TowRig.com - Diesel Discussion and Tow Tech

Need some advice fellas... I'm considering replacing my 265/70R17 Michelins with 285/75R17 Toyo Open Country A/T's. Any comment on the size and choice would be appreciated but that isn't my question.

What I would like comments on is the fact that this tire will be 2" taller which (unless I figured wrong) will make my effective gear ratio 3.50.
How much will that affect my towing? I don't tow much more than a Massey tractor on a trailer but I have thought about getting a 30' travel trailer in the future.
Any comments will be appreciated! Thanks!

That's not enough to worry about. The cummins won't even notice, plus you might get a little better MPG with the lower RPM you will be turning.

dont forget about the speedometer being off.

Good catch on the speedo! I had thought of it, just didn't mention it. The GPS will take care of that until I get a Smarty or pay to get it corrected.

That's not enough to worry about. The cummins won't even notice, plus you might get a little better MPG with the lower RPM you will be turning.

And I REALLY need the extra MPG! These common rails don't get nearly the milage that the older ones did. Hiway mileage is ok but I'm too tempted at the redlights to keep offa da pedel! lol

I’ve got those same tires on my rig, good choice…I love them. If my memory serves me correctly jumping up to that size only throws the speedo off something like 3.5%....as others have mentioned, it’s not really enough to make a difference in power let alone the speedo read out.

I'm running 285's with 3.54 gears. I only wish I had more gear at the very top of hills while pulling my TT, I sometimes have to drop out of OD to keep the egts down. Go for it! waytogo

I just put 305\70\17 on my dodge and they turned out great. speedo is off about 5 mph at 70. I don't think you will have any problems.

I'm running 315/70's with 4.10's. It equals out close to what 3.73's and stock tires would be. I also had the dealer reflash the computer for the tires. Just find out the revolutions per mile for the tire.

285s will be fine. I am running 33s and my speedo is off by 4 mph. you shouldnt be off more than 2.5.

you wont even notice

You'll love the open country's! I put them on my truck as soon as I bought it and it's not an issue for towing power. Great tire choice! Enjoy!!!

Sorry to hijack the post but what is the easiest way to determine what gear ratio I have in my truck? I know some mfg's put a sticker in the glove compartment with codes indicating the vehicle options but mine doesn't have that. I know I can pull the rear diff cover and count the R&P teeth and divide one into the other...but that's a lot of work. Any ideas?

Babastard,

I would think the easiest way would be to run by a dealer and give them your VIN number. They can run you a printout with all the equipment both standard and optional that your truck came with. In fact, that's how I found out I have an 04.5 which was a nice suprise.

I bought my truck used and it had nothing with it so that what I did and it took about five minutes.

Hope this helps.

If you have the time and are willing to put forth the effort then just jack up the rear of your truck so that both rear tires are off the ground. Place the truck in nuetral and then turn the drive shaft by hand. Count the number of turns on the driveshaft. If you turn the driveshaft 3 and 3\4 turns for every one turn of the rear tires then I would say that you have a 3.73 ratio.

The 285's will be an awesome choice for your rig.