Gear ratio RPM ???? - Ford Powerstroke Diesel Forum
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Old 02-23-2011, 02:00 PM
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Gear ratio RPM ????

Ok guys not new to this but finally sat down and ran some calculations on my gear ratio.

tire ratio 285/75/r16 = 32.8
O/D ratio 0.76
RPM 2650 or so
Speed is 75 MPH verified by GPS and speedo

When i punch all this in i get a gear ratio of 4.56. I used this calculator it is one of the most acurate. Last on on the page.

Gear Calculator

From what i can find all 97 standards 3/4 and 1 ton trucks had ZF5 tranny and the O/D ratio is 0.76 if this wrong let me know. I am pulling my cover tomarrow to see if it has after maket gears. Since ford never put this ratio in a truck. The front and the rear r the same due to it does not buck in 4x4.

Input please
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Old 02-23-2011, 05:43 PM
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I'd be willing to bet you have 4.10's. That's what I have in mine, and though I have 35" tires, at 75mph, I'd be in the 2100-2200rpm range. If you allow for your smaller tires, I'd be surprised if you had anything other than 4.10's.
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Old 02-23-2011, 05:52 PM
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Add to the fact that i have only owned this truck for a short time and the front bumper was at some point trimed to clear larger tires. Im still betting some one did a gear swap. Unless my Tach is way off, some how, but it starts at 0 and makes no crazzy jumps or jerks. Is there any way to test the tach. If u use the calculator i included in my post with ur own ratios u will see why im confounded. thanks for the input.
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Old 02-24-2011, 01:59 AM
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I've got an AIC in my truck as well as the factory tach in the dash. The two don't agree on my RPM, so one isn't totally accurate. When the AIC says I'm doing 2000rpm, the tach reads approximately 2100. Since the display on the AIC is digital, I assume it to be more correct than the analog tach.
I'll be interested in hearing what you find when you pull the differential cover.
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Old 02-24-2011, 06:48 AM
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My money is on the tach being wrong. I have the same issue with mine, I have exact tire height, gear ratio, tranny ratio and gps speed but my rpms dont jive up with what the calculations say. The only variable left is the tach.
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Old 02-24-2011, 07:08 AM
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Idk Ive had a snap on MT2500 plugged in and its usually within about 50rpm of the tach. Either way I used to it do this and found i have 4.30 gears.
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Old 02-24-2011, 12:18 PM
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Yea kanuck15 we have had this discusion before about a year ago. We both decided we had 4.30s. I just got back from the ford dalership tach checks out good. They also pulled my vin and the truck was manufactured with 4.10s. I have a buddy that works over there. I have $85 of lube in truck still debating on if its worth pulling the cover and wasting that money. I have a call into another friend he used to have an engine scope hopefully he will get back to me by tomarrow.

The main reason I am concerned with this is i am going to do a 60 swap. I need to know if i need both axles or just the front. If i have after market gears it will be cheaper to get both axles instead of changing the gears in the 60.

Last edited by tracker8wr; 02-24-2011 at 12:21 PM.
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Old 02-24-2011, 12:28 PM
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you could always do the wheel turn VS drive shaft turn method. but you would have to mark things well and keep a close eye on it
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