I need help. I have a 1983 ford f150 with a 302 5.0L. It has a 4 speed manual transmission witht eh granny gear or compound gear in it. I am looking to convert the 1/2 ton axles to a 3/4 ton. My current gear ratio is a 3.55 and I found a guy who will sell me 3/4 ton axles but they have 4.10 ratio. My question is, IF i put the 4.10s underneath my truck will it kill my top speed or will I still be able to go 75 on the freeway. It is a 4x4. I have heard that if i put these axels in i will only be able to go top speed of 50 mph. It that is the case will putting in a 5 speed help?

 

Yes, it will cause the rpms to be higher, but if you plan on installing larger tires, the gears will make up the difference. 4.10s will pull a 35 in tire on a Ford truck just like it still had the stockers on it. With the four speed, it will almost be enough for 38s. I have a 79 with 4.11 and 40 inch monster mudders. It also has a granny. Pulls great. Other truck has 4.56 and 40 super swampers and a granny.

 

It would cut your top speed about 20% unless you go to the taller tires mentioned above. You will still be able to do 75, the engine will be going at higher rpms. .