Gear ratio opinion with current setup

[guidoverduci]

I’m stuck between 4.56 and 4.88 after reading all the threads. I’ve got a mild 351w, T170 trans which is a 4 speed - 3.01 1st gear, 0.78 OD. 35” tires with a 4.5” lift. Mostly street driving, occasional 65mph cruise down freeway, and some light trails. Leaning towards 4.88 and hoping that 65mph is not too high RPM

 

[El Kabong]

I like to use the grimm jeeper gear ratio calculator:
http://www.grimmjeeper.com/gears.html It allows you to compare 2 different combos side by side with results displayed in multiple ways. And to change the mph or rpm for the resilts if you want to see it at a different setting than their norm. It's not the complete answer to gear choices, but helps to understand where changes will put you.

 

[71broncman]

I have a similar on my 71, which I just finished putting back together. I have 302 and a C4 in mine. I did the calcs. and found that 4.56's would return the gearing close to stock equivalent. With the OD trans and 351 in yours, I think the 4.56's would be good.
Mark.

 

[Apogee]

The OD on that trans isn't that tall, you have a relatively torquey motor and only 35" tires, so I'd probably do the 4.56's if it were my rig. Now if you see yourself moving to a 36 or 37 inch tire in the future, then take the extra 7% gear reduction and go 4.88 or 5.13 now.

FWIW, RPM at 65 will be 2219 or 2375 with 4.56 and 4.88 gears respectively per the Grimm Jeeper calculator, either of which would work well given your engine. You can enter in the 2nd and 3rd gear ratios as well to determine likely shift points, which may help with your decision.

 

[blubuckaroo]

Whenever using any of these tire size/gear ratio calculators, it's important to use the effective tire diameter rathan the static tire diameter. Load and tire pressure will affect the calceulation quite a bit.

 

[guidoverduci]

Thanks everyone. The calculator and some more searching has got me leaning towards 4.56 now. The local shop wanted $1300 for drive in / out parts and labor just for the rear end. I’m thinking I’m just going to buy a 3rd member and research doing it myself.

 

[g-71]

With your setup and use i would definitely go 4.56. I have a roller cam 351, NV 3550, and 35" tires in a "71. I went with the 4.88, love them on the trails, don't like the 2500-2600 rpm on the highway.

 

[Apogee]

Whenever using any of these tire size/gear ratio calculators, it's important to use the effective tire diameter rathan the static tire diameter. Load and tire pressure will affect the calceulation quite a bit.

You should actually use the published "revolutions per mile" on the tire manufacturers website to calculate the effective diameter, as there's a certain amount of tread squirm going on that you can't really account for with just a static measurement. I think at the end of the day, it doesn't really make all that much difference on the calculators, but if you're a stickler for the details, that's where I'd start.

For a 35x12.5R15 (#33572) BFG KO2 A/T tire, I've noted how I would do that below using dimensional analysis.

602 rev/mile and 1 mile = 63360 inches, so the tire spins .00950126 rev/in

We inverse that to get inches per revolution, which is 105.25 in/rev, and since 1 rev = Ceff = pi*d where Ceff = effective tire circumference, deff = effective tire diameter and pi = 3.14, we can divide 105.25 by 3.14 to get an effective tire diameter of 33.5 inches.

Of course, if you don't run the same load or pressure as the manufacturer when they determine the rev/mile spec, then your results may vary.

Tobin

 

[blubuckaroo]

You should actually use the published "revolutions per mile" on the tire manufacturers website to calculate the effective diameter, as there's a certain amount of tread squirm going on that you can't really account for with just a static measurement. I think at the end of the day, it doesn't really make all that much difference on the calculators, but if you're a stickler for the details, that's where I'd start.

For a 35x12.5R15 (#33572) BFG KO2 A/T tire, I've noted how I would do that below using dimensional analysis.

602 rev/mile and 1 mile = 63360 inches, so the tire spins .00950126 rev/in

We inverse that to get inches per revolution, which is 105.25 in/rev, and since 1 rev = Ceff = pi*d where Ceff = effective tire circumference, deff = effective tire diameter and pi = 3.14, we can divide 105.25 by 3.14 to get an effective tire diameter of 33.5 inches.

Of course, if you don't run the same load or pressure as the manufacturer when they determine the rev/mile spec, then your results may vary.

Tobin

The difference between 35" diameter and 33.5" effective diameter would certainly be important when trying to decide between two different gear ratios, if you were on the fence.