Originally Posted By: azsynthetic
You are comparing apples and oranges. Of course there is a difference between 75w85 and 75w110. Within the same weight (i.e. 75w90) there is no significant difference (if any) in fuel economy at 40C. Lubrizol test is for corner cases and not for the other 99% of the general public.
First off, what's a "corner case?" I may respond again to how you define it because what lubrizol is doing has already translated to the "real world" in the form of oils like the 75W85 that Indy mentioned. More is on the horizon.
Anyway, if you are talking comparing 40C (100F) specs, the variations in fuel economy among a group of 75W90 oils would indeed be insignificant. But that's not what we started the debate about. Here is what you said:
Quote:
I doubt and willing to bet that you will not see a difference in fuel economy with the variation in 40C viscosities. There are too many variables that can effect the fuel economy of the short hoppers for the gear oil viscosity to make a difference at 40C. No one is blending gear oil specifically for 40C usage and most of the time your differential will be over 40C.
I contend that in a cold climate (not where you are if that's AZ, at least very often) the 40C number is important. And I also disagree on your contention that diff oil is mostly going to be over 40C (100F) in cold weather. Here's why and how:
For the better part of a decade, I have been running diff oil temp gauges in three vehicles (all light trucks, two half-tons and on 3/4-ton) and monitoring them closely in a variety of tests for magazine stories. I also have a few industry tests in my files that list common diff oil operating temps (and mine are typical).
I could give you chapter and verse on how speed, gear ratio, hypoid offset, ring gear diameter, etc, effects diff oil temp, but suffice it to say that on a cold day (let's say 35-40F) it takes a good 25 miles of continuous running at 55-60 mph to get the diff temp up to 120 degrees. I noted that yet again just yesterday on a short trip in my F150. It takes a good 40-50 continuous miles to stabilize it at a maximum of about 140-150 degrees at around 35F ambient at that speed. In that long warmup period, drag is increased dramatically. Some years back, I ran some coast-down tests and the differences between 100F and 190F gear oil were significant... e.g. more drag.
Now the mpg "hit" is going to be variable according to the actual vehicle and situation, and a diff with a smaller capacity will warm up more quickly than one with a larger capacity, but if MPG is the goal, and you are a short hopper in cold weather (as many are) then the 40C spec could be important. That's one reason why a 75W90 would be a better choice than a 85W90 for a cold climate.
I'm going on about this for future generations who will read this more than just to convince you because I don't want this thread left with your dismissal of the importance of the 40C spec. My mind is open is you have the means and data to convince me otherwise.
