Dodge Cummins Diesel Forum banner
1 - 14 of 14 Posts

·
Premium Member
Joined
·
86 Posts
Discussion Starter · #1 ·
i just bought bigger tires, 305/70R17's. I have 3.42 gears, 6 speed auto. I have lost around 1.5 MPG's according to the computer. I didn't expect this. Is this normal?
What can be done to get the MPG back?
I know my speedo is off about 7 mph slower than i am actually going. So is my computer just not compensating for the new tires? How can I adjust the speedo? If I install a K&N cold air intake, will that help with the mpg?
Any input would be greatly appreciated.
 

·
Registered
Joined
·
818 Posts
First, you speedo and odometer can only count wheel revolutions - change tire size, and what the truck thought was a mile is no longer a mile. In the case of bigger tires, you go *farther* than a mile when the truck thinks you only went one.

So, figure out the difference in circumference between the old and new tires, apply that ratio as a correction to both speed *AND* distance, and check your numbers again . . .

Although I would expect some reduction in mileage with bigger tires - more rolling resistance as well as drag . . .

- Tim
 

·
Registered
Joined
·
66 Posts
^^ beat me to it ha. ditto

Any time you add rotational mass it will effect your MPG. The dealer can re-"calibrate" your ecu or you can go the aftermarket tuner route for the speedo. As far as recovering the lost mpg, intake, exhaust, deletes. It all adds up but it depends what you want to do with your truck and warranty.

try a gps and hand calculation for mileage for now.
 

·
Registered
Joined
·
870 Posts
Yup. More rolling mass heavier tires will all have an impact on MPG.

If you want it back drop the tire size back down, or put 3:73's in it. My mileage went up when going to 35's


-Powder Keg
 

·
Registered
Joined
·
3,514 Posts
easiest thing to check first.....what is the air pressure in the tires......?
 

·
Registered
Joined
·
2,835 Posts
Yup. More rolling mass heavier tires will all have an impact on MPG.

If you want it back drop the tire size back down, or put 3:73's in it. My mileage went up when going to 35's


-Powder Keg

Everything being equal, calibrated speedo, gearing etc, etc, adding more rolling resistance will NOT give better MPG. Physics 101.
 

·
Rescue Tower
Joined
·
10,810 Posts

·
Registered
Joined
·
3,514 Posts
g56 IS THE MANUAL 6 SPEED TRANNY DESIGNATION.
 

·
Registered
Joined
·
870 Posts
Everything being equal, calibrated speedo, gearing etc, etc, adding more rolling resistance will NOT give better MPG. Physics 101.
When the taller tires bring your rpms down to where the tq is the best then yes it will add mpg for on the highway. My spedo I calibrated and verified with radar. I'm not saying it went up all around but on the highway it did. At 75 mph now I'm not screaming at 2250 - 2300 rpm now it's down to a more acceptable 1850-1900 at the same speed.

Also the manuals have a much lower final drive than the autos IIRC

-Powder Keg
 

·
Premium Member
Joined
·
86 Posts
Discussion Starter · #13 ·
When the taller tires bring your rpms down to where the tq is the best then yes it will add mpg for on the highway. My spedo I calibrated and verified with radar. I'm not saying it went up all around but on the highway it did. At 75 mph now I'm not screaming at 2250 - 2300 rpm now it's down to a more acceptable 1850-1900 at the same speed.

Also the manuals have a much lower final drive than the autos IIRC

-Powder Keg
what kind of mpg are you getting on the highway?
 

·
Registered
Joined
·
870 Posts
It really depends on how I drive it. @ 75-80 maybe 18-19

@ 60-65 a pretty solid 22

Keep on mind that when I get these numbers there is very little if at all city driving


-Powder Keg
 
1 - 14 of 14 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top