You almost cant even go by that because my door sticker says 225/75/16 but my build sheet says 245/75/16.

 

I call BS.

 

Not to call anyone out, but I'm right there with 95Ram360. I have a 318 in my dodge and right now i'm running 265/75R/16 with 4.10 motive gears and I get better highway mileage than I ever did with my 3.55. That may have something to do with the hills on my drive on the highway. Also, I run around 2000 RPMs at 70 and about 2200 at 80 MPH with 4.10. I have done nothing but gain gas mileage in the city and highway with my gears and that is over the course of 5 years now. I get about 16 in the city and about 18-19 on the highway.

 

Have you adjusted for the odometer difference in your calculations? If not, your numbers will be 15% higher than they actually are. There's a reason that none of the manufacturers use 4.10 gears as standard equipment.

 

You are going to have to edjamacate me on how having a larger tire effects the actual number of miles recorded by the odometer. Speedo I totally understand, but the odometer is counting the number of revolutions that depending on year model of your Ram either A) Output shaft of tranny or B) Ring gear makes and uses math to calculate miles. I could be wrong on where it is exactly that each year model's odometer counts.

Heck it is Wednesday I could be totally wrong on what my name is right now for all I know.

 

Larger tires will have a larger circumference than what the PCM is basing its calculations one. So, for each tire revolution, the truck will actually travel FURTHER than what the PCM really thinks. (it will underestimate mileage....) So effectively, he is getting MORE miles per gallon that the calculations would indicate.

 

Well, at least on my Ram ('94), according to my gps, my speedometer and odometer are off by the same amount. I am not familiar with the mechanism behind them.

 

@ HeyYou, Hellz yes! I wouldn't think the calculation would be too much difference especially when we are talking these gas guzzlers, but every little bit counts

@Grox, I work with GPS daily and unless you have a Survey grade GPS, You GPS is probably off by 2MPH and some where around a tenth of a mile or so by default. That is why hand helds and touch screen GPS cost a couple hundred as opposed to over 20 grand for a survey grade! As long as they get you where you going though!

 

Over the course of many miles, the error will become negligible.

Unless the gear ratio is changed proportionately more than the tire diameter.

 

And there it depends on the year of the truck, on the 98.5 and newer, vehicle speed is read from the tone ring on the carrier, so, gears have zero effect. Only tire size.