Tire Size and MPG with 3.42's

[Nobby]

My truck came with 265 size tires on it with a 3.42 rear end. In spite of the further lower gearing change I am just wondering if this is now tipping the balance the wrong way for MPG.
Does anyone have any experience going from 245 to 265 on a 3.42 and hence know any figures.
Sure 265's look better but that in the end is not my primary goal.

I am also considering going back to 245's as I will doing some towing as well.

cheers
Nobby

[Robyn]

I have two 94 Burbs (2500) with 6.5's and one has 4.10 gears and the other has 3.73's
The mileage is about the same on the highway, 20 and about 18 around town.
Both have 245 tires which I believe is the stock size.
Running 3.42 is going to raise mileage some on the highway but will cut the towing power some without dropping it into the gears on the hills.
The 265 is going to raise the ratio a tad further and the results will be the same.
Lower RPM at cruise but less power available to pull without dropping out of overdrive.
What RPM do you run in OD at 55 MPH My Burb with the 4.10 is turning right at 2000 the 3.73 will run about 1800 Now these numbers are by the tach and subject to scrutiny, but they are a good starting point.
The only real way to knopw for sure is to figure the tire diameter and run the math to see what it should be.
The 6.5 is a very happy little camper to hum along at 2000 or so.
Running 70 it still gets around 19 highway and the RPM's are around 2350
Depending on what you are going to pull with the truck I would not be too concerned with the 265 tires.
Tires are cheap compared to gears and such.
Try it and se how it works.

[More Power]

Larger tires could contribute to more rolling resistance (especially if run on the soft side), which ultimately affects fuel economy. Running near the max rated pressure as listed on the sidewalls can help mitigate that.

Jim

[Nobby]

For 55mph a calculator gives me 1400 my tach is showing around 1500 or so. 65mph gives me 1655 by the calculator about 1750 on my tach.

Thing is my mpg is around 19-20 average and I do a greater share of highway miles. I was expecting a little better than that, so I am now wondering if 265's are making the gearing a little two tall and hence now I am using up fuel to overcome this. Strikes me that best mpg will be around 1800 rpm at the peak of the torque curve. 245's will address that balance somewhat.

Bigger more open exhaust would no doubt help things too. Maybe i'll invest in that first as the tires are recently replaced.

oh well
cheers
Nobby


Nobby

[DA BIG ONE]

W/3.42's the 265's dia. is about max w/o highly modded power train & controllers. I got to the point that throwing $$$$ to increase mpg's actually resulted in loss of mpg in some cases........

IMHO, 33" require 3.73's for best all around driving and mpg's,
35" require 4.10's for best all around dirving and mpg's, keep in mind that ride weight, height, tire width/compound and drag come into play running tires larger than stock.

Has your speedo been hacked to the larger tire dia, or? If not your mpg figures can be off?

Going to freeflow exhaust, aftermarket air filter will help.

[Robyn]

If you are getting 19-20 with a 6.5 TD Be happy
I have owned several and thats about it.
The 6.2 engines would do much better in the older rigs.
I had an 86 2500 Burb with 4.10's and stock tires that would do 22-24 on the highway with ease running 60-65
That rig had a TH400 and no OD
The 6.5 is just not going to get too much better than what you are seeing.
As was pointed out tossing $$$$$$$$$ at it may just reduce the mileage.
The early 6.5's 92-93 with the mechanical pumps got somewhat better but not a lot.
I know this is not what you wanted to hear but its pretty close to the mark.

Robyn

[Nobby]

Yes I am compensating for speedo error.
I am not looking for miracles here just was expecting with 342's to be over 20 not under 20 and in the end it is what it is.
I see others have posted 21-22 mpg with 342's so am just curious.
I may well try a tire size change and definitely see a bigger exhaust in the future .
Thanks for the input.

cheers
Nobby

[Warren96]

It has a lot to do with how much towing, and how often.Some of the members here tow some very serious loads! For example i have 342's and it has 285x85's,but it never hauls anything except my 200 pound keester.As you can see MPG's are all i care about.If push came to shove and i had to tow a house someday, i would probably buy a pair of smaller than stock to lower the stock effective ratio.

[Nobby]

It has a lot to do with how much towing, and how often.Some of the members here tow some very serious loads! For example i have 342's and it has 285x85's,but it never hauls anything except my 200 pound keester.As you can see MPG's are all i care about.If push came to shove and i had to tow a house someday, i would probably buy a pair of smaller than stock to lower the stock effective ratio.


So what are your average mpg's? Need to take into account here tho that yours is a blazer.

cheers
Nobby

[moondoggie]

Good Day!

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