89 3.55 gear rear what rpm?

[gsmcsw]

i'm new so i will say hello first....

i have a 89 bronco that i just got thru putting together and getting running and i have a question. i got a rearend from the local junck yard due to the one that was in it was trashed. and it seems that the rpms are higher then i would expect. the bronco has a 351w, c-6, brand new 31 10.50 tires, and 3.55 gears. the question i have is what should i see for rpm's at 65-70 mph, currently i'm reading 3000 rpm at 65 i think that is high for the combination

 

[Roadkill]

Try this site. Just fill in all the information for your truck and it will calculate for you. You can also experiment with different combinations of parts if you are thinking about making changes.

 

[Blue beast]

Your gear ratios front and rear have to match or you will damage the transfer case!! Chock the front wheels, Mark the u-joint, mark the tire, Jack

up the tire, put it in neutral, spin the tire one rotation while counting how many turns the driveshaft makes. 4 turns plus a few degrees is 4.10:1

3 turns and a half is 3.50:1 which is a common ratio!! Pretty much it is a ball park the axle tags are not always correct if someone did a gear swap so

at least you will have an idea. Do the same with the front axle and see what you get you can do it without jacking it up just turn the driveshaft and

count the number of turns to one roatation on the front half shaft. Mud run trucks sometimes run different ratios, but if you try it your transfer case will eat itself in a short time. That Jyard rear probably has 4.10 in it which with 31 will leave your motor screaming on the highway at least for my tastes.

I prefer running a too high(numericaly lower) ratio for better road mileage (mpg) When you get off road and the going gets tuff you just have to use

4 lo more often, But that's what it is there for!!