Help choosing new gear ratio!

[Robster]

Hey guys I need your help. I have a 74 ford bronco with a ford AOD transmission. It has a 300 hp engine and 35" tires. It is a weekend driver only and I never off road it. I currently have 3:50 gears and need them to be lower. What is the best for my situation. Thanks in advance for all your help.

 

[garberz]

The AOD has a tall overdrive. 4.88’s at a minimum. 5.13’s is you don’t plan on putting lots of highway miles on it.

Mark

 

[Broncobowsher]

5.13s if you see a lot of highway, 5.38s if you want more fun around town.

You didn't say how you were making that 300HP, for a 302 to make that it will need a bit of RPM to be in the happy band. 35s are a bit big, and the AOD has a really tall overdrive with a torque convertor that can't unlock. To use that OD you need a lot of gear. That gear sure makes things fun around town.

 

[chuck1022]

Www.4lo.com

Gear ratio calculator

 

[chuck1022]

35's with 5.13's and an AOD (.67 OD) at 65mph will be turning 2144 rpm.

 

[chuck1022]

I dont know enough about Motors. 2100 RPM might be perfect or too low.

 

[Broncobowsher]

Need to know more about this 300 HP engine.

A bronco on 35s takes a good bit of grunt to push down the highway at 65. Does this mystery 300 HP engine have enough grunt to cruise at ~2100 RPM? Maybe if it is a nice mellow 300 HP 351W it might be OK, but if it is a 300 HP 302 it will want a bit more RPM to be happy cruising. Probably closer to 2500 RPM.

2500 RPM cruise with an AOD in OD will be a whole lot of passing fun. Roll into the throttle enough to get a kickdown and you will be right at 4000 RPM. A 300 HP 302 will be running strong at 4000 RPM. Lift and back to a happy cruise.

 

[chuck1022]

Need to know more about this 300 HP engine.

A bronco on 35s takes a good bit of grunt to push down the highway at 65. Does this mystery 300 HP engine have enough grunt to cruise at ~2100 RPM? Maybe if it is a nice mellow 300 HP 351W it might be OK, but if it is a 300 HP 302 it will want a bit more RPM to be happy cruising. Probably closer to 2500 RPM.

2500 RPM cruise with an AOD in OD will be a whole lot of passing fun. Roll into the throttle enough to get a kickdown and you will be right at 4000 RPM. A 300 HP 302 will be running strong at 4000 RPM. Lift and back to a happy cruise.

according to the gear ratio calculator, you would need 5.98 gears to turn 2500 rpm at 65mph with 35s

 

[chuck1022]

I punched in my stock bronco specs with 5.98 gears and my motor would be doing 4700 rpm at 65mph. I like to keep it at peak power at all time

 

[rguest3]

300HP 302 motor most likely needs high RPMs.

35" tires and AOD needs Lots of gear.

4.88 gear ratio minimum.

 

[Broncobowsher]

Although there are lower ratios, 5.38 is about as low as practical. Some would argue that is getting pretty small on the pinion gear, but I think it will be fine for a mostly street truck. An abused trail rig that is overweight and hammered on with a heavy foot it probably isn't strong enough.
5.13 is the next highest gear set, has a little larger pinion. Depending on the engine it is probably a bit on the high side.