I have a 67 f100 with a 240, 3 speed and 3:70 rear end ratio. seems low geared to me. its wound pretty tight at 60mph. I need advice on how much to step up. im thinking 3:50 or 3:25 may work.
any advice appreciated...thanks

 

If it's a 9" rear just swap the pumpkin to one with the 3.25 or 3.50 gears . My 68 F100 360v8 had factory 3.25 I would have swapped for free .

 

These trucks were not Hwy vehicles, 45-50mph was like the limit since these were basically farm trucks.
The easiest switch is tire size. If you change the rear end gears that little 240 may not have enough power to get you
up an over a small hill or a head wind an have to down shift.


Orich

 

didn't think about it being a little 240 .

 

The first I'd do is install a tach so you know what your RPM really is and do before making any changes. .
Back in the day when these truck were taken on the Hwy's they poked along behind the 18 wheelers.

Orich

 

its has plenty of low end now, I can take off in second gear easily. not sure which rear end I have, the specs on this pickup says its "light duty". not sure if that means it has a 9 inch or not.

 

wrong....the light duty is the transmission but it is a 3 70 gear

 

If the warranty tag on the back edge of the drivers door states "09" under the Axle heading, the truck has a 9-inch 3rd member with a 28-spline open (single track) differential with 3.70:1 gears. --this is assuming the tag is still on the door and that the door is original to the truck.

As you go higher in rear end ratios (lower numerically), the truck will have less take off/acceleration ability and less ability to pull/haul heavy loads.

If you're going to make a change, the best change would be to swap in an overdrive transmission and keep your current rear end ratio. This would give you the best of both worlds in greatly reduced RPMs at highway speeds but have the gearing if you're pulling a load.

 

As Orich said use larger tires.

 

tag says 09 on it. you think 3 70 to 3 50 would be too much? I have access to two rear ends but don't know what gears they have, taller tires would be much easier, it has 235x70's on it now
should have got 235x75's I guess

 

If the tag is original to the truck, and if the rear end ratio hasn't been changed, "09" would be a 3.70:1 ratio.

 

My '66 has a 240, 3 speed, 3.50 rear, and 235/75/R15's. I can start in 2nd if I have too but it likes starting in first better. If you're set on changing gears, I would go to 3.25. Maybe 3.00 if your planning on rebuilding your motor soon and upgrading it to a 300.

 

I didn't do the tire-math, but it's often wrong from the real world anyway. Basing my gear vs speed on the 235/70 (assuming a 15"?) being a 27" tall tire, your 3.70 puts your engine speed at roughly 2700rpm at 60mph.
Probably a necessity for pushing a big square bodied (poor aerodynamics in other words) truck through the air at over 60 mph.
If you measure your exact tire height from the center to the ground (rolling radius) and double it, that's your rolling diameter and what the speedometer and engine effectively see. Tire/Gear calculators will do the rest.

Not sure about the 240, and sixes tend to rev lower than eights do, but that 2700 is nothing to a 302. My Bronco cruised at near that same rpm all day long on trips (4.11 and 31-ish tires) at 2600-2700 rpm.
Is 2700 high for an engine like yours? Would lowering that down 100 rpm help at all? Seems a lot of work and money to go to a 3.50 and 100 rpm lower. A 3.20-ish would put you at 2400-2500 rpm. I would think that's as low as I'd want to go with the rpm. Still not a lot for your money, but letting it go too low might create other issues.
A 3.00 would net you approx. 2200rpm. Nicer sounding on paper (and to your ears) but is there enough power to pull higher speeds with that engine? Maybe, but you'd have to make that decision.

Do you haul stuff with this truck? Take it on long trips loaded for bear? Not hauling a camper or trailer, right?
If it's always empty and just a cruiser, then the 3.00 might be a good choice. But if you use it as a truck much, to haul the bacon, so to speak, I would not go too tall on the gearing.

But that's me. Guess it comes down to how strong your engine runs and whether it would appreciate the lower rpm range for cruising.

Good luck. Almost as tough a decision as paint colors!

Paul

 

Others have asked what do you use the truck for and how long are you going to keep it. A 31" tire will lower your RPMs to 2400 @ 60, which is reasonable for a stock 240" motor.

 

Once you make up your mine on hwy. gearing then you should also install a vacuum meter to monitor your low vac. in low rpm your lugging the motor depending on how low of gearing to go..
Plus you may have to do some timing adjustment if you have any piston rattle or pinging at a certain speeds.
Just throwing this out that could happen.

Orich