Switch Trans or gears?

Hey fellas...new guy on the block here.

I have a 1978 Bronco that I have been restoring. Interior is completely new as well as paint and body. I have a 4 speed manual transmission. I just rebuilt the engine (351-400) and it runs great. I am running 35" Goodrich tires. My problem is this....

I would like to get a higher top end without turning 3500 rpm. The granny gear is pretty much useless to me. The truck is rarely if ever off of pavement. I assume the gears are 350 from what I have read. I will verify this.

Sooooo.....am I smarter to switch out the transmission or change gear ratio's in front and rear. 

Any and all comments are appreciated.

Hi Jeff, welcome..!  

Is your speed @ 3500 RPM  around 70 MPH..?

Most all trannies are 1-to-1 in hi gear.  You would have to go to an overdrive 5 speed.., the 5th being overdrive.

If you go too hi in gearing, you could lug the eng.

Neither option is cheap or easy exactly.  The diff. gear swaps have to be properly tuned to prevent howl noise.

Google the power curve charts for that engine. It may be the optium RPM/performance is  at that 3500 RPM.

If those were not the original tires, and I'm pretty sure they are not, changing the differential gears would be the thing to do.  This chart is for a cruise speed of 55 MPH so for 65 you would shift left about 3 to 4 columns http://www.supermotors.net/registry/media/280159  Or to be more accurate you can use the formula at the top of the page, to get the performance you are looking for.  Like the chart there are considerations for mileage, power and a good balance between the two.  Mileage will never be a real consideration in a Bronco, they were not designed for that. 

Also, consider the running gear.  You may want to beef up the drive shaft and universals to a stronger type since the larger tires will put a lot of stress on them if you get into a condition that will require a lot of torque.

Good luck and keep us posted,

>-

PS see the next chart after the one above.  You will need to change the speedo gear to get the right speed indication on the speedometer.

Yep..,  That was my point about speed.....

At 70 MPH --- on 35's --- with 538 gears --- would get you +- 3500 RPM..!!

Discover what gears are in your diff's  AND/OR  check/test your tach.