tire size

[chrisalma]

is a 18.4.34 about the same diameter as a 16.9.38

 

[Fordfarmer]

I don't have my chart handy to tell you the exact difference, but the 16.9x38 is somewhat taller.

 

[Bob]

A chart is linked below.

Pick your style of tire, then click for full details.

(In the chart I looked at, the "38" looks to be about 1-1/2" larger in overall diameter.)
Firestone

 

[zap]

18.4-34=284sq in. 16.9-38=252sq.in.

 

[INCase]

Thanks Bob, A VERY handy chart inside that link.