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This got me interested and was going to work out the relationship between engine speed and vehicle speed but of course there is a website calculator for that.
Combined with your tire size, you can check your what your final transmission gear ratio must be for the combination of rpm and speedometer reading.
For example, for 235/75R15 tires, a final rear-end ratio of 3.73:1, final transmission ratio of 0.7:1, an engine speed of 2000 rpm corresponds to about 66 mph.
I respectfully disagree with Hanr3. When I am going freeway speed ~ 70 mph my Blazer is cranking out about 2400 rpm's and gets 16 mpg. The other s10's I have driven with overdrive as indicated by the D with andO around it hit between 1500 and below 2000 rpm's at the same speed. I have an automatic WITHOUT overdrive.
The specs on a 2001 Blazer state mileage at 16 city/23 highway - I get 11 city/16 highway. Don't try to convince me I have overdrive. (Not trying to be sarcastic.)
Yes, I know that 1st gear and first and so on, I don't require remediation in that area.
Regarding mileage, I would only expect 16 mpg at 70 mph on a good day. These are not very aerodynamic vehicles with mileage for me dropping once I get much above 60/65 mph.
Also, I have always attributed the 23 mpg on highway for the 2wd, 3.08:1 rear-end ratio Blazers.
You answered what no one else came up with and one that I suspected. Apparently I have a GU6 3.24 from the RPO code. Certainly explains the rpm's. But, why change the rear differential? You are not going to do serious 4 wheeling in 4th gear anyway. Leave it for gas mileage.
If that is the RPO sticker from your truck, you have GT4 - 3.73 gears. 3.73 gears are typically used for towing applications. G80 just means that the rear axle is equipped with the gov-lock differential (an automatic locking differential).
My Blazer has the GT4 ratio and G80 locking rear end. I'm glad I have it even if it means not so great mileage as I have it for towing and hunting/fishing.
Yep, that's the differential I have. Explains the whole mess.
However, this is what I got from the online calculator. Not exactly positive on the transmission ratio and the speed calculator does not match the RPM's I experience.
Yep, that's the differential I have. Explains the whole mess.
However, this is what I got from the online calculator. Not exactly positive on the transmission ratio and the speed calculator does not match the RPM's I experience.
I see your point. Those numbers are not what one would expect given the raw data available.
What size tires are you running? If not stock, the speed reported by the speedometer will be off.
For instance, with my 30" tires my speedometer under-reports my speed by about 3%.
Have you checked your speedometer against a GPS or could the tachometer be off?
I have an OBD-II monitor by Ultra-gauge that I use to monitor the transmission temperature. I've never bothered to look but I'm sure it and other similar monitors can independently check engine speed.
Engine RPM vs. Speedometer speed calculation
