Hey guys I have a 1956 mercury 312 eng didn't think my tranny was going in to high gear hears what I got used tack and gps for speed at 55! I was at 3000 rpm .At lower speed was watching tack no fluctuation in rpms thinking its in second ? Need help .

If you can give us the actual tire diameter and the rear axle ratio, we can calculate what rpm should be at a given speed.

John - "The Hoosier Hurricane"

If you are really going 55 MPH and your tack is reading right you are not in second gear. Aside from that it is hard to figure out what you are asking. Can you spend more time explaining your issue. Pete

I believe you are correct that 3000 RPM is too high for 55 MPH, but it would be helpful to know your exact tire size and axle ratio. I have made some assumptions based on the 56 Ford axle ratios in my Fordomatic manual and based on an approximate tire size of a 6.70 X 15 tire (the 56 Fords still had 15 inch tires).

At 55 MPH, you are traveling 4840 feet per minute. If your 6.70 X 15 tire has a 27.50 diameter, your wheel travels 7.199 feet per revolution. If you divide 4840 by 7.199, you get 672.3 RPM for the tire. If your axle ratio is 3.22:1, the driveshaft (and engine in high gear) would be turning at 2164 RPM. One of the tables that I have shows that second gear has a ratio of 1.47:1, so the RPM at 55 MPH in second gear would be 3182 RPM, so that is fairly close to what you measured. Of course, these calculations are approximate, but I believe that 3000 is too high for the engine to be in high gear (3rd gear) in the Fordomatic.

When you are driving the car, you should be able to tell when the transmission shifts from 2nd to 3rd, even if it is at a fairly low speed. Sometimes it is hard to tell when the shift occurs, but it should be noticeable and if the shift never happens, you are probably staying in second gear at road speed.

Hope this helps. Good luck with your diagnosis.

Dave

Will get tire size tommrow. I also adjusted the linkage as the manual said made no difference in the rpms still 3000 .good down shift if you put it in low gear also good up shift when
you put it in drive just not a third gear up shift I don't be leave.used my gps for accurate speed.
Some one suggested pull the govener ? Car is not a overdrive tranny.Running out of ideas lol
Thanks
Bob

Pete what gear would I be in?

I hate to belabor the obvious but is the rear end original to the car? I was thinking of someone somewhere having installed one of the 4.10 (or thereabout) overdrive gearsets??

As far as I no its the original rear end .
The tires are 215-75-15
Manual says the gears are for the car 3.15 or optional 3.54
The rest of the car is original as far as I can see.

Dave you are on with the rpms that's about what I am at.
Bob

Lawrenceville, GA