Hi guys, writing from New Zealand and regular listener to the show!
Q: Am considering replacing the old three speed gear box for a four speed - have received conflicting advice on doing this!
1. the vehicle was designed to run the three speed so don’t mess with the setup!
2. the three speed is an ancient (although reliable) configuration but by changing to a four speed you will achieve better fuel economy (i drive it every second day to work) and a generally smoother ride at less revs
Both pieces of advice are wrong. The vehicle was designed to run with either a 3 or 4 speed. How ever, the top gear in both transmissions that are designed for it are 1:1 so cruising rpm will be the same.
Generally the 4 speed will have a lower first gear than the 3 speed. Some truck 4 speeds have a very low first gear, commonly referred to as a “granny” gear. It is for starting up a steep hill with a full load, otherwise you just start in second and treat it like a 3 speed.
Now you can take a car type 4 speed, where the gears are a little closer together, but first is still lower than the 3 speed tranny and add a higher gear ratio in the differential. You don’t mention which engine you now have, but lets say you have the 240 cu in inline six, you probably have 4:11 gears in the rear. With a 4 speed, you might go to a 3.55 or even a 3.23:1. If the four speed’s first gear is low enough, you might even get by with a 2.92:1 gear set from a 66 Galaxy with that engine.
You could just change the differential gears without changing the transmission, but if you regularly haul a load and drive in a hilly or mountainous area, the clutch probably won’t last and you will have almost no power.
The Ford F-100 models in the U.S. did offer the Borg Warner overdrive through 1969. This was an electrically controlled planetary gear set that was in a unit behind the transmission. My first wife father had an F-100 with this transmission. This would reduce the rpm of the engine at highway speeds.
However, you might have to change the rear axle ratio to a lower gearing for the truck to pull well while in overdrive. As I remember, the overdrive truck came with a higher ratio (lower geared) differential than the turcks equipped with the 3 speed that did not have the overdrive. However, even with the higher differential gear ratio, the overdrive did reduce the rpms of the engine. As another post noted, the 4th speed is the same direct drive in the 4 speed transmission as the third speed is in the three speed.
The Borg Warner overdrive would give you 5 forward speeds: 1) low; 2) second; 3) overdrive second; 4) high; 5) overdrive high. In overdrive high, the output shaft is turning faster than the engine.
Hey Keith,
thanks for the sage advice, suspected more to the process.
Actually running the 390 big block - regular day travel with the occasional junk load to the tip (or helping move stuff for friends) Could you suggest a configuration for the four speed?
thanks again!
Cheers for this, i’ll investigate the Borg Warner overdrive option, see if still avail anywhere!
thanks for the time
The overdrive mentioned below (I had forgotten about that) would be good, one thing though, the ones I remember would free wheel when coasting. This is super good for gas mileage, but if you are going down hill with a load, you’d better disengage it.
Lets see, a 66 F100 with a 390, Borg Warner T10 and 3.55:1 limited slip, HOT. Four barrel, headers and a good cam, WAY HOT.
Edit: Back to the real world, rear gear selection is dependent on rear tire diameter as well. You would need lower gears for larger diameter tires. If you have 15" rims, then you would go to a higher gear (lower numerically). If you have the 16.5" rims, then lower gears would be called for. If you are planning on large diameter custom wheels, like 20" or larger, you will need to be careful about changing the rear gears or putting on an overdrive.
Legend, great tips n advice!
Moving forward with confidence…
The Four barrel, headers and good cam may be a little way off just yet (dreams are free right)
thanks again
It was just barely available in 1966 and never with 390 engines which were not used in P/U’s until much later than 1966…The Big Block truck was a 352…No matter WHAT you do to that truck, it’s not going to improve gas mileage much other than leaving it parked…The BW overdrive was a rare option in 6 cylinder and small V8’s…It won’t mate to your transmission…
I have seen and ridden in a 1968 F-100 with a 360 V-8 that came from the factory with the Borg-Warner overdrive. My first wife’s father had a 1969 F-100 shortbed with the 302 V-8 and had the Borg-Warner overdrive. That truck ran very quietly at highway speeds with the overdrive engaged.
Heh heh, think you might be right on the gas mileage thing! However, it’s a supreme drive just be great to bring the RPMs down a tad on the open road.
NB: yep funny thing on the big block - the truck had to undergo pretty stringent inspection through customs and then registration into New Zealand and we listed it as a 352 only to be ‘informed’ that it was ‘actually’ a 390 and the numbers all check out with the original rego paperwork! Nuts huh.
thanks for the feedback