Well, made it back from the Great White North - was up in SoDak to spend
Xmas on the farm, now I'm back at the good ol' computer in Abilene, TX. :)
My mileage wasn't so great this time around. I thought it would be better
since I put taller tires on (235/75/15) vs. the shorter tires I had on
before (225/60/15). Only averaged 19 going up (had a light tailwind) and
17 coming back (bucking a 15-20mph wind). In both cases, I was averaging
67mph, which means I was actually running close to 80mph most of the time,
and the little towns and stop signs pulled my average speed down to 67mph
(based on time between fill-ups). I took the backroads (no interstate), so
had to slow down for little towns. Also, was hauling a coupla hundred extra
pounds on the return trip.
I believe the poorer mileage was due, in part, to the larger-diameter tires
I put on the truck. You would think the opposite would be true, since
larger-diameter would give higher effective gearing. However, the extra
inch-and-a-half of clearance under the truck lets a lot more air under the
chassis, substantially hurting the aerodynamics. You could actually hear
the air rushing under the truck after I put the taller tires on (same
driving conditions with no wind). The truck also felt less stable in
crosswinds, but it did feel like a TRUCK with the tall tires on. My poor
mpg could also be due in part to the high mileage on the truck/engine
(146,700k), but the truck still has good power and only burns a quart every
5-6000k.
I guess the moral of this story is, if you want higher MPG, go with
low-numerical gearing in the rear-end (3:21) and small-diameter tires on the
truck, as opposed to 3:9's with large-diameter tires. Assuming overall
ratio is the same, the former will get better MPG due simply to better
aerodynamics. I've heard from different sources that the worst aerodynamics
on a truck are underneath the chassis. Blocking airflow under the truck
will improve aerodynamics more than a tonneau cover, etc. Problem with air
dams on the front is that you're increasing overall frontal area of the
truck (bad for Cd.) Problem with small-diameter tires is that some people
don't like the look (it does look a little ricey :( ). Advantage is that
you can lower the truck without altering the suspension, and it DOES improve
handling quite a bit - trust me on this!
Also, you need to correct your speedo/odo for different overall gearing. I
use a bicycle computer, which allows me any size wheel/tire I want with a
speedo/odo that's more accurate than the factory unit ever was...
Sorry for the ramblings......
-- '92 Dak CC 2wd 318 3.55 '84 GoldWing Interstate
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 11:46:24 EDT