Bigger tires on a diesel truck

[Big Dave]

Stay with the 235s. 265s are about the same.

Going bigger will result in lower MPGs. Hundreds of guys have tried it and bigger tires are your enemy.

Big tires have much higher rotational inertia and hurt you every time you pull away from a light. Slowing down the engine for a given road speed is a good idea, but anything that adds to the "flywheels" reduces MPG.

This is one thing the factory engineers did absolutely right. Leave it alone.

[Sludgy]

Originally Posted by Big Dave

Stay with the 235s. 265s are about the same.

Going bigger will result in lower MPGs. Hundreds of guys have tried it and bigger tires are your enemy.

Big tires have much higher rotational inertia and hurt you every time you pull away from a light. Slowing down the engine for a given road speed is a good idea, but anything that adds to the "flywheels" reduces MPG.

This is one thing the factory engineers did absolutely right. Leave it alone.

I'm way jealous of your 24-27 mpg..... from a 1 ton. Where did you get the fastback fairing? I can't find one. Snugtop makes a rounded cap, but not for F350s.

I'd love to re-gear the truck to 3.08 too, but I'd have to do both axles since my truck is a 4x4. And maybe put in a front positraction unit. How much did a single ring and pinion cost?

[Big Dave]

I made my fastback fairing out of stuff found in my garage. Plywood, one-by lumber and rubber floor runner turned upside down.

If all goes well I'll make a better one this winter. Probably fiberglass with faired in mounts for CCTV so I can lose the elephant ears.

For a4 the best ratio you can get is 3.55:1. Not worth it unless you are ruuning OEM 4.10s.

But for a 4x4 maybe a GV overdrive would work. Those are good for instant 4 MPG. They are salty but if you are high-mileage they pay off fairly quickly. I drive 25,000 miles per year, but the 4x2 adapter is low-volume therefore outrageously expensive. 4x4 adapters are high-volume and semi-reasonable.

[GasSavers_SD26]

Originally Posted by Big Dave

But for a 4x4 maybe a GV overdrive would work. Those are good for instant 4 MPG. They are salty but if you are high-mileage they pay off fairly quickly. I drive 25,000 miles per year, but the 4x2 adapter is low-volume therefore outrageously expensive. 4x4 adapters are high-volume and semi-reasonable.

Finally went to the GV site. Pretty cool stuff, but enough to make my head hurt. Someone put 285/75's on my X, and I'd prefer to be back to stock (265/75) or narrow (235/85). But I like the RPM's at 60 to 65. Would be interested in a little more ground speed with the same RPM's. Hmmm...

[kamesama980]

keep in mind when going to narrower tires that, assuming same PSI, they're gonna belly out more thus effectively eliminating the small % gain in diameter. if you went from 265s at 35-50 psi to 235s at 90 psi then it may not matter.

[Daveedo]

Originally Posted by Sludgy

Well, I've tried downsizing tires on the Blue Beast, from 265-75R16 to 235-85R16. The new tires are a hairsbreadth taller than stock but narrower. I haven't seen much improvement in mileage.

I have been thinking of going in the opposite direction. BFG makes huge all-terrain 305-75R16 tires. Now I know that the tread on these tires is not aerodynamic and they are heavier. But it will effectively gear the truck about 8% lower for a given speed. The effective axle ratio will go from 3.73 to 3.47.

What's the group's opinion? Will big mud tires make my mileage go up or down?

To echo the others...down 4sho. you could try a 255/85/16. Be careful of the wider taller tires. Any overdrive gains are negated by the increased RR and weight.


I've got a '97 PSD 1ton. Since 2002 I've increased the fuel mileage doing a number of things specific to this vehicle.

Learned to drive speeds that lock up the torque converter. Mine locks at 45mph but after it locks it won't unlock until I drop below 40mph.

Went from 285s to 265s at 80psi.

I added an open element intake (tymar),

a chip made a big diff too. Weird enough the best fuel mileage gain is on the hottest setting of the chip.

Removed my bug guard (not really vehicle specific).

and I don't drive on trips of less than 10miles unless I have to. The owners manual actually says not to cause the motor won't get up to operating temp.

I use a block heater on a timer for at least 1.5hours before starting it on days I have to drive it (generally only in the winter). Cold truck really gets back mileage.

One other thing...I try to never get the rpms over 1600. I believe that my motor has 425ft/lbs of torque at 1600rpms


Good luck with it.

PS The best fuel mileage I ever got was a trip to SD from WA in a really hot July a few years back I was getting about 560miles before each fillup whereas usual around here is 425 before a fillup. Extended freeway driving in hot weather seemed to be the ticket (I was generally doing about 80mph too as the montana speed limit signs are just there to satisfy the feds )