if i did it right.. my tach however says 1850ish at 65 mph. however, might not be that accurate of a tach and i coulda done the math wrong.. but should give you a ball park idea.
Postmaster
Joined: May 2004
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18% reduction is incorrect. My 83 Ford Light Truck Spec Book says the OD ratio in an AOD is .66....a 34% reduction. FYI... it's .71 in the 4OD manual tranny with the 4.9 and larger engines.
My calculation in my post is based on my 4OD turning 1600 at 60 mph, then adjusting for the higher speed and higher axle ratio.
My calculation in my post is based on my 4OD turning 1600 at 60 mph, then adjusting for the higher speed and higher axle ratio.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
AOD tranny has a 0.67 overdrive gear, so engine speed at 70mph in top gear would be 1948rpm.. assuming 30" tires.
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Hotshot
Joined: Sep 2001
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My truck has the same setup pratically except for 255 70 15 tires that are just barely bigger than oem 235 75 15. I turn just over 2K rpm at 70.
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Posting Guru
Joined: Jun 2002
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From: Suffolk, VA USA
I just tried mine...86 302 AOD 4x4 with 3.55s and 32s. Speedometer and tachometer have been independently verified as accurate (both by other electronics and the local PD). I'm turning 2500 rpm on the nose at 70 mph.
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The calculator shows this combo to produce 2609 at 70 in 1:1 third gear, and
1748 in .67 fourth.
So maybe your 32's aren't 32. They usually run less diameter than the stated size, but your 2500 figure would put them closer to 33.39". Maybe they 'grow' a bit at speed....
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
How did you verify the tach? Either you did this in D and not OD, your tach is way off, or you don't have 3.55 gears.. more like 5.10!!
Posting Guru
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From: Suffolk, VA USA
The tach matched the rpm readout on an engine analyzer hooked up to it a few months back....I made a point of looking at the time because I wanted to make sure about the tach. That's not to say it still can't be wrong, but it did match the figures on a fairly high end machine. Thinking about it, 2500 does sound pretty high, but the truck runs so well that I really didn't give it another thought. This is my 8th truck from this generation and the best runner of them all. I've got another tach I can put in just to make sure. There's no question I was in OD at the time...I started from dead start and can count to four. I was in the rear differential just yesterday changing out the lube. I didn't count teeth, but can tell it's tagged as a 3.55 and certainly looked to be in the mid-threes. The truck runs great and gets about 15+ mpg average.
Are we sure the calculators are right?
Are we sure the calculators are right?
Posting Guru
Joined: Nov 2004
Posts: 1,137
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From: Asheville, NC
Not trying to hijack your thread, but this topic has me thinking. And thats rough. 
My '83 with C6, 235/75R 15's indicates 3000 RPM at 70 mph. Any suggestions for slowing the old girl down? I can go for a new rear end. EDIT: Its a 2WD, and I need to get an idea of what gear set to go with. This truck never tows, and is rarely loaded. LWB, and alot of highway driving. Thanks.
My '83 with C6, 235/75R 15's indicates 3000 RPM at 70 mph. Any suggestions for slowing the old girl down? I can go for a new rear end. EDIT: Its a 2WD, and I need to get an idea of what gear set to go with. This truck never tows, and is rarely loaded. LWB, and alot of highway driving. Thanks.
Last edited by Holmesuser01; Nov 11, 2008 at 06:17 PM. Reason: mistake in syntax
Post Fiend
Joined: Nov 2005
Posts: 7,437
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From: Charles Town, W bygod Va
I came up with 3.7 gear ratio...Im not sure what rear you have and what ratios were available in it but with a 3 spd trans youll lose a lot of off line perfomance if you try to get your rpms down to low at higher speeds, I say let the motor rev a little.
Posting Guru
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From: Asheville, NC
Mines an 8.8" standard rear end. I had a chance to switch out rear ends a few years ago, but was mis-guided by someone with his "fact" that the 6 cylinder rear ends were built weaker than the V8's... It was a 300 six with the top shifter overdrive. Now, its gone. It was ordered by the original owner to be thrifty with gas. It averaged 20mpg for me.
Id check the door sticker but it is long gone.
The tag on the rear end cant be found either. This old truck sat for awhile before I got it.
Id check the door sticker but it is long gone.
The tag on the rear end cant be found either. This old truck sat for awhile before I got it.