change in tire, affect gas?

2442 Views 25 Replies 15 Participants Last post by pyrojoe22, Apr 24, 2008

flyingfirefighter

Discussion Starter · Feb 18, 2008

i have an 87 with the 22rec and i currently have 33x12.5x15 on it, it gets roughly 16-17 mpg. it i were to put 31x10.5x15, theoretically what mileage do yall think i would be looking at? thanks.

1 - 20 of 26 Posts

fourwd1

· #2 · Feb 18, 2008

When you put taller than the original stock (27-28") tires on, it throws the engine out of it's power band. MPG and HP suffer.

33's/4 cyl/5 spd is a common combination. Power and MPG go down a bit, but most people live with it.

As far as MPG, you have to take in to account the fact that the taller tires throw the speedo and odo out of calibration, so if you don't take that into account your stats are off.

80_pickup

· #3 · Feb 18, 2008

Bigger tires = higher to end speed

Smaller Tires = More low end tourque

If you have a 4 speed which maxes out pretty low on the highway, adding bigger tires will boost up your maximum Mph. In which could save you some mpg's.

Putting smaller tires on it will lower your miles per gallon, and raise power and torque. Because the fact that is it will be easier to turn the tires around, but when you get on the highway you are going to be running at higher Rpms because in order to get up to speed your engine has to turn the smaller tires around alot faster, in which once at highway speed, larger tires are going to be easier to turn and lower rpms at higher speeds.

I could be wrong feel free to correct me.

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DCM

· #4 · Feb 18, 2008

This in an age-old discussion. Bigger tires may reduce RPM's at highway speed, but how much harder do you have to push on the gas pedal to keep it there? My arguement is that smaller diameter tires (or deeper gearing) need less power to spin them. Because of this, the fuel economy increases.

flyingfirefighter

Discussion Starter · #5 · Feb 18, 2008

haha i appreciate everyones response. i know that bigger tires throw the speedo off, and i also know that smaller tires would increase rpm for the speeds im at now. i was hoping for a ball park figure from someone who has done this tire swap. if what was posted is all anyone can let me know, i appreciate yalls input, i guess when it comes time for me to change my tires ill know anyways, thanks again everyone

seandnw

· #6 · Feb 18, 2008 (Edited)

80_pickup said:
Bigger tires = higher to end speed

Smaller Tires = More low end tourque

If you have a 4 speed which maxes out pretty low on the highway, adding bigger tires will boost up your maximum Mph. In which could save you some mpg's.

Putting smaller tires on it will lower your miles per gallon, and raise power and torque. Because the fact that is it will be easier to turn the tires around, but when you get on the highway you are going to be running at higher Rpms because in order to get up to speed your engine has to turn the smaller tires around alot faster, in which once at highway speed, larger tires are going to be easier to turn and lower rpms at higher speeds.

I could be wrong feel free to correct me.

first off, how do bigger tires give you more top end speed?

and going to a smaller tire won't reduce your top speed, or "gain" any hp/torque. it will just get all that back from when you had the big tires...if the gearing is stock, which I assume it is, you won't lose any top speed, your RPM's would be higher if you drive it exactly how you drive your 33's, but then again I'm guessing you stay out of 5th gear quite a bit. With the 31's you will probably be able to use it more. Also you will "feel" more torque, and get slightly better gas mileage due to the reduced weight of the tire size.

its too early, but i think i covered a few there.

jowdba

· #7 · Feb 18, 2008

I agree with DCM.

But, unless can adjust the odometer for a change in gears or tire size, you can't be accurate with with the mpg calculation.

Or have a navigation system to aid in the correct calcs.

80_pickup

· #8 · Feb 18, 2008

first off, how do bigger tires give you more top end speed?

If you have a 4 speed, and on regular sized tires and it tops out at 65mph at high rpms, Theoretically if you put bigger tires on it once you get into 4th gear the taller tires are go farther with less rotations needed.

bad85toy

· #9 · Feb 18, 2008

I have 31" tires on my truck and i get roughly 18-19 mpg, as for speedo being off, i guess the guy that had the truck before me could have adjusted it but i dont think so and mine is dead on, but i dont know how much you can go off me cause i have the 4 speed auto with overdrive and it still wont hit 65 on the highway with 31's so you guys are all lucky if you can hit that will 33's.

jijiandfarmgang

· #10 · Feb 18, 2008

Hey,

If your running 33x12.50 tires, from stock yes it will change the final drive ratio by quite a bit, but it won't help you get better mileage with stock gears for sure.

