Going from 30 inch to 31 inch tires…power loss? (1 Viewer)

[sproggy]

Hi all,

I've got some 30 x 9.5 x 15 inch Goodyear Wranglers on my 71 fj40. Lots of power from my stock F engine and I assume stock 4:11 gearing and acceleration is great but they are….wait for it: over 30 years old. So I have an appointment this afternoon to install some 31 x 10.5 x 15 Ohtsu AT4000 tires on these same rims. I have never heard of this brand of tire (made by Falken) but the price is only $90 per tire and since I don't drive the truck much at all (I live out of the country) it seemed a good option to be driving on something safer than 30 year old, worn out Wranglers. Plus, the bigger tire will fill out the wells nicely. I do have some lift on the truck so they will certainly fit height wise. Will they bump the TRE? Hmm, I will have to test it.

I'm wondering about power loss. I have read some threads on here indicating that it will not be significant but I would feel even better if some folks could chime in about this. I don't want to lose the snappy feel and power I have right now. The guy at National Tire and Battery (where I'm getting the tires) mentioned that they are heavy…something about the load carrying capacity. I don't know how much heavier they are than the Wranglers. Is that heaviness an issue?

Thanks so much….Sproggy.

 

[JohnnyC]

you'll never notice any power loss

 

[sproggy]

JohnnyC....just the words I was looking for! Cool! Thanks for responding.

 

[Downey]

You are going 1/30th larger, but tire tread pattern/grip factor/amount of air in tire could all distort any direct relation between tire diameter and usable h.p.???

 

[x463]

Actual tire sizes can vary quite a bit from the "stated" size. Your current 30" tires may be larger or smaller then 30"....same with your new 31" tires. Difference could be more or less then 1" in reality....but regardless I agree getting off those 30 year old tires is the most important!

 

[65swb45]

My best fuel economy was with 31s. Economy is related to efficiency, which is related to power.

 

[Output Shaft]

You'll feel the difference slightly.
I can definitely tell the difference between new 31" tires with full tread vs same tire when 1/4 tread is left. I like the feeling better with less tread ... Smaller circumference.

 

[skidplate]

You'll be very happy with the 31's.

 

[sproggy]

Thanks all. I bought the truck when I was 17 I guess, or perhaps even 16, in 1985 I believe, and put the new tires on right after that. So they are actually older than 30 years. Then it just sat in a barn for decades since the mid 90s as I was living away and now out of the country. I got it back on the road a few years ago for a brief time when I was home. I was planning to sell it but...fell in love all over again. I didn't realize the old tires were so dangerous until I saw the dry rot cracks and read up on old tires. Yeah, happy to get some new ones and only gonna be about $470. Nothing fancy and the wheels aren't going to win any ribbons but it will be a great improvement, no doubt.

 

[JohnVee]

I doubt it's likely you'd notice any possible difference if you're away from it for long periods of time.

 

[JohnnyC]

if you do notice a difference it's because your looking for it... And the slightest thing that could be ... Will be... But rest assured it's all in your head ... There is no difference ... Your just loosing it


It's the fumes

 

[sadam_husain]

its all pretty subjective and comes down to personal opinion but with 4.1 diffs I've always found 30" ish tyres too lower geared, around 33" tyres gives a good gearing and performance to match the torque curve of the engine, I'm not saying you should put 33" tyres on it but that's where I've always found the sweet spot is

 

[Tancruiser]

Your coolness factor will far out way any power loss.

 

[sproggy]

Got the new tires....ride looks...COOL! For me the look and size is pretty perfect. The fill the wells really, well, well. The ride seemed a lot bouncier than on the old 30s. I only drove it home about 10 miles from the tire place but that's what I noticed. Handling was better and power seemed absolutely unchanged. I'm loving them, so far. I will say that I was hoping the bumps would be friendlier but that didn't seem to happen. The smaller ones are bouncier but the bigger ones are still rattling my bones. I have to figure out why it is coming down so hard on them. It never used to be so hard. They aired the tires to 35 psi, I believe. Should I run them at a lower psi? I'll post a pic tomorrow.

 

[JohnnyC]

35 is high

28-30 is where I am

 

[tstepp920]

Photos please? Any rubbing on TREs or rear wheel well? Are they on stock 15"wheels

 

[shawnfj40]

If your 1F has
"plenty of power" you will never notice a change.

 

[sproggy]

No, they are not stock wheels, yes they are 15 inch, 9 inches wide. The pics are with the new 31s. I don't believe they are hitting the TREs. We checked in the shop when the truck was up on the rack and they did not touch. But maybe under load on a turn…I don't know. To tell you the truth, I am not completely sure what I am looking for when checking for hitting. but they are only 1 inch bigger than what I had on there for years so I don't notice any difference as far as turning radius, etc… I do have Rancho springs on the back and some shackle lift and I think…a helper leaf in the front. All this was done so many years ago I can't really remember. I do need some new shocks, however, and I'm wondering how I'm going to get the length correct…not knowing my lift.

 

[shawnfj40]

If you approach it from a reasonable position, you could probably say that you have a 2 1/2 inch lift. Even if you are a little off (doubtful) you will still be in the ball park.

 

[sproggy]

Is that based on looking at the photos, Shawn? I would think...the Rancho springs in the back were a two inch lift plus a 2 inch shackle lift. And the front, ya, I would say 2.5 as you suggest. Would being off by 1.5 inches be enough to ruin new shocks? I'm not really doing any heavy off-roading in the rig. Those days are on hold for now.