Hello,

I have a question that no one can seem to answer. My truck is a 17 F-250 XL, it came stock with the 245/75R17 tires. The old steel work truck tires. According to my research so far those are 31.5” in diameter.
Recently, I came across some blacked out 20” pull off’s with 35’s from a guy I know who purchased aftermarket rims/tires. With almost brand new Toyo Open Country tires on them. I gave him the $1.2k he asked for them and put them on my truck. Truck looks great and all is well until my gas mileage dropped from 16.5mpg to 13.2mpg. Throttle response is no longer like it was either. I tow a 12k lbs camper and I’m worried the new wheel/tire upgrade could possibly be to much strain on it. Stock gear ratio I’m 99% sure is 3.73. With the new 35” tires it’s 3.36. A local off road shop is recommending I swap out differential gears to 4.33 I think. My question is.. wouldn’t 4.33 be to low? Because I don’t really want to hit 3k RPMs to go 65 or 70mph. What gear ratio would I need to get it back closest to stock? As of right now my speedometer is 4mph faster than what it says on the gauge. It is a 6.2L gas engine.

reference: stock ratio is 3.73. With 35’s it’s now 3.36. Gauge is 4mph off. Any help from someone with experience in this or just simply smarter than me is greatly appreciated. Hope I posted this in the right spot. Thanks!

Matt

 

Do a google search for "gear speed calculator", you can plug all the numbers into that to get the gear ratio needed to bring the speedo back to correct and this will also fix your gas mileage calculations and the poor take off.
Any change beyond that will require some sort of recalibration of the PCM to correct for the different gearing.

 

You could easily have the dealer program it to the stock 275/65/20 stock size and it would only be .4 inch off.

I am running bigger tires on that programmed size and its pretty close.

 

Thanks, I got my answer. Ratio now is 3.34 and to get it back to the way it was the needed gear ratio is 4.16. Thanks!

 

understood, the speedometer isn’t the only real issue though, I’m honestly worried about my GCVWR being 18k lbs up that mountain I go up for vacation. If it were a diesel I doubt it would matter so much with those torque levels.

 

You could also put stock sized tires on it and solve most of your issues. Regardless of gear ratio or speedo correction, it's going to take more power to turn larger diameter, heavier tires and wheels.

 

Put the 17" wheels/tires back on for towing?

The heavier wheels and tires will affect MPG, not matter what the axle ratio is. The higher axle ratio will help get the truck/trailer up the hill.

 

So your calculation says you need 4.16 gears with the 35" tires to equal the ration you had with 3,73 gears on the stock tires? That is good to know as I ordered my truck with the 4.30 gears with the idea of moving to 35" tires someday. I will also be towing a 7,000 lb camper.

 

Gotcha. I would throw a 4.30 or 4.56 in it if you need to.

I was pointing out you are still pretty close (hair out) of what a factory option was.

 

For one, your mileage is looking worse than it is because the larger tires are skewing your odometer a little...hard to get correct mileage unless you are calculating the % difference between the two tire sizes...

Secondly, I would recommend the 4.30s absolutely. Maybe even 4.56 if you don't do a lot of empty highway driving. With 4.30s and 34" tires, my truck turns ~2100rpm at 72-74mph. With 35's, yours will be a touch lower. High gears equaling high RPMs are old ways of thought left over from the non-OD transmission days. We have 2 and 3 OD gears now.

A 18k GCVWR means the trailer you're towing is around 10-11k? That's nothing for the 6.2.