overdrive

gareth · #1 03-05-10, 21:44

Is there an overdrive available to fit on the back of my F15 4 speed?
what about different diffs?
I am looking to cruise at about 40 mph, sounds like i'm about to blow the engine at 35 at the moment, Gareth.

Bob Carriere · #2 03-05-10, 22:04

Hi Gareth

A quick search of the web should produce suppliers of generic aftermarket overdive systems....but most will be USA located.... and they are expansive.

Remember that overdrive means overdriving other components as well..... brakes must be top notch, gears/bearings need to be in good shape and will make more noise, steering components....tierods...kingpins.... radiator capacity.....etc.....

I would suggest you install a temporary tachometer and record RPM at various speeds and see if the engine is really screaming at 40mph.... you might still be within range albeit at the top end.......

Consider the expanse of the conversion versus the times that you will actually be cruising at 40 MPH and see if it is worth it.

Rear axle gear ration probably can be changed but the savings in rpm may not be as great as an overdrive system which usually saves about 25 to 33%.
Find alternate gearing that will fit the existing axle casing may be difficult in the UK....... somewhat easier in the USA/Canada with more old vehicles around......... I would try to search Ford shop manual-parts manuals to see what gear ratio was available in the 1937-39 eara for the civilian market...... 1 1/2 to 3 ton range......

The other choice is to enjoy the scenery while driving slower.....

Keep us posted of your discoveries......

Bob C.

cletrac (RIP) · #3 03-05-10, 22:17

Is your F15 a 4x2 or 4x4? On my C15 4x2 the rear end is the same as a 1 1/2 ton civvy truck and they made one around 4.25 to 1 but I haven't found one yet. I'm sure the same would hold true for Fords. On my C8 I got a 4.11 gear set from a civvy pickup.

Keith Webb · #4 03-05-10, 22:33

Quote:

Originally Posted by gareth

Is there an overdrive available to fit on the back of my F15 4 speed?
what about different diffs?
I am looking to cruise at about 40 mph, sounds like i'm about to blow the engine at 35 at the moment, Gareth.

The White scout car diff centre fits and will gear it up comfortably but 40 MPH should be easy in a F15 without blowing anything up. The fan does make a lot of noise though. Having said that, most of the 4X4 CMPs are happier slower.

You should ask Euan his cruising speed in the CGT with the 292 Chev engine, and I recall a friend being amazed when he was passed by a cab 12 Ford doing over 60 MPH, although he did find out it had a Windsor. The higher speeds tend to flog the transfer cases out.

japes · #5 03-05-10, 23:18

Haven't done the math but what about the easier alternative of fitting 20 in. wheels? They fill the wheel wells better also (in my eyes at least).

J.P.

gareth · #6 04-05-10, 05:54

Many thank's for your input guys, being a 4x2 I thought it would be easy just to bolt on a overdrive unit and make a new prop does anybody know of such a unit, maker, website link?
Rear axle swap?
I have thought about fitting 20" rims so anybody in the uk have a set for sale?
I am not looking for higher speed just to give the engine and my nerves an easier time, Gareth.

Ganmain Tony · #7 04-05-10, 12:59

Gareth,

I know this is not the answer you're looking for, however....

Your F15 2wd should easily do 40mph as it is. The gearbox does have an unsettling note, but you'll get used to it. (at any rate they only get really noisey above 55 mph).

I'm posting this to give you some positive reinforcement about the ability of your machine as it is in standard form & to maybe, prevent you making a costly & unnecessary modification.

If you really really want to do something - go ahead with 20's but dont make the tyre diameter too big (as I have done) or you will never get up a serious hill without electric fuel pumps etc. 10.00X20's should be fine.

You've got a great truck.......enjoy