For those who have switched from 3.73 to 4.10, same tires before and after. what was the difference in mpg?

I have 4.10 and 32" tires. at 75 mph i am running just under 3000 rpms, I am getting very poor gas mileage at that speed.

I have been thinking about going to 3.73s (would 3.55 be overkill?) for better gas mileage.

Is re-gearing going to save enough money in gas to justify itself?

The reason I ask is I am going to be doing a lot of commuting soon.

 

Hmm..you must have a stick. My Auto @ 75MPH is right @ 2500RPM.

Rick

 

same with me but I'm more at 2400 or so.....

I also have an auto, 4.0, 4.10 gears, stock tires

 

Did you CHANGE your gears to 4.10 from a factory 3.73 ratio? Or have you always had 4.10's?

The reason I ask is this: when you went to 10% larger tires than stock (approx), your VSS now turns less for the same distance, making your miles traveled seem 10% lower than it is.

If you were factory 4.10, and went to 32's, you should be mutiplying your miles traveled by 1.1 before calculating your miles per gallon. OR, you can just multiply your miles per gallon by 1.1 after you calculate it.

I would point out also, that a factory 4.1 with 32 inch tires is actually traveling at 82 to 83 mph when the speedo reads 75.

 

I bought the truck used with 14k miles, as far as I know the gears have always been 4.10. I am using my GPS device to determine the speed that I'm running, which isn't much different than what the speedometer is reading. I'm pretty sure that I'm not going 80+ mph because most people seem to pass me. The tires are actually a little smaller than 32", 235/85/16.

 

[QUOTE=n3elz]you can just multiply your miles per gallon by 1.1 after you calculate it. QUOTE]
How did you figure 1.1? I came up with 1.05.

I have been playing with one of the RPM and MPH Correlation calulators, and the numbers just don't mesh. Either my GPS is way off on the speed, or I have gears other than 4.10.

 

Okay, well that fits this then actually.

The truck was converted to 4.10's, then you added tires that were bigger and the two cancel out giving you stock calibration (more or less) on your speedo.

If so, then your mileage sucks as bad as you say.

Also verify that the speedo matches the GPS both at 25mph and 75 mph. If it's off by the same percentage, more or less, at each position (or not off) then you're fine.

If it's off by the SAME amount at 25 and 75, you could have an "offset problem" (basically meaning the speedo needle is on wrong, lol).

 

Good Idea. I will verify...

 

At lower speeds the speedo and GPS are nearly concurrent. Above 50mph the error noticably increases and seems to max out at 80mph (speedo) to about 3 mph difference.

in 5th gear
2000 rpm ≈ 55 mph
2500 rpm ≈ 67 mph
3000 rpm ≈ 79 mph

 

Not sure about your GPS, but the Garmin I have is useless above 55mph. Or so that's what the directions tell me.

 

That's really a trivial difference. I'd consider your speedo to be % on" for the most part.

 

i have 32s an 4.56s and at 70mph i am at 3000-3200rpm. my gas milage but i love the power

 

Is there a way to fugure out what gas mileage I would get with different gears?

 

100% sure that I have 4.10s in the rear; just took off the cover to verify. I don't know what is wrong with the calculations. Maybe my Tachometer is off by a large margin.


Anyway, Should I get 3.73s?

 

You will actually lose mileage and create more work on your engine going to 3.73's. they will make the RPM lower sure but you are also making it harder to turn the tire and as load increases, requirement for work increases. You are getting about as good as you will get right now.

And why take the cover off? You can either "a" read the tag on the diff and run the number to see what they are or "b" jack the rear end, index the pinion to the housing and rotate the tire one full turn counting how many times the driveshaft rotates to get your axle ratio. Lot easier than removing the diff cover and then having to refill with lube man.

 

Well, after using the RPM/MPH calculator, the math only worked if I had 4.56 gears. I was pretty sure that I didn't, just wanted to be certain.