Newbie understanding gear ratios

[TimP-69 Bronco]

So i have been reading a lot trying to understand what gear ratio would be best for me, which is 90% street driving. I have all stock dana 30, 9" with stock 3speed. From what i have read many people are moving to a 4.11 or 4.56. I also read that stock is 3.5, if that is true wouldn't moving up to a 4.11 ratio slow me down?

Please let me know what i am missing here, Thanks

 

[mudstud]

What size tire?

I also read that stock is 3.5, if that is true wouldn't moving up to a 4.11 ratio slow me down? If you keep the same tire size your RPMs for the same speed would go up. Here is something to play with http://www.grimmjeeper.com/gears.html Enter your data and make the comparison

 

[Timmy390]

Going from 3.50's to 4.11's doesn't slow you down it just increases the RPM you have to turn to run the same speed. This is a GOOD thing as stock 302's love higher RPM's. That's where the power is.

I run 32's and with the stock 3.50 gears my rig was a slug. I went to 4.11's and the thing is alive. At 65-70 I'm turning 2850ish RPM's and when I put my foot on the floor, the rig reacts and goes faster where as before, it did nothing.

Love my 4.11's so much, I often think 4.30's would have been even better.

Tim

 

[Hozr]

You definitely have to take into account the entire system from tire to engine. Many people look at Just the Axles, or Just the Transfer case. Everything in the train matters from engine to tire.

I have ZF S5-42 5 spd, T-shift Dana 20, 4.11/4.10 axles, and 35" tires and I cruise ~68mph @ 2000 rpm in 5th (OD)

There are some REALLY good drivetrain calculators available but you need to have all the information to be effective.

 

[NYLES]

What size tire?

I also read that stock is 3.5, if that is true wouldn't moving up to a 4.11 ratio slow me down? If you keep the same tire size your RPMs for the same speed would go up. Here is something to play with http://www.grimmjeeper.com/gears.html Enter your data and make the comparison

Ya beat me to it

 

[Rustytruck]

6 cylinder or V8 they make a difference. 6 cylinder came with a 27 inch tire 4.11 gearing with larger tire and 4.56 gearing optional. V-8 came with 27 inch tire 3.50 gearing with larger 28" tire and 4.11 gearing optional. The weaker 6 cylinder motor needed lower gearing to pull the same load than the v-8. You change tire size you may need to change gearing to put the torque produced from your motor in the sweet spot. Change parts around and you can pull the torque out of its sweet spot and you greatly change the efficiency of the whole system. The limitation of the system is the torque produced from your motor. Torque is what makes the whole thing go. Changing motors and knowing where peak torque is produced is extremely important. Keeping your motor in your peak torque area while its doing its work is the responsibility of the gearing both in the transmission and axle gears. Unfortunately Broncos were designed in a time where they only put enough motor with its available torque ratings to just get by. There was no extra torque, simple tire size changes and a trailer or heavy loads in the Bronco negatively effected the drive ability of the Bronco. The stock Bronco is a great piece of machinery and will out last its peers. Start modifying the system and you have to deal with the whole package to maintain its proper function and utility and keep its systems in proper balance.

 

[TimP-69 Bronco]

Answers to questions...

28" tires
J shift t case
302

Is it unreasonable to think I can travel high way speeds (60-70) with a stock tranny? I'm hoping with the right gearing I could get there.

 

[TimP-69 Bronco]

Going from 3.50's to 4.11's doesn't slow you down it just increases the RPM you have to turn to run the same speed. This is a GOOD thing as stock 302's love higher RPM's. That's where the power is.

I run 32's and with the stock 3.50 gears my rig was a slug. I went to 4.11's and the thing is alive. At 65-70 I'm turning 2850ish RPM's and when I put my foot on the floor, the rig reacts and goes faster where as before, it did nothing.

Love my 4.11's so much, I often think 4.30's would have been even better.

Tim

What transmission do you have?

 

[Rustytruck]

You should get there right now with what you have. If its a stock early 302 you will be winding 3000 rpm or so using most of what you got. Automatic or stick?
Maybe time to look into a good tune up. Special attention made to ignition , timing and advance systems. I would run a compression test on all cylinders. maybe a leak down test and pull and verify that all plugs are functioning properly. I would attach a vacuum gauge to the vacuum system and verify the you have a very steady vacuum pressure. If the needle isn't steady then you have some work to do. You could easily have a plugged fuel filter limiting your top speed. Allot of variables but in the end 60-70 on flat ground you should be able with a v-8 and what you nave.

 

[thegreatjustino]

Answers to questions...

28" tires
J shift t case
302

With this setup, I'd stick with the stock 3.50 gears

 

[TimP-69 Bronco]

It's a stick 3speed. I had an unbalanced fly wheel that was causing some vibration and that kept me more pushing it over the 50s. Now that it is fixed I will push it a little more when Im on the road again in spring time.

Thanks for the input

 

[americanhorses]

(sidenote) Here is a equation I used to find my speed since my speedo is way off.

 

[TimP-69 Bronco]

(sidenote) Here is a equation I used to find my speed since my speedo is way off.

Thanks! i know mine is off as well this will help

 

[Hozr]

GPS iPhone Speedometer app! I had my speedo rebuilt and it's never been close to anything correct or consistent.

 

[jckkys]

I've gone cross country at least 5 times in EBs with 29" tires, 3.50 gears, and 302s. The RPMs at 65 were about 2650. This was nearly ideal for the 302 torque curve and drove comfortably. Later I bought another EB with 4.11 gears and found it unpleasant at highway speeds. I recommend keeping the 3.50s for your present combination. If you go to larger tires, like 31 10.50s the 302 may prove inadequate. At that time a 351w swap would be less $ than new gears for both axles. The added torque more than compensates for the larger tires.

 

[Timmy390]

What transmission do you have?

Stock 3 speed off the tree and in the floor.....

Tim

 

[blubuckaroo]

To use a tire/gear ratio calculator or formula properly, you'll need to use the loaded tire diameter. Otherwise your RPM is going to be higher than you had expected.

http://www.vibratesoftware.com/html_help2014/2011/Diagnosis/Tire/Static_Loaded_Radius.htm