I need some help in deciding what is the best gear size for my setup. I have a 92 F150 4X4, 4.9L, 5 Speed, 6" Pro Comp lift with 35" Tires. The stock gear set is 3:08. As you can guess I don't have a whole lot of power right now. Anyway I am ready to replace the gears. I am not very good with the math, but I think I need 4:88's to get me back to where I need to be. Does anyone have any suggestions?

Kevin

 

Your truck most likely had 29" tires. I like this calculator http://www.et-studios.com/motorsports/gears/gears.html

I have a goal of 60-65 mph in high gear at 2000rpms. Because you have overdrive, I think a 4.56 will be better suited than 4.88, and you could get away with 4.11s. Find your 5th gear ratio and plug into the calculator and expieriment with different ratios.

 

If you arnt running on highy at all and wheel hard go with the 4:88 rears . I run 4:11's with 33" tire rpms worked out about same. But thats highway use. If you want the 4:56 would work decent but the lower rear would really pep up your truck . Good luck

 

Thanks for the replys...I think I'll go with the 4:56's.

Thanks again

Kevin

 

if you do a lot of driving that involves a lot of driving 70 mph or faster then i would say 4.10s. if you do a lot of driving around 60 or slower with a lot of hills then i would say 4.88s that would give you a lot of off road power to. and if you do the 60-65 then in the hills then i would say 4.56 or if there arent many hills at all and you stay on the road and dont tow a lot then i would say 4.10.

yes the 2000-2200 rpm is a good way to look at it lower rpms if everything is flat but 1800 rpm at 60mph is the low side i would say


I have 33" tires and i cant decide weather to go 4.10 or 4.56 i wish they 4.30 but i cant find them for the front.

 

4.88 all the way, I have 4.56 with 31" tires and its great, with 35's you you will be dissapointed with anything less than the 4.88's

 

Here is a formula...

MPH x GEAR RATIO x 336 / TIRE SIZE * OVERDRIVE RATIO.


This forumla will allow you to plug in any MPH and gear ratio you wish to play with and it will compute the RPM in 5th gear you will get.

The RPM at 70-75 mph is what most people will determine their gear ratio by. On a 4.9, you make the most torque at 2 grand.

If you drive on the highway you will want to seriously consider what target RPM you wish to achieve.

I have a similar truck, 35 tall tires, and at one point I had the same tranny (OD ratio on yours is .80).

I'm running 4.10s as 4.56s were too high for me.

Stock, my truck was a 3.55 gear on 29" tall tires. With a 35" tall tire, to achieve the same final gear ratio...you'd need a 4.30 gear. They don't make a 4.30 and 4.10 was close enough so I went with the 4.10.

70 x 4.10 x 336 / 34.8 = 2800 RPM in 4th gear....* .80 = 2250. So I turn about 2250 RPM at 70 mph.


A 3.08 is a high (numerically low) gear for a stock truck...it's more towards the fuel economy side of things. But with a 35" tall tire you'd only be able to get 3.73s or 4.10s to come close to your factory gearing...definitely not 4.88s. But your factory gearing was crap...and you wouldn't want factory gearing with those anyway...so you'll want a lower (numerically higher) final gear ratio...more power for those tires and the weight of the vehicle.

So a 4.10 would be on the conservative side...if you don't run on the highway much or don't care about highway mileage than you don't need to be as concerned and you can go with a more steep 4.56 gear. I would only use a 4.88 if I wanted all out power...with little/no consideration for highway mileage or RPM.

I like to run 75-80 on the highway sometimes so 2500 RPM is about as much as I want to spin on the highway so I went with 4.10s.