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will I lose mpg with a 31x10.5?

B

black dawg

1/2 ton status
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On my 91 1/2 ton 4x4 burb, it is getting time for some new tires, I would really like to put 31x10.50 on it. It has the stock 235/75-15 pizza cutters on it now. Do you guys think there will be a noticeable mpg reduction?
It is 3.73 geared with a 700r4 and a 350
 
You will a little not much.now if you going to 35's then you would see a difference.make sure you run the psi that tire requires for the best mpg
 
Hmm, my '89 has 31/10.50/15 as a factory sized tire on the sticker, I could have swore. 3.42 rear gears, 700r4 trans,, yadda yadda yadda...

Used to get about 14-16 highway. a lot less now with the 35's and 4.10's.
 
dont know what ya get now but my setup gets 14 with the same drivetrain just 33's instead.

you have to run the psi the vehicle requires not the tire. larger/wider tires can be run a few psi lower than your vehicle requires due to larger surface area. 4 wheel n off road mag has a formula you can use, but just reading the drivers door jamb for the requirement and subtract 2-3 you'll be ok.
 
I wouldn't go by what the plate says on the side belowe the striker bolt is.I just took a look at mine is say to run tires that came with truck at front 28psi and rears at 30psi looked at my 35's there rated 42psi.
 
tire gets what the tire wants
all there is to it (IMO)

I'm sure super swampers weren't on Chevy's mind when stamping that label. If that's a bit rash, where do you draw the line? :)

My previous setup was 3.73, 31", 700r4, 400sb. Ran like a champ everywhere. Dunno mileage.
 
A little taller gearing would probably be better for you given your location. In my drive through Montana i learned that you folks drive FAST and LONG distances being the 4th largest state in the Country. Lower rpms at the top end highway cruising should be good for you i would think.
 
You will have to change the DRAC to get the speedometer to read right with the different tires, if you are really concerned with economy. Otherwise, you'll switch tires and see a more drastic change in economy than really occurred.

I doubt you'll see anything noticeable in economy. I've had 235's, 31's, and the current 33's, and the "freeway best" was never noticeably better with one or the other. City might be different, but city driving is impossible to replicate mileage, all you can do is a best guess or average over multiple tanks.
 
Thanks for the replies guys, and I will re cal the drac.

As for mpg now, I have only checked it twice, once all in town no highway and the other time all highway. 12 in town (in the winter) and 15 on the highway (with lots of hills) at 70.
 
crash k5 said:
I wouldn't go by what the plate says on the side belowe the striker bolt is.I just took a look at mine is say to run tires that came with truck at front 28psi and rears at 30psi looked at my 35's there rated 42psi.
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The stock tires have a smaller contact area so they need a higher psi # to get correct tire to pavement contact. Larger tires have a larger contact area so they need LESS psi to maintain the same contact. Think of an 8" wide tire with we'll say a 5" contact area front to back. Thats 40 square inches. At 30 psi(POUNDS per SQUARE INCH) thats 1200#'s of capacity per tire. now lets say a 12" wide tire with a bigger diameter so.. a 6 inch contact area front to back. Thats 2160#'s of cap. per tire. WAY more than needed for your vehicle. Actually in this general example 16.7 psi will give you the same weight capacity as the factory recomendation. The vehicle recomendation is the HIGHEST you should go unless you go with smaller tires. The rating on the side of your tire is the MAXIMUM psi for that tire. Not what you should fill them to. All that will do is give you a worse ride and lower tire life. Look it up on google it is stated everywhere. even on manufacturer sites

http://www.michelinman.com/tires-101/tire-care/tire-maintenance/how-to-check-pressure.page

Sorry for the long winded somewhat rant, but its a safety thing that many people are misled on and I'm just trying to help.

tires.jpg
 
shady said:
The stock tires have a smaller contact area so they need a higher psi # to get correct tire to pavement contact. Larger tires have a larger contact area so they need LESS psi to maintain the same contact. Think of an 8" wide tire with we'll say a 5" contact area front to back. Thats 40 square inches. At 30 psi(POUNDS per SQUARE INCH) thats 1200#'s of capacity per tire. now lets say a 12" wide tire with a bigger diameter so.. a 6 inch contact area front to back. Thats 2160#'s of cap. per tire. WAY more than needed for your vehicle. Actually in this general example 16.7 psi will give you the same weight capacity as the factory recomendation. The vehicle recomendation is the HIGHEST you should go unless you go with smaller tires. The rating on the side of your tire is the MAXIMUM psi for that tire. Not what you should fill them to. All that will do is give you a worse ride and lower tire life. Look it up on google it is stated everywhere. even on manufacturer sites

http://www.michelinman.com/tires-101/tire-care/tire-maintenance/how-to-check-pressure.page

Sorry for the long winded somewhat rant, but its a safety thing that many people are misled on and I'm just trying to help.
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This is the right general philosphy, but technically it's the volume of air the tire holds. 20 psi in a 35x12.50 is a lot more air than 20 psi in a 31x10.50 tire. For the 31 to have the same volume of air you have to increase the psi. Think of a kid's swim tube....you get a really small one and a really big one and blow them up with your mouth (basically the same psi). A grown adult and can float around on the big one no problems but the small one will just sink when you get on it.
 
