Do you loose some power when you go from 235/75 15 size tires to 31x10.5x15?
I'm debating whether to get 30x9.5x15 or 31x10.5x15, I hear different things that when you go to the 31's you loose miles per gallon and you loose power on taking off, any truth to this and is it that noticeable?

 

i had 31s on my 89 when i had it and the only thing i noticed when i put them on versus the 235s was the speedometer was off like 7 mph. other than that they were good. and only a couple more bucks a tire.
'75 F-100 4x4, 4" lift
16x38.5x15 super swamper TSL
mini-spooled 9" rear end
390 w/ cam,headers,
and 4 barrell edelbrock

TRIED TRUE and FORD BLUE

 

You will barely notice a power decrease with 31s, the speedo will be off, but can be corrected with a new plastic gear on 91/lower trucks or just easily recalibrating the 92-96s. Go with the 31s hands down. And you will actually get a power increase. The larger tires lower the RPMs in a certain gear. If you were going 65 at 2100rpm, now you will do it in 1900 or so.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

It depends. If you have a tired old 300 or 302 you will notice it, going from 235/75R15's to 31x10.5's as a slight loss of power and mileage. You will definitely feel it. Little more sluggish on takeoff, and probably also on steep hills, and a loss of about .5-1 mpg. If you have a strong motor you will probably not notice it at all. Also, there is pretty much no difference in size between a 235/75R15 and a 30x9.50R15 so you won't feel that.

 

[updated:LAST EDITED ON 05-Sep-02 AT 03:29 AM (EST)]I have 33x12.50x15's on an 89 Bronco with a 4.9 and a 5 speed. Since I just bought the truck I really can't speak about the loss of power. It had a 4 inch body lift and the only rubbing come in a full steering lock. It rubas just a little on the radius arms. If you go to www.superford.org and go into the calculator section it will give you alot of information including a formula for calculating speed when putting on larger tires. I hope this helps

 

Thanks for all the info everyone!

 

I repeat myself, you will NOT see a power decrease with larger tires, it will increase your gas mileage. Same reason why 3.08s get better gas mileage than 3.55s!


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

Not necessarily! Two reasons. First of all, a bigger and/or wider tire has more rolling resistance. It takes more power to flex the tire with every revolution. Secondly, simply reducing the engine RPM does not always increase mileage either. OTW we would have really tall overdrives that make the motor turn about 1200 rpm on the freeway. You have to keep the engine at an RPM where it is most efficient, if you gear too tall you fall out of the band and mileage drops. If these weren't true, people with 44's and stock gearing would be getting the best gas mileage of anyone. Right?

 

I'm just trying to find a tire that looks good, I'm not going to be doing any off roading.
I will be taking some 3 to 4 hours trips on the truck once a month that's why I don't want to loose to much on the gas mileage!

 

You are correct in certain circumstances, but 29 vs 31" tires, you will see an improvement in gas mileage. The extra weight doesn't decrease the gas mileage as much as the lowered RPMs increase mileage. There are CERTAIN situations where bigger tires will decrease mileage, but OVERALL, bigger tires will increase mileage. You'd be suprised how much the lower RPMs will increase your mileage. Believe me an engine will use more fuel at faster RPMs than you think. Where your gas pedal is, means squat because your computer automatically feeds the engine fuel. You use more fuel at 3K RPM with your foot off the gas, than you do at 1500 with your foot on the gas. You are correct, in that if you get to big of a tire for the gear ratio, you have to use say, 3rd gear at 35 mph at 1500 RPM, vs when before you could use 4th gear at 1,200 RPM. That part you are correct on, but that part doesn't happen as often as you think, and keep in mind you're going faster per RPM with larger tires. OVERALL, 31" tires will get better gas mileage than 29" tires. Even if you had 35" tires on the same 3.55 gears, you will see an improvement in mileage, but your OD would be useless and your truck will be slow. The reason people change gears for larger tires is to restore their power, not for gas mileage. With larger tires on the same gear ratio, all you do is lower the RPM in relation to each gear and speed, you go faster per gear. Say at 4k RPM you could go 25mph in 1st gear, with larger tires, you can now go 35 at 4k RPM in 1st gear. Thus, an increase in mileage. Believe me, you will see an OVERALL improvement in gas mileage with 31" tires over 29" tires. My 32" tires get better gas mileage than my 29" tires, so there is your proof. However, when towing my 32" tires do worse than my 29" tires because of the power loss.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

On highway driving, with w/o towing, you will see a definite gas mileage increase.



Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

This is the exact same principle on gear ratios. If you have 3.08 gears, you will have better gas mileage than you will if you have 3.55 gears. It is exactly the same concept but with tires and not gears. Tires change the final drive ratio. 32" tires on 3.55 gears brings the final ratio down to 3.15, and the lower the number, the lower the power, but better the mileage, (to an extent). If you had 3.55 gears, and you changed to 3.15s (don't exist, example) than you will see the exact same changes as you would if you had the same 3.55 gears and put 32" tires on it. The added weight does not become as much of a factor as you think, it does when you get into much larger tires, but not these sizes.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

Thanks for all the info Mustang!

 

Sorry for the mult. posts, i have the habit of thinking of something else to say after i posted it.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

Sorry, for my first two posts, i ment you won't see a mileage decrease. You will see a power decrease. My mistake.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.