For a very precise answer, there is actually a paper published in the American Society of Agricultural and Biological Engineers journal ('Water Absorbtion Characteristics of Wheat and Barley During Soaking' Transactions of the ASABE. Vol. 46(2): 361–366 . (doi: 10.13031/2013.12916) @2003) The abstract reads:
Water absorption characteristics of wheat and barley during soaking
were measured at five temperatures ranging from 10.C to 50.C. From the
water absorption characteristic curves, it was determined that water
absorption of wheat and barley was in the second falling rate period.
Using the measured data, a non–linear least squares method was applied
to an approximate solution of the diffusion equation MR = B1 exp(–Kt).
Values of parameter B1 for the diffusion model were estimated to be
about 0.6 for wheat and about 0.8 for barley. Therefore, the measured
data were fitted to the exact solution for the sphere diffusion model
for wheat and for the infinite plane sheet diffusion model for barley
by a non–linear least squares method. The measured results agreed well
with the calculated results. The values determined for the diffusion
coefficients were 1.1 . 10–12 to 1.0 . 10–11 (m2/s) for wheat and 3.5
. 10–12 to 3.9 . 10–11 (m2/s) for barley. An Arrhenius–type equation
was used to relate the diffusion coefficient of wheat and barley to
temperature (T), and the energy of activation (E) for wheat and barley
was estimated. The values determined were 44.0 kJ/mol for wheat and
45.9 kJ/mol for barley.
A hopefully more useful answer for those above who don't understand what the above really all means is that wheat (and rye for that matter) don't have the same fibrous husks that Barley have, and therefore absorb less water. I've heard numbers (unsourced, from memory) of about 0.07 gallons / lb of wheat or rye malt.