I usually use the door sticker as a pretty rough guide. I just went and looked on this burb, and the door sticker says 35 in the front and 41 in the rear. I have 45 all the way around, tire wear is even, and it drove like crap with only 40psi.

I cant imagine driving this thing with p-rated 235/75-15 at 35psi.
 
Last edited:
black dawg said:
On my 91 1/2 ton 4x4 burb, it is getting time for some new tires, I would really like to put 31x10.50 on it. It has the stock 235/75-15 pizza cutters on it now. Do you guys think there will be a noticeable mpg reduction?
It is 3.73 geared with a 700r4 and a 350
Click to expand...


any thoughts on either a tbi 250 or tbi 292 for mpg?
 
Most here do a 5.3 swap for power and MPG. a good straight 6 could be cool though:thinking:
 
black dawg said:
any thoughts on either a tbi 250 or tbi 292 for mpg?
Click to expand...

As I recall, those motors didn't get any better economy than the 305's or the 350's. It probably all comes down to the weight of the vehicle, gearing, and aerodynamics. A small motor needs to use more fuel to get the same amount of work out of it as a larger one.

The same would hold true between carbed and TBI trucks, even with same displacement. Economy gains didn't happen. More gains came from OD and better gearing than the motor changes.
 
dyeager535 said:
As I recall, those motors didn't get any better economy than the 305's or the 350's. It probably all comes down to the weight of the vehicle, gearing, and aerodynamics. A small motor needs to use more fuel to get the same amount of work out of it as a larger one.

The same would hold true between carbed and TBI trucks, even with same displacement. Economy gains didn't happen. More gains came from OD and better gearing than the motor changes.
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everybody tells me the 292 wont do good on fuel, and I have never been around one, but everything I have heard and seen says that the 250 will do pretty good.

The 250 I have experience with is in a 80 k10 4 speed and 3.73 gears.
gets above 14 in town (wich is what this burb does) and 17 on the highway. Is this not normal?
 
The '79 C-10 I had wuth a 250 six got between 14-16 mpg,it had the camel humper integrated head engine with the 2SE Rochester carb ,but I put a 600 Edelbrock on it with a home brewed adapter,and dissconnected the secondary linkage..that got a bit better mileage,about 16 - 18 mpg.....................................................................................................After that engine lost the top ring land on the #6 piston,I put a 305 in it from an '86 G-10 van,the gas mileage was no different,and it accelerated like a dragster compared to the six,though the six seemed to pull hills better................................................................................................All the 292's I knew of that friends had,didn't get much over 14 mpg,they had SM465's and low rear end gears,but any long stroke engine usually isn't a mileage champ--the torque is nice though,and worth the extra cost for gas,IMO....but a 454 will get the same or beat 14mpg if its driven and tuned right too...I've had two of them with 450 Holley Economaster carbs and they would do 15 mph on a trip if you didn't use the secondaries--that was before we had ethanol gas too though............................................................................................I think a 4.3 V6 is probably better for mpg than a straight six is,and they come with EFI ,so thats probably an easier swap in the long run....how a chevy six would do with EFI mileage wise is an interesting subject....GM had some in Brazil I think,there must be a reason why they dropped the straight sixes and went to V6 engines I assume..maybe they didn't pan out emission wise or get the MPG they hoped for...or they just plain lasted too long!..those sixes run forever with any care...
 
black dawg said:
The 250 I have experience with is in a 80 k10 4 speed and 3.73 gears.
gets above 14 in town (wich is what this burb does) and 17 on the highway. Is this not normal?
Click to expand...

No offense, but I'm pretty leery of anyones claims about mileage. I just don't know the vehicle enough to know that the odometer works as it should.

But, if that mileage is true, that's pretty good in one of these trucks. 17 on the highway without OD and 3.73's is actually pretty darn good. I can barely equal that with 3.42's. But I can, so is it worth losing the power of the V8? I don't have any numbers in front of me, but I remember seeing a comparison somewhere, and I don't remember any of the smog era straight sixes beating any of the V8's offered at the same time, to include the 305.

People have spent a lot of time and effort on injecting the sixes, and I think any "off the wall" swaps like that are cool, but unless you ARE going solely for the cool factor, the V8's just make more sense all the way around.
 
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