Compare a stock tire to a 33x12.50 tire, theres so much more weight and rolling resistance for the 33'.

All you can do is guess what your mileage would be with different size tires because there are so many variables. I would guess 1-2 mpg difference.

flyingfirefighter

Discussion Starter · #11 · Feb 18, 2008

thanks

80_pickup

· #12 · Feb 19, 2008

bad85toy said:
I have 31" tires on my truck and i get roughly 18-19 mpg, as for speedo being off, i guess the guy that had the truck before me could have adjusted it but i dont think so and mine is dead on, but i dont know how much you can go off me cause i have the 4 speed auto with overdrive and it still wont hit 65 on the highway with 31's so you guys are all lucky if you can hit that will 33's.

hahaha, i can not even hit 60, you got me beat ! :thumbsup:
I have a 4 speed manual.

bad85toy

· #13 · Feb 20, 2008

80_pickup said:
hahaha, i can not even hit 60, you got me beat ! :thumbsup:
I have a 4 speed manual.

actually yeah thats about right i can sometimes get to sixty if im lucky..lol i have had it almost there but i was on a slight downward angle lol.

fourwd1

· #14 · Feb 20, 2008

Bottom line is, going from a 33 to a 31" tire gets you closer to stock, so the closer it gets to the engine's optimum power band.

Or you could keep the 33's and regear to 4.88, putting you back to "stock" and back in the engines power band. But you'd still have the increased rolling resistance of the larger tire.

I run 36's and 5.29 gears. The result is a stock final drive ratio, the speedo is dead on, and I can hit 65-70 on a straight flat road (with a worn out 22R) if I push it.

DCM

· #15 · Feb 20, 2008

^ which is a prime example of proper gearing and tires. There's nothing worse than big tires and tall gearing to make your 4-banger seem gutless.

turbo87toy

· #16 · Feb 20, 2008

Yall seem to have it.

I have a 5speed manual and 31s and regularly drive 70 according to my spedo and still get between 16-18 mpg.

bad85toy

· #17 · Feb 21, 2008

Well i guess gearing has to be my problem then. I guess when the truck gets parked ill just have to save some cash and get some different gears and get me away from stock gears cause my truck is deffinetly gutless

tooldudetool

· #18 · Mar 14, 2008 (Edited)

If you're wondering exactly how far off your speedo is depending on your tire size, here is a handy tire size calculator. I believe the stock tire size for my '87 was 225 75 R15, and my 31 x 10.5's make my speedo show 60mph when I'm actually doing 65mph... seems like a good thing to know!

It also shows you the percentage that you're speedo is off, so you could use that percentage to calculate your true miles per gallon, which may not be as good as you thought!

turbo87toy

· #19 · Mar 14, 2008

Im not 100 percent sure but I think if you just calculate your MPG without correcting for tire size increases it will give you an MPG lower than actual because your tire is covering more ground per revolution of the axle. If you use the calculator above and find the percentage your speedometer is off add 1 to that number then multiply that by your calculated MPG you should get your actual MPG.

wudshapr

· #20 · Mar 14, 2008

I feel for you guys not getting above seventy on the hwy. I run something close to stock on my 83, man. 4x4 and push it over 85 sometimes. I have been able to pull out of most situations in the mud, except for when I idled into a monster pit in four low just for shits and giggles, then sat and spun in the dead center of the pit. I go through up to about 18 inches of snow like its blacktop, but the five foot snowbanks still stop me. Point is, except for looks, or extremely serious mud bogging, what really is the point of huge tires? Old jeeps with little donuts got the military through just about anything. IMHO, i prefer all around performance for my DD, on the road, hwy, trail, or pits, and a good set of winter weather radials covers all those bases fine for me. But then again, this thread doesnt have anything to do with what I like to do, or small tires, so um, I guess I'll cool it.

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