Gear ratios

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Oct 15, 2003 | 08:46 AM

bilmac

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Gear ratios

I have a 91, 150, inline 6, 2wd that I changed from a manual 5speed tranny to a much older 4 speed transmission because I needed a grandma to keep from burning my clutch when towing. Now I'm not quite happy with my performance when I am in high gear. Don't have a tac, but I would guess I am turning 2200 rpm or so at 55mph. I would like to slow the engine down some for the sake of gas mileage. I've been told that changing from 235 75R 15 tires to 31x9.5 can make quite a difference. That doesn't seem quite possible to me. Can I have a good useful grandma gear, and still be able to get decient gas mileage? How would you recommend getting there? The numbers on my differential are F638A.

Oct 15, 2003 | 11:36 AM

MustangGT221

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From: Topsfield, MA

MPG X GEAR RATIO X 336 / TIRE SIZE

Formula to determine the rpm at a given speed in 4th gear. That number from the diff doesn't tell us what ratio you have, that only tells us what plant it came from. There should be other numbers on the tag, or you can get the code off the door jam sticker.

Oct 15, 2003 | 11:37 AM

StrangeRanger

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From: Copley, OH

Make that MPH not MPG

Oct 15, 2003 | 08:10 PM

MustangGT221

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From: Topsfield, MA

oops...good eye.

Oct 15, 2003 | 08:40 PM

bilmac

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Thanks Mustang. I have a 3.55 diff. and my tires are 27.5" high. According to my calculations changing to the 31 9.5 tires will be close to changing to a 3.09 rear end. Thats assuming the tires are actually 31" high. This all begs the question what is a good rpm to run the inline 6. I seldom drive the truck over 65.

Oct 15, 2003 | 10:35 PM

kspilkinton

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From: South Kitsap County, WA

You'll need granny with a "rear" like 3.09. When I went to 31's from the stock 29.5" tall tires on my '82, I actually loss mileage. Heavier tires and overcoming the rolling resistance are suspect.

I'd try for 1850-2000 rpm.

-Kerry

Oct 17, 2003 | 09:36 AM

bilmac

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Thanks guys. I believe changing rear ends may be the way to go. If I could find someone who wants to take his ratio the other way I could do it for just the labor involved.

Last edited by Boss_358; Oct 18, 2003 at 12:27 AM.

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Oct 17, 2003 | 06:14 PM

MustangGT221

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From: Topsfield, MA

Well, if the tire size you have is the 235/75/15, they are not 27.8" tall, they are 28.9 or something of that nature. Go to summit racing and do a search for 8.8 ford racing gears, they will give you a list of available gearsets. They are about 190 bucks, and you need an install kit which is about 80 bucks. I would also have the outer axle tube bearings changed as well as the seals, so another 20 bucks. If you have 31" tires and 3.55 gears with no overdrive, than your running about 2500 RPM at 65 mph. A gearset of 3.08 will give you 2170 rpm at 65, and a 2.73 gearset will give you 1923 rpm @ 65. Just keep in mind that going from 3.55 to 2.73 is the equivilant of about a 35- 40 hp loss and 3.08s are about 25-30hp loss, so you will loose a significant amount of power by doing this. This is why they use overdrive, so that you can still run a low gearset in the rear for more power.

Last edited by MustangGT221; Oct 17, 2003 at 06:16 PM.