285s - MPG dropped... Normal?

[98chevy2500SS]

Most of you know that I went to 285s for the first time on my 97. It looks great, but, here's the kicker. I don't know if anyone has this much mpg loss, but when I got the truck with 265s, I was getting the same if not better than my 98. But since I went to 285s, mpg dropped... quite a bit. I just filled up Monday and have already sucked half a tank, this is a mix of town and highway driving.

Now, my 98 does the exact same mix of town and highway and I am getting 15 mpg with 4:10s! What gives? I know when going to 285s with 3:73s, the gear ratio goes to 3:42s, but is this normal mpg loss? I also know these trucks don't get the best mileage to begin with, but it should be getting better than this. I am about ready to sell my 285s and get the 265 version of these Dick Cepek's, because I do like the tire. I'm not sure if losing torque, and mpg was worth it for looks... Plus, the cruise control has a hard time as well.

I am going to be selling my PY0 rims off of my 98 because I got some polished 8 hole rims that finally fit, would it be a good idea to put the 285s and PY0 rims together to help sell better?

Edit: before my gas tank started leaking, I was only able to get around 170 miles out of the 97 (going from F to 1/4), whereas around 250 miles out of the 98 (going from F to 1/4).

 

[TheAutumnWind]

285 and 265 only refer to tread width. How tall are your tires?

Wider and taller and heavier tires definitely negatively effect mpg. More so if gearing puts you out of the efficient rpms of your motor while cruising and the trans is constantly hunting for gear.

 

[98chevy2500SS]

They are 285/75r16's, they are almost 33x11.5's. What you said is just what I was kinda dreading about. I do find that when cruising on the highway, when a hill approaches, it will always downshift, with there is 15mph wind, it seems to struggle quite a bit, especially when pulling my lawn mower.

Edit: 98 has 265/75r16's, but I would've thought the 4:10s would decrease mileage, but it really didn't.

 

[kennythewelder]

285 - 265 is an aspect ratio. As asked, what size are these tires. The complete size. My 275/60/15s are just a little shorter than the OE 235/75/15s. Its all relative. Higth VS width. You would think that a 275 is taller than a 235, but its a percentage of the width of the tire. The first and second set of numbers play off of each other.

 

[PlayingWithTBI]

Here's something to play with - Tire height based on sidewall designation, among other fun stuff...

https://www.tciauto.com/racing-calculators

 

[98chevy2500SS]

I did play around with that, nice little tool.

 

[andy396]

Edit: before my gas tank started leaking, I was only able to get around 170 miles out of the 97 (going from F to 1/4), whereas around 250 miles out of the 98 (going from F to 1/4).

Did you correct your distance traveled for tire size in your mpg calculations? In other words, with 285s you’re actually traveling more distance than your odometer says.

 

[98chevy2500SS]

Hmm, I didn’t know the 285s made the odometer incorrect, but for some reason when I go 62 on the Speedo I am going 60, kinda weird, it should be the other way around. It went 5 over with the 265s, which was weird because that’s what came on the truck from the factory. I am guessing because the transmission was rebuilt, so the shop that did it must have screwed with something When they installed it.

Sort of off topic here: how much would a speedometer calibration be from the dealer?

 

[andy396]

If I had to guess it would be at least $100. They seem to want that much at a minimum to just plug in there diagnostic tool.

I guess it’s possible that the transmission shop recalibrated the speedometer. Did you have it rebuilt or did the previous owner? If you did maybe that transmission shop would do it for you for less than the dealer.

 

[Hipster]

I noticed quite a difference when I went to 285's from 265's myself. In my situation there wasn't a tremendous difference in height, but a significant difference in weight, wider tread, more rolling resistance, effectively lower rear ratio, all added up to more throttle to get it and keep it rolling. On my truck there is also a huge difference in consumption between putting along 65-68 down the highway or running 75. On my regular 12 hr round trips 5-7 mph faster works out to be almost 2 tankfulls round trip.

I like the 285's with the 3.73's towing in 3rd puts me better in the torque curve about 2.6k rpms running about 62-65mph and it actually doesn't do bad mileage wise towing 4k lbs.