I was having some steering issues with my 99 dodge 4x4 2500ctd. So I put in a new ps pump and steering gear. Steering problems cleared up like expected. The next day i went and replaced my stock tires and wheels(245 series) with a set of 285/75/r16 toyo mt's and some allied wheels. I decide go with the 285's because i only have 3.55's for gears. The problem is that the rig is now guzzlin' the diesel. I went from about 21-22mpg at 60mph to 13-14mpg at the same speed. As far as i know all the install for the pump and gear went good. I know my little trip computer is going to be wrong because of the speedo being a little off, but that is a big loss and i would like to get this problem under controll as diesel prices being so high. Would a programer help? I am mechanic by trade but the diesel world is still new to me so any help would be greatly appreciated. This seems to be a great site and i hope to be able to contribute sometime soon.

 

How are you calculating you MPG? I hope you are not going by the overhead console. Because with the tire size off, the speedometer and trip are off, so therefore the overhead MPG readings will be off. So if your odometer is off, you have no way of knowing how many miles you put on a tank. You need to get the speedo corrected and then you will more than likely realize that your MPG did not change that much at all.

 

Get a Smarty and you can correct your speedo, and get some mileage back.

 

usually with a larger tire size on the diesel the fuel mileage goes up a bit or atleast stays the same. You havent done a drastic change in tire size. Run that exact size on my ram

 

I have the same gear ratio as you and have put on the same size tire. You are getting 7.5% more miles than your odometer is showing. Also, when your spedo shows 60mph, you are doing close to 65. These larger tires will hurt bottom end torque, but you will run less RPM on the freeway. I have noticed a drop in mileage as well, but have no solid figures to work with. Just lost my lift pump, so perhaps the weak pump was causing some loss in mileage. You mentioned that you only have 3.55, but that gearing actually has less torque than the 4.10. So adding larger tires has hurt bottom end pulling even further. The tires are 33" compared to the stock 31". Mud tires might further reduce the mileage. Based on the info you provided, I'm estimating a corrected mileage of 14-15MPG.

 

I have roughly the same issues with my 285's. I'll probably go back to the 245,s when these wear out. Increasing the tire size 2" in height equates to 9 to 12 inches in rollout. It screws badly with gearing. The engine works harder to get the weight moving thus more fuel. My gas ram 1500 got 10.5 mpg with these same tires. Talk about crying at the gas pump I have an HO 6spd. so I think that helps but I can tell when pulling the boat and the economy drops to around 15.5. Im at 18 or so here in town stoplight to stoplight without any towing.

 

I'd look at the calculation method also, and then consider the tire size. I have noticed on my 99 that I have more rolling resistance with my 315/75's than with my 245's, and I believe this affects the mileage a bit, but I have never calculated the difference.

Chris

 

I lost no mileage when I went to Michelin 285's, after calibrating the speedo for them... but they had a max inflation level of 65 psi.

Lost .6 mpg (also after correcting speedo), on a 1,000 mile test, going to present 315 Toyo A/T's, but they are limited to 35 psi. If someone would make us an E rated 315 tire (80 psi) tire, I believe no mileage would be lost! Just my guess.

I may go back to the 285 Michelins when these Toyo's are worn, but the 315's look so 'right' in that big wheelwell that I'm not sure. Mileage isn't everything, right? And I love being 2000 rpm at 80 mph.

RJ

 

mud tires are not the best for mpg.
winter diesel does not help much either
you are going farther then the odo says so you must add that in too

I run 285's and get awesome mileage. I would be checking filters and trying a different fuel station. and last but not least do not believe the console odo.

 

Your tire width has gone +1.57 inches (not good for mileage)
Your Tire rollout has gone +7.42 inches.
Your Speedo and Odo are both reading off by 7.20%

The more agressive tread, probably heavier tires, and increased rolling resistance are a triple threat for bad mileage.

You effective gear ratio has gone from 3.55 to 3.29
It's working harder to get those big meats turning from a stop.

 

Doubt that will ever happen.........too bad because you are right about the mileage. The problem is that the load carrying capacity of a tire that large in a D rating is as great or greater than an E rated tire in the 265 size. The amount of rubber on the ground negates the need for more plies. I don't think anyone would like to see the price if they did make one. Look at the Toyo M55s in 285, expensive but well worth the extra plies. IMO

 

'Boondocker' Just stumbled on these... 65 psi and 35"... with 3646 lb load limit!!
Last time I looked, there was no such animal!

Interco SSR M/T's
SSR-41R - 35x10.5x16's - D Rated - 8 ply - 65 psi - 3415 lb
SSR-58R - 35x12.5x16's - E Rated - 10 ply - 65 psi - 3646 lbs

http://www.intercotire.com/site34.php

The narrower 35's have me interested. Wonder how loud they are on pavement?? Got a price of $280 each.

RJ