View Full Version : Tires vs Mileage
01-18-2010, 09:16 PM
I haven't posted here for a while as I have been to sea or one thing or the other. While I was home this time, I did a couple things to my truck and am putting myself out there for you guys to tell me what I did wrong, if I did anything wrong or whatever. Here is the story: My fuel mileage has dropped from 22mpg to around 17-18mpg. Before, it was like clockwork. I drive this truck back and forth between Charleston and Baltimore at about 72mph on I-95. I have photographs of this trucks speedometer, GPS and tachometer all at the same time. I have been aware that the truck would be going about 72 mph at 1800 rpms. Now, after the changes, it seems that I have to do 2200rpms to go 70mph. It used to just "drop in" at 72... now it does not seem so comfortable at 72..more comfortable at 70.
I went around and around with Sears over the new tires that I had put on her. The tires were exactly the same size as the ones that I had replaced. I think. The tires are in Charleston and I am in Baltimore. I won't be going home for another couple of months. I will know more when I get there. The other thing I did was install a 3200 spring. The truck runs great and I am not really driving any different on the highway that I was before. Therefore, I think that I have a smaller (circumference) tire than before. I remember measuring the tire height, then calculating the gear ratio (3.54 w/5spd) and it all coming out exactly as it should as far as rpm was concerned... that was with the other tires. If anyone has the final drive ratio with the overdrive xmission, I would appreciate it. Has anyone had a similar experience with a normal change of tires??:([verymad]
01-19-2010, 02:27 PM
Wow 1800 RPM vs. 2200 with just a tire change is a HUGE difference. I can see maybe 100-200 RPM change from a 215/85R16 vs a 235 or 255/85 but not 400 RPM... Something is amiss and we need more info.
01-19-2010, 03:30 PM
I'm just thinking out loud here.... Could be that your getting more fuel from the spring, that would change fuel mileage, but not explain the the 400rpm difference. Maybe its the 400 rpm difference that explains the fuel mileage. Do you think that your clutch may be slipping? From all the air resistance at those speeds and additional power from the spring it might be too much for an old clutch? If you jump on it does the rpms climb more rapidly than the speedo? A slight slip could cause the rpm rise. I bet you'd be able to smell the friction material though.
01-19-2010, 04:15 PM
OD on the Getrag is .77. That number times your rear diff ratio of 3.54 equals 2.726. That is your final drive ratio. I am also running the 3000GSK with no adverse effects in relation to mileage. With my NVs final drive ratio of 2.62 - the NV OD is .74. In mixed driving 60% city verses 40% highway I see 19+ MPG. Pure highway runs at 22+ MPG at 65 MPH. The 400 RPM difference does not make any sence. As far as the drivability at 70 vs 72 that could be poor balance by Sears. I would take it to a GOOD tire shop and see how the balance is. If it is off that would affect drivability. My truck likes MPH up to 70 but no higher.
01-19-2010, 05:04 PM
I'm wondering also if your clutch disc is slipping. I'm also wondering, how long did the truck sit? Your brakes could be dragging or a little cheese thief (mouse) could have built a nest in either your air filter or exhaust pipe. If the tires are the exact same size (you did check the tire itself didn't you, not just take their word for it) they should have the same circumference. As they wear down, that will change but not for many miles.
01-20-2010, 10:44 AM
Going down the interstate not pulling anything you're not putting much stress on the clutch, if it's slipping there you'd be having all sorts of problems just getting up to highway speed.
Did you change tread designs? I lost 1-2mpg when I went from a strictly highway tread to A/T's.
Still, that doesn't account for the 400rpm, I don't have an answer for that one.
01-20-2010, 04:07 PM
I think i may have figured out your problem.... It has to do with your tire size, i've used a tire size calculator and this gear ratio calculator http://www.grimmjeeper.com/gears.html and have come up with the only possible outcome that your previous tires were larger than stock. because stock tires with your trans and rear comes out perfectly that at 2250 rpm in 5th your truck speed should be 71MPH at 1750 rpm you would only be doing 55 mph and that seems about right. All this is given that you have the stock 235/85R16 tires. If you opted for larger tires before and went to a shop where they have to install whats on the door this would be your problem. I know sears and bjs must install only the tires listed for that vehicle, otherwise the warranty doesn't apply and they get their pp smacked.
01-20-2010, 08:38 PM
I have my wife checking the size of the old tires back home. The clutch isn't slipping... checked the air breather and cleaned it before I left, as you are right.. those little critters will build a nest in there when it sits a while. I think JustRamIt19 is right. I think the circumference is smaller on this tire. I will let you know tomorrow what I find.http://i258.photobucket.com/albums/hh279/rbarrett/Gauges-1.jpg
It is hard to see but this is a before the tires picture...[guitar]
01-21-2010, 06:40 PM
JustRamIt91, I think that you have put me on to something... I think that I may have been ripped off by Sears!! I'll have to do some more research but the tire calculator indicates that the diameter of a 235/85-16 is 31.73 inches. That is about (almost exactly, in fact) what my old tires were. I just went out and measured the ones that are on the truck right now, the new Wranglers, and they are definitely less that 31.00 inches(30.8750). I measured them the same way I measured the old ones, and the old ones were about wore out... the old ones measured 31.75"... almost exactly what the "calculator" says they are supposed to be. I am gonna do some more research before I go off half cocked. I just talked to my wife and she measured the old tires laying there on the trailer without any air and not on a rim and they measured 30.500". Pump 80#'s in those things they are sure to be > 31. 5"...[tapdshut]
01-21-2010, 09:33 PM
Before you go off like kozmo kramer at the sears store, take a look on the side of the tires, there are some numbers. Rather than measuring the over all diameter there will be a tire size 235/85R16 is the size for most of our trucks. This number is usually in smaller raised rubber letters just under or opposite to the brand name of the tire. I think that your older tires may have been slightly bigger(explains raise in highway speeds, maybe a 245/75R16 or so.
There is also another thing to consider... Load range/capisity. You may have had the proper rated tires before, basically this means that its rated to hold more weight than regular tires(weight of a cummins motor). The new tires, you said are wranglers, i am assuming that they are regular suv or light truck tires and aren't load rated for the 3/4 ton truck. This will mean they are going to look deflated(smaller diameter) when filled to the manufacturer rated psi(35-40).
Another thing to think about is the price you paid. The correct tires are usually more expensive, i think they are about 150 around here, and thats cost price, and is too much for me.
Just a couple things to think about, let me know what you think. Just make sure you have everything correct before you go in screaming at everyone in you way... it usually gets the cops called and THEY DON'T CARE if you think your right... they will still arrest you, or at least escort you from the property ;-)
01-22-2010, 04:48 PM
The tire size is right. I checked them last night. 235/85-16. They are the proper 10 ply tire with 75 psi at this time. I will take them up to the limit when I am hauling anything heavy. The tires at home (size) are still in question. Not sure. I know that these are proper for this truck and the right load range. Will keep you posted... Rockfoot
01-22-2010, 07:22 PM
What is on the spare? Is it the same as the old tires?
01-22-2010, 09:14 PM
Not sure about the spare but my wife called and they were 235/85-16's on the old set. I have done the math and there is not that much difference in the size of the tires...1 inch on the diameter does not make a 300rpm difference...(only about 56rpm at 60mph) I guess I am stuck with it for now. I can probably reason with the mpg difference in that the 3200 spring has me running a little higher rpms through the gears... it is the 300 rpm at the top that has me baffled.
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