68 F100 Top speed issue

[Fordkidd]

Hey I got a 67 f100 longbox 240 six motor and a three on the tree , now my question is whats causing my top speed to be around fifty and thats screaming the motor out loud enough to where I can even hear my cell phone go off . my gears to speed go like this , 0-10 in first 10-18 in second and 18-50 screaming in third , is my rear end ratio the cause of this ? what could I do to remedy this fairly quickly because I daily drive my truck 48 miles to work and then to jobsites all day and its really affecting my gas mileage im getting roughly 8 miles to the gallon and burning a tank everyday and a half just getting to and from work , any help or observations would be greatly appreciated

[basketcase0302]

Fordkidd on Mon Sep 24, 2012 11:30 pm
Hey I got a 67 f100 longbox 240 six motor and a three on the tree , now my question is whats causing my top speed to be around fifty and thats screaming the motor out loud enough to where I can even hear my cell phone go off . my gears to speed go like this , 0-10 in first 10-18 in second and 18-50 screaming in third , is my rear end ratio the cause of this ? what could I do to remedy this fairly quickly because I daily drive my truck 48 miles to work and then to jobsites all day and its really affecting my gas mileage im getting roughly 8 miles to the gallon and burning a tank everyday and a half just getting to and from work , any help or observations would be greatly appreciated

Yes, it sounds like it's the low gears in your rear end. Here's the axle ID tutorial:
http://www.fordification.com/tech/rearends_ford01.htm
There's also an "old school" way to find your ratio, (or real...close to it) by jacking the rear / putting the tranny in neutral / spin one wheel on full revolution / count the revolutions of the driveshaft. Four complete wheel revolution(s) + ten driveshaft revolutions = 4.10 gears (or close to it). You could change the ring and pinion for less than $200 but by the time you buy the new bearings and seals and gear oil tour now into the axle around $300-$400.
I'd look for a higher geared rear axle to bolt under the truck if it were mine. That way you can always put your old axle back under the truck if your not happy with the new ratio. Keep in mind you will lose some low end pulling power when going to a higher ratio rear axle.

[Fordkidd]

I use the truck to carry at most 500 pounds of tools and concrete samples for work so pulling power I dont really need , what I need is to be able to get up to 60-70 without screaming the motor and get some decent gas mileage out of the truck because right now Im getting like 8 mpg and I know I should be getting alot better than that I mean its a straight six and a smaller one at that, my dads 460 carb f250 gets 11 mpg and were talking almost twice the displacement

[elgemcdlf]

I use the truck to carry at most 500 pounds of tools and concrete samples for work so pulling power I dont really need , what I need is to be able to get up to 60-70 without screaming the motor and get some decent gas mileage out of the truck because right now Im getting like 8 mpg and I know I should be getting alot better than that I mean its a straight six and a smaller one at that, my dads 460 carb f250 gets 11 mpg and were talking almost twice the displacement

Small engine + heavy truck = poor mileage

[Fordkidd]

yeah I suppose your right but my trip to work the truck is empty and im barely into the throttle at all I mean like my big toe is about all the force i need to stay constant at speed I just dont get it , what could I do to get better mpgs out of the truck would a tranny swap be in order to remedy this ? or should I just swap the rear ends out and what rear end could I use for it ,

[elgemcdlf]

Personally I would check into a 5 spd swap. The 240 is the same block as a 300 so a later model 5 spd from a truck with a 4.9 should work fine. You may find yourself swapping in a hydraulic clutch though.

[rdaniel911]

Had a similar issue with the 68, my problem was the warmer flap in the exhaust under the carb. There is a spring loaded flap that when the motor is a idle it partially blocks the exhaust manifold to keep the warmth under the carb, when you give it throttle or it warms up, it should fully open out of the way. The one on the 68 240 has froze up in the closed position, wouldn't open, thereby choking the engine, no power, poor performance. I took some PB Blaster, and a hammer carefully applied to the counter weight, with caution and freed it up. What a difference, ran much better, and more power.

[ultraranger]

Keep the rear gears, lose the (non-overdriven) transmission. An overdrive transmission will take care of the high RPMs at highway speed and the lower rear end gears will still allow you to start off/accelerate with no problem.


http://ultrastang.com/info3.php

[Red Mercury]

I have two 1967 F100's with 240 engines and 3 speed transmissions. The trucks are completely original. They both came factory with 3.70 gears and they rev high at highway speeds. Gas mileage is awful, always has been (My father bought one of the trucks new in 67). We often expect that the small 240 6 cylinder should be good on gas but they simple fact is that they are not. I have experimented with swapping gears in one of them by putting in a 3.00 set and also a 3.25 set. The truck is driveable, but there is a significant power loss with the taller gears. Because of the taller gears, the little engine revs lower, but seems to work harder so the gas mileage increase is really small. The truck really loses passing power with the 3.00 gears. Ford put the low speed 3.70 gears in those trucks for a reason. I think that the 5 speed O/D transmission swap is a good idea so you get the benefits of a low speed rear axle but the O/D from the transmission.

[TNIceWolf]

Welcome to the reality of 40 plus year old Ford trucks. They were built when fuel was cheap....plentiful and produced to a much higher octane rating in some cases. The point being that fuel mileage wasnt an issue. And squeezing MPG out of one isnt always cheap or easy.