Overdrive for a T-18?
 

Is/was there a bolt-on overdrive unit for the T-18 Ford tranny?

Mine is in my '66 F250, and it occured to me that waaaay back in the 60's the US highway system speed limit was 70, so there was probably a market even back then for an effective overdrive system on these trucks.

Any body run across one?

check out advance adapters they have over/underdrive units that are totaly seperate from your gear box they go between the trans and the bellhousing like an auxilery trans the have there own input shaft that gos threw the clutch the the trans input gos into the od unit only 2 draw backs to this style and thats an extra shifter coming threw the floor and youd have to shorten the driveline

But were most of these trucks orginally intended to travel at 70 mph? For the most part my dad and my granddads used their trucks around the farm or to take an occasional load of crap to the dump, not for highway travel.

naw back in the day 45 mabe 50 was the intendede speed for these old trucks after all how could a 240 pull a heavy load hmm a set of realy low gears did the trick witch meant a slower top end

You can always find a T89N and bolt it in. That is a factory Ford overdrive transmission that was optional on the 66 model. Very good trannys. I love mine. I think they are getting a little rare but they are still around. You can rig them up to work as a six speed.

Hey, careful about 240 engines. My dad drove a 200 mile round trip at 70 mph once during harvest (yep, farmer) to get parts for his combine. 240 coupled to a t-18 and 3.50 rear. I figure that engine was doing about 3500 rpm. No trouble in 1972. Engine still running in 2005.

Clearly, t-18 was intended to pull and be a truck tranny.

Close ratio t-19 is the same tranny, but has a higher ratio in the low gear. Couple that with a higher rear, and you have a "3 speed with overdrive".

There is no magic to overdrives. It is all the distribution of the gears and the rear end that is coupled to them.

The 60's was an era when engines could far out-perform the suspensions and braking systems they were mated to. Even though these were trucks in the true sense of the word, the speed limit was 70, they would spin at 3 grand or better all day, and even with a 4.10 rear end, that was pushing 70.

Gregtruck: The T89n sounds exactly like what I'm looking for. Was yours stock in your truck, or did you swap it in?

Gregtruck:

I'm coming up with a big zero in searching for a T89N tranny. Are you sure that's the model number?

Look for a "T-85" tranny with an R-11 overdrive unit affixed to the back of it. This tranny was available in Ford PUs from early 60's thru the '72 model year behind the "FE" big block engines. The shifter cover may say T-89 but the OD trannys were really T-85s.

On second thought, if you are using your F250 as a "real" truck, the 3 speed w/OD may not be ideal for you as there is no "granny" gear with which to start a heavy load without some slipping of the ol' clutch.

I have references that I have saved refering to both a T85N and a T89N. I'm not sure which is correct. Both files I have saved seem to be refering to the same transmission or at least some similar variation. There is supposed to be an R-11 casting on the extention housing.

I feel like Fordknut is right about T85 being the correct number. However they came with 6cyl engines as well as FE's. I own one so there is no doubt about it. Warranty tag says it all....

No WAY-

I just looked at the exact same transmission reference that I looked at yesterday, and there it was: T85N, 3 fwd gears, electronic OD unit on 3rd gear.

I couldn't get the gear ratios, but is this the same basic 3-sp that is used on the 3-on-the-tree trannies?

Realistically, I need my truck for "real" truck duties, and the 3-sp isn't for me. Seems odd that there wasn't an option for the 4-speeds, too. Work trucks sometimes go down the highway empty, and it would bbe nice to drop the R's to 2500 or so at highway speeds.

I'm afraid your only option may be to install an "add-on" overdrive unit. The type "Wizzard" speaks of is probably the best for heavy duty applications but there are several units built to "stand alone" between the transmission output and the differential input. Some are advertised for use in large RVs so they should work in your situation. The big drawback for most people: quite "pricey"!!

The bottom gear in a t-18 is mostly worthless, unless you pull really big stuff and regularly. Thus, the T-18 is in essence a transmission with an "underdrive". So, its a 3 speed, with a crawler gear.

You want a 4 speed, with the gearing distributed such that you can use all four. You need a close ratio t-19.

Much cheaper than an overdrive unit. May require swapping out the rear (9" Ford I assume) to a different ratio as well. But all can be done for far far less than the prices for those aftermarket OD units.

I have not yet followed the advice I just gave. Plan is to slip the clutch and live with high RPM until the existing tranny and clutch are history and then convert to a t-19. For now the old t-18 just refuses to die, so I just live with it.

Wellllllllll......

The T-19 comes with the SAME ratios as the T-18, it's benefit is the synchro'd 1st gear. Both trannies come with the same (3) different forward gear ratios, identified by either a 4.02, 5.11, or 6.32 low ratio. Curiously, the reverse ratio for the T-18 is DIFFERENT in each ratio-range.

So a close-ratio (4.02 1st) T-19 WOULD be a little more street-able, 4.02 is still too low to use as a normal gear. You'd end up using 2nd thru 4th.

AND, here's the crux of the biscuit: both trannies have a a 1:1 final ratio, meaning the ONLY benefit of the T-19 is the synchro 1st.

In my case, I need the granny daily to crawl up the hill from the shed in the back yard where my wife makes me park it. And for some reason, maybe it's the broad torque range of the 300 six, but I don't feel the need for any more than those 3 fwd gears.

So you mentioned the best solution, change the rear end ratio.

BTW, I think you will be old and gray before the T-18 wears out.