True enough. With an extra 2" in diameter, everything is going to be off. Have it reprogrammed and go from there.

And for determining a discrepancy in gas mileage, you really should know the exact mpg. Approximation of so many miles on a tank can be very subjective.

 

Too tall of a tire diameter will cause you to use larger throttle angles to hold the same speed.

if it slows 1/4 mile times it will for sure reduce fuel mileage.

recalibrate the PCM for tire height and gear ratios then see how the mileage is.
I have always noticed better fuel economy with 3.42 over 3.08 gears with 28" tall tires at legal speeds.

My 1997 truck with 28" tall tires and 3.08 gears got better fuel economy crusing at 130 kph than crusing at 110 kph. the high gears made my truck crusie at too low an rpm.

You want to cruise your truck at whatever rpm the engione makes its best vacuum at. High vacuum equals goof economy. If the truck is crusieng out of its ideal rpm, like when lugging the truck with too high of gearing and too tall of tires, the throttle is open more and teh vacuum reading will be lower, the fuel economy will also be lower.

Gear and tire your truck to cruise at whatever rpm makes the MAP sensor read the lowest pressure (lowest kpa or psi) which is the highest vacuum.

peace
hog

 

I'm getting the speedo recalibrated to match the tire size so I will be able to get a more accurate mpg reading. Although, I dont see how the bigger tires would actually make the mpg show up differently.

Think about it, One revolution of the output shaft on the tranny is one revolution. It's all the same. The computer still sees every revolution the same. My truck should just be going further than it used to for every rotation due to the incresed diameter. Thats the way I see it anyway.

NOW, what hog is saying is that Im making the engine work harder for every rotation, so its using more gas and I have to increase the amount of throttle. I can understand that exactly. Reguardless, I still am going to reprogram it. It seems to be shifting slghtly different so that might also be part of my problem.

 

Just FYI, I have 4.11's, not 4.10's. They are Yukon gears, and Yukon makes a 4.11 ratio, not a 4.10. I have no regrets going to the 4.11's, very happy with them. I knocked down 21mpg on a trip home from dallas one night, did 65mph, 3.5 hours, all interstate.

 

You just gave yourself the answer right there: Truck is going further. the odometer says you went 300 miles, when in reality you went 320. Do you see how that can affect gas mileage calculation?

Make sure you use the correct diameter for the reprogramming - don't assume the manufacturer's stated diameter is correct. When I reprogrammed mine, I used a calculated value of 29.94", even though BFG says it's 30.5" - My speedo is spot on!

I taped a toothpick to the sidewall (make sure it is perpendicular to the floor) and marked the floor. Rolled the truck for one revolution and marked the floor again. The distance between the two markings is the circumference, so divide by Pi to get the correct diameter.

 

Your truck does not know that you are getting the extra distance from each revolution until you recalibrate your speedo. Right now you are moving further with each rotation than your computer thinks, so it is reading a lower mileage rate with the larger diameter tires.

 

I second that!

My 275/55's calculate to 31.9", but they actually measure 31.0" tall. I used a different method to measure though, I used a level and tape measure. I set the level on the top of the tread extending out away from the tire, held it level and measured the height. My speedo's right on, I had it checked at 30mph and 70mph with laser

 

Because of the tire bulge, due to the weight of the truck, this method could come up smaller than reality. With that said, you will have a more accurate value with a performance tire than a stock tire (or more correctly: a smaller profile). Which is probably why you are good to go.

 

HAHA. I know that man! No way in hell i would take some of those classes! Thanks for the compliments. I would go with 4.10s or at least 3.73s. Im having the same problem with my 3.42s. at around 55mph it bogs like crazy! I cant wait to get my 4.10s, trutrac and 3000 stall in there! Its killing me.

 

My 05' VHO gets an average of 19-20mpg on HWY(65-70mph tops) w/ AC on. That was also with a full tank of gas and three people.
My in town city mpg will be between 15.5-16.5mpg (2,000-2,500rpm's)"usually"
That's with the 275/ 55/ 20's, 3.73 gearing, 6.0L
That should get you an idea to start at.
You will definately get better fuel miliage and acceleration with better gearing.
I get about 340-360 miles a tank right when the low fuel light comes on.
That's using 93oct fuel all the time.

Jim