MTFCA Forum

I guess my brain is in a cloud this morning. Im trying to determine or confirm the approx ratios of my TT Auxiliary transmission and then approx mph per/1000 engine rpm based on rolling radius of my tires. One rear tire is jacked up with a flag attached so I can see one revolution while hand cranking with the spark plugs out. I am making the assumption that I have the slow TT 7:1 final.

With the aux in “straight thru” and the Ford transmission in low it takes 7 engine turns to get 1 tire turn.
With the aux in “straight thru” and the Ford transmission in high it takes approx 2 3/4 cranks to get one tire turn.
Low is achieved with a board cut to length holding the low pedal tightly forward.
High of course with the brake lever forward and no pedals.

My results seem perfectly backwards…..why? What obvious factor am I missing?

If you have only 1 wheel jacked up, or if one wheel is dragging, you're seeing the ratio within the differential at play.

Given that, I'd say that Ford Hi & straight through is a 5:1 ratio. This is based on the assumption that with 1 wheel stopped, the free wheel is turning at double speed.

A std. TT axle is 7:1 or close to it, so with the Ford trans in high (1:1) and the aux in straight through (1:1), it should take close to 7 turns of the engine to make one turn of the rear wheel.

Ford low is about 3:1, so with the aux still in straight through, it should take about 21 turns of the engine to make one turn at the rear wheel.

Other combinations would depend on the ratios of the aux in gears other than 1:1. Are you sure you have the aux in 1:1, or perhaps in some other gear, like an OD gear?

Pat, 7:1 would take 7 turns of the engine to turn BOTH wheels 1 rotation. Mike has 1 wheel on the ground, if I understand correctly. A 7:1 would take 3 1/2 turns of the engine for 1 wheel rotation.

After reading Jerrys post eluding to a high speed rear and to eliminate one of the variables, I just pulled the axle housing cover, thrust washer asby and roller bearing. SURPRISE (6) teeth on the worm! I have a high speed rear end! YeeHaw.

So factoring the need for two turns of the wheel I can just multiply cranks 2x.

2 3/4 turns(2x) in high is close enough to 5:1
7 1/4 turns in low(2x) is 14 1/2. In Ford low at 1:3 thats close enough for me. Something around 14 1/2 / 3 is darn close to 5:1 again.

I appreciate the help in figuring this out. I’m happy that the math makes sense now and am thrilled that I have the high speed rear.

I may even sleep tonite!

This chart is where I was headed with this whole ratio exercise. I just wanted to know the expected range of MPH in my Twin-High TT. I believe the math is correct because the proportions coincide perfectly with the advertising literature stating 33% overdrive and 66% underdrive. For those with high speed TT rears, do my straight Ford numbers look ballpark? Its looking like my intended/hopeful 20-25mph on Forest Service roads is quite realistic.

Looks pretty much 'in the ball park' to me.

On the road you will find certain gear combinations are 'good' and some you will avoid. Not sure why, but I find some work and some don't depending on the road and I guess the 'fettle' of the engine.

It is fun sometimes to work up the ratios like a semi, my 8 forward options (Ruckstell and axillary over drive combinations) sound cool but requires a lot of lever (x2) and pedal gymnastics

You gave me a chuckle Adrian. I picture you making engine noises in your head as you upshift.

OOh it does sound pretty neat