gear ratio difference on 28/29s vs 30/31s?

[eagle]

I have a question.... is there a different gear ratio in the 30/31 transmissions or rear end to accomodate the smaller wheels versus the 28/29 larger wheels? Seems to me it would make some difference. Some run 16" wheels on a stock A, that would be even worse.

[Jim Parker Toronto]

No, other than the early rear ends, the standard was a 3.78 to 1

[Jim/GA]

If the tire sidewall height is taller, it can offset the rim diameter being smaller and the overall dire diamter can be more or less the same.

Very early cars had a slightly different rear axle ratio from the rest of the run, but other than that, the rear axle ratio was the same across all the years (unless someone wanted different gears for mountains, etc.)

[jkeesey]

My boss has a very early 28 roadster pickup, it still has some ar parts would that be a different gear ratio?

[Benson]

Originally Posted by jkeesey

My boss has a very early 28 roadster pickup, it still has some ar parts would that be a different gear ratio?

Early ratio was 3.70. Drive shaft and pinion gear design was changed early on to make it stronger as breakage of early driveshaft was a problem. Drive shaft keyway hole would break and pinion would get loose on the driveshaft.

Later on the ratio was changed to 3.78.

Not sure if this redesign was before the change to 3.78 or at the sametime as change to 3.78.


NOTE: Service bulletins page 327 says change from 3.70 to 3.78 was March, 1929.

[eagle]

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Has anyone ever figured out the engine RPM difference running different size tires at a set speed? It would be easy enough to calculate... maybe I'll check it and post the difference. I have a '34 wheel with a 16" tire on it so I'll check that as well. Obviously it would make some difference with different brand and style tires but would be an interesting comparison anyway.

[Ned]

Some calculations of engine RPM vs tire diameter, rear end ratio and speed. Includes asection for a 26% OD. Hope it this works. I have the Excel file for this.

Engine RPM for Various Differential and Overdrive RatiosTire SizeSectionRim DiaTire
DiaTire
Rev/MileDiff RatioEngine
Rev/MileHigh Gear - w/o ODHigh Gear - with 26% OD30 mph40 mph50 mph60 mph70 mph30 mph40 mph50 mph60 mph70 mph4.50 x 214.5021306723.2721971099146518312197256481310841355162618974.50 x 214.5021306723.5423791189158619822379277588011741467176020544.50 x 214.5021306723.7825401270169321172540296494012531566188021934.50 x 214.5021306724.11276213811841230227623222102213631703204423845.00 x 195.0019296953.2722731137151518942273265284111211402168219635.00 x 195.0019296953.5424611230164120512461287191112141518182121255.00 x 195.0019296953.7826281314175221902628306697212961620194522695.00 x 195.0019296954.11285714291905238128573333105714101762211424676.00 x 166.0016287203.2723541177157019622354274787111621452174220336.00 x 166.0016287203.5425491274169921242549297494312571572188622006.00 x 166.0016287203.78272213611814226827223175100713431678201423506.00 x 166.0016287204.1129591480197324662959345210951460182521902555RPM = MPH x Ratio x 336 / Dia Ratio - Diff ratioMPH = RPM x Dia / (Ratio x 336)Dia - Diameter of tireThe measure height from the axel to the ground is slightly less than the dia.

[Ned]

Looks like the RPM chart didn't work. I'll try another method.

PM if you want the Excel file.

[Ned]

Engine RPM for a given tire size can be calculated by this -

RPM = MPH x Diff Ratio X 336 / Tire Dia.

Use 29.75" for a 21" tire and 29 for a 19"