091 Transmission Data
by David Schwarze
"All T4 trannies have the same ratios in 1st, 2nd, and 3rd gears. The differences appear in 4th gear and final drive ratios. From '68 through chassis 213 2068 548 (mid '73) the 4th gear ratio was 0.82. From there on to '79 the 4th gear was 0.89."
I've seen that published too, but my experience didn't bear it out. I have a late '73 bus with chassis number well after the one above, but my 4th gear was .82. Didn't find out till it was too late. :-(
"The final drive ratio changed also, but at a different chassis #. From '68 through the end of the '73 model year (chassis 213 2300 001) the final drive was 5.375."
I think it's till the beginning of the '74 model year.
"Starting in 1974 chassis 214 2000 001) the final drive was changed to 4.867 through the end of the breadloaf series."
Not true - only '74 and '75 had the 4.867. 76-79 has 4.57 final drive (with the .89 4th gear).
"I happen now to own one of those oddball 0.89 4th gear, 5.375 final drive trannies."
I used to think that too. I'd be really suprised if you have one (I think they don't exist). Knowing the final drive, you could put it in gear, mark the shaft and prove me right or wrong by counting the number of turns on the input shaft vs. the output shafts.
"I also own a '78 tranny Tranny with the 0.82 4th and a 4.867 final drive, which allows 70+ cruising speeds, but quickly forces a drop in gears when you enter the mountains."
I think you have .89 4th and 4.57 final drive on that tranny.
"The advice I was seeking, since I am planning a cross country trip this June, is on which way to compromise. Should I seek the higher "flatland" speed across the midwest, or would the lower gearing of the late '73 tranny allow me to maintain speed in the mountains. The powerplant for this assemblage, BTW, is a '78 2L FI engine in a '73 body."
I still say keep the higher geared box. You'll do a lot more flat interstate than you will mountain passes. Here's a clip from a spreadsheet I made to help figure things out. The bastard transmission is a 78 case with the .88 4th gear and an oddball European ring and pinion (5.43/1) which combine for a serious stump puller. I used .88 for the 4th gear ratio in the later trans, but it may be .89. I've seen both in print and I haven't bothered to find out which it really is yet. Is it any wonder I don't trust anything I read about these tranny ratios anymore? The ideal trans is one I'm going to have built from my original '73 box with the .82 4th gear and the 4.86 ring and pinion from the '75 installed. I feel it will be a good compromise between freeway cruising speed and hill pulling in the lower gears.
Ring Final 4th gr 4th gear
and ratio ratio
Pinion Drive 0.88 0.82
----------------------------------------------
79 trans 32/7 4.57 4.02
75 trans 34/7 4.86 4.27
late 73 trans 43/8 5.38 4.41
bastard trans 38/7 5.43!! 4.78
ideal trans 34/7 4.86 3.98
And since that may have been a little confusing,
here's another one that's a bit easier to understand. The first
column is overall ratio in 4th gear, and the second column is how
many RPMs you'll be turning at 60 mph with brand new 215/75R14
tires in back. Note that these tires are rather large. When I had
205/70's on the rear (which are comparable diameter to 195/75/14
btw) the RPMs were about 75-100 higher.
4th rpm@60mph
----------------------------------------------
79 trans 4.02 3326
75 trans 4.27 3534
late 73 trans 4.41 3644
bastard trans 4.78 3950
ideal trans 3.98 3293
Some of you are thinking that 3293 is a bit too low for the motor to be turning on the freeway, and you're probably right. It's okay though, because I'll be going back to smaller rear tires when I get this trans, which should put me at about 3375-3400 @ 60mph, which is exactly where I want to be (I'll normally be going 65ish anyway). One final thing to note is that putting bigger tires on the back of your bus isn't going to mess up the speedometer, because the speedometer is driven off of the front wheels.
The 091 trannies started in 76 in the US. At this time my best guess is that sometime in 74 they changed from 5.375 to 4.86, still in the 002 case. In 76 they started with the 4.57 and the 091 case. The 75 case was one year only, 002 number but with 091 style ribs on it.
This is one area where the Bentley appears to be a poor source of information. You really can't be sure what you've got until you open it up and look. You can just make an educated guess.
