rear end gearing

[69f-100]

I have a question on some rear end gearing, that I need some thoughts on. I have a 69 f100 2wd short bed with a 390 C6 and I will be running 275/60/15 tires . I'm stuck between these three gears 3.25,3.50 and 3.70. When I'm done with the restoration I plan on just driving to shows with not too many long distance highway miles. I was wondering if anyone has ran any of these 3 gears and thier thoughts on them.

[HIO Silver]

3.50 is a good compromise btwn street, more spirited driving, and towing.

A 3.25 is good if you do more highway than street and tow infrequently.

3.70 is more for the strip, better towing/hauling heavy loads, and hilly terrain.

Pick the gear that best suits your probable driving habits and needs.

[tnlprt]

With the spec tire of 275/60/15 at 27.99 inches tall {this will vary according to wheel width}

3.25 gears 60 MPH 2350 RPM

3.50 gears 55MPH 2350 RPM

3.70 gears 53MPH 2350 RPM

3.89 gears 50MPH 2350 RPM

If I take my truck on a long cruise over 100 miles I put my 3.25 gears

for every day use around the house IE trips less than 50 miles I run 3.70 gears

If I want to have fun I put in either my 3.89 or 4.11 gears

The only difference is I run a 31 inch tall tire

Remember you have a poor mans quick change in a 9in rearend

2 people and air tools and you can change the gears in under an hour

[sargentrs]

I had 3.25's in my truck to start with. They locked up on me and I replaced the whole 3rd member with one from a slick that had 3.89's. Man, they really woke her up! She'd burn rubber in 1st and 2nd. However, out on the highway, above 60mph you couldn't hear squat. She was wound out so loud she screamed. I'd think the 3.50 range would still give you something out of the gate without blowing your ears out on the highway.

[gregamorton]

It sounds like you wont be driving much but its all about what u like. With 390 c6 I would have 3.00 or higher. My first truck I built I had 4.11s in it and it was fun but it got old. My daily driver has 3.70s but it has a 240 in it also. I have had luck with really high gears behind a healthy 390 and 460. I love the 2.47s and 2.50s behind the auto 460s. So with my experience I would say with your setup I would still go with the 3.25s to make it a better driver unless you have a huge cam and stall.

[ultraranger]

Having an overdrive transmission would greatly simplify things. You could keep your gearing in the rear end and still cruise at a reasonable RPM on the highway. --best of both worlds.

The 1st half of the page, in link below, is a very close approximation of what the engine RPMs would be at 60 MPH with a given rear end ratio and tire height, without an overdrive transmission.

The 2nd half of the chart gives the same, only, what the RPMs would be WITH an overdrive.

http://ultrastang.com/info3.php

...as additional information, from more of a performance standpoint; how to figure your vehicles torque multiplication factor:

http://ultrastang.com/info4.php

[Mancar1]

I have 4.10s I would like to have 3.50s.....to me that would be perfect....in a few months I will have higher rear end gearing.

[gregamorton]

Mancar1 3.54 rearend gears are nice in 3/4 ton cruisers and still do ok towing lighter loads with the bigblock!

[67dually]

I have 4.10s I would like to have 3.50s.....to me that would be perfect....in a few months I will have higher rear end gearing.

I'm on the same page as you, 50mph is the cutoff for me Though It sure is nice when you have weight on it or behind it.

[colnago]

Wow, that all makes sense! No wonder I can't get over 50MPH without going deaf! I have 4.11 gearing in the rear (F250 Camper Special)! Great fun in town, though.

Joseph

[ToughOldFord]

When I had 4:11s I maintained a steady 70mph easy. Could've done more, but 70 is my cruising speed. I went to 3:73s in an effort to save some fuel economy. Dropped a whopping 200 RPMs at freeway speed. Saved approximately nothing on fuel economy, give or take a few nothings either way.

Running stockish size tires, 238/65-16s.