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Old 07-05-2011, 01:26 PM
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I Need input on gear ratio & tire size

Here's what I have:
86 swb
350
flat tops pistons
Edelbrock rpm air gap intake
vortec heads
comp xe268 cam
long tube headers
2.5" flowmaster super 44's

Stock 350 tranny

Here my problem: The truck was an original 305 with 2.56 rearend.
I just had a 3.73 posi unit installed

now the truck is screaming at like 50 mph. I dont have a tach so cant give rpm numbers and i need to change my speedo gear so mph is off.

im running 22" rims with 305/40/22 tires. I went with a 3.73 cause of the bigger rims and tires and I plan on either going back to my stock rallys or down sizing to 17's or 18's.

I cant even image how bad it would be if I had the smaller wheels on it right now.

any input on why the truck is screaming at such a low speed of only 50-55 mph.

THANKS IN ADVANCE!!

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Old 07-05-2011, 01:28 PM
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i had intentions on going with a 700r4 but turns out the tranny was bad.
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Old 07-05-2011, 11:05 PM
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Quote:
Originally Posted by TX512
Here's what I have:
86 swb
350
flat tops pistons
Edelbrock rpm air gap intake
vortec heads
comp xe268 cam
long tube headers
2.5" flowmaster super 44's

Stock 350 tranny

Here my problem: The truck was an original 305 with 2.56 rearend.
I just had a 3.73 posi unit installed

now the truck is screaming at like 50 mph. I dont have a tach so cant give rpm numbers and i need to change my speedo gear so mph is off.

im running 22" rims with 305/40/22 tires. I went with a 3.73 cause of the bigger rims and tires and I plan on either going back to my stock rallys or down sizing to 17's or 18's.

I cant even image how bad it would be if I had the smaller wheels on it right now.

any input on why the truck is screaming at such a low speed of only 50-55 mph.

THANKS IN ADVANCE!!
Often, changing from 2.5-ish gears to 3.73 gears will seem like you're riding around in second gear- because you almost ARE, considering the difference in ratios! This can take some getting used to.

You need to get a tach. Then use an online calculator to see that it agrees w/what the rear gear is supposed to be, and that the torque converter or tranny isn't slipping excessively.

BTW, the nominal diameter of a 305/40R22 tire is about 31.5"- so it's a "big" diameter tire, relatively speaking.

Last edited by cobalt327; 07-05-2011 at 11:13 PM.
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Old 07-06-2011, 08:07 PM
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Those gears aren't very highway friendly.
If you swap in a 700R4 Trans, it's .70 4th/OD will knock the 3.73's down close to what you had. 3.73 X .70 = 2.61
Best of both worlds, accelleration & crusing rpm.
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Old 07-06-2011, 09:15 PM
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You can go to 4lo.com and they have a ratio calculator.

(New tire height / old tire height) x old gear = new gear
example
( 32 / 28 ) x 2.56 = 2.92

So as you can see a 3.73 would be a bit drastic.
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Old 07-07-2011, 01:34 AM
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Quote:
Originally Posted by cobalt327
Often, changing from 2.5-ish gears to 3.73 gears will seem like you're riding around in second gear- because you almost ARE, considering the difference in ratios! This can take some getting used to.

You need to get a tach. Then use an online calculator to see that it agrees w/what the rear gear is supposed to be, and that the torque converter or tranny isn't slipping excessively.

BTW, the nominal diameter of a 305/40R22 tire is about 31.5"- so it's a "big" diameter tire, relatively speaking.
Looking at this another way, if you had 2.56 rear gears and a TH350 tranny, putting it in second gear would be the same as a 3.89:1 rear gear in Drive. Obviously a 3.89 isn't that far from a 3.73, so the difference between the two is nearly the same, and I would expect it to take a while to get used to.

OTOH, there are a LOT of guys running non OD trannys w/3.73- and deeper- gears. It just depends on your tolerance to it and how much highway driving you do.
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Old 07-07-2011, 05:38 PM
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you have to get off the line too. anything you give up on top end will make you pay for it on the bottom end. for a 31 inch tire i dont think you are too far off the mark. my ol bronco had 3.50 gears with a 33 inch tire and seemed just right. with 35's it was a dog on take off. btw 3:50 was stock in many 4x's

but i know what you are talking about believe me. i have 4:56's now and a 35 inch tire so i am limited to going about 60-65 on the freeway at around 2800. and i could actually still be happy with 4:88's and just going 60. i would want bigger tires tho to even it out.

with a 3.73 stock ratio, stock tires are usually 31-32" so you shouldn't be complaining too hard.

Last edited by bullheimer; 07-07-2011 at 05:46 PM.
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