I have a '90 F150 4x4 equipped with the 5.8L, E4OD transmission, 3.55:1 R&P's and stock size 235/75r15 tires.

My future plans for the truck are to install Moog cc844 coil springs to enable fitting larger tires. At a minimum, I will be buying 31x10.5x15 tires. I know there will be an overall reduction in RPM's and the effective R&P gearing will essentially be 3.35:1. If I ever wanted to correct it, I could change to 3.73:1.

Telling my better half about my overall plan, she suggested going bigger with 33x10.5x15 tires. I know these would fit but I am concerned with the potential loss of drivability. Instead of 3.55:1 it would be more like 3.13:1; making overdrive nearly useless unless going downhill at high speeds. I don't have intentions of offroading the truck but would like to be able to tow a small boat or tent trailer or haul whatever I need to in the bed without the truck being a worthless on the road.

So my questions:

With said stock gearing, what is the max size tire you would recommend before regearing?

Is regearing an automatic requirement with 33x10.5x15 or larger tires?

 

There are too many variables involved for any gearing choice to be "automatic." Mostly it comes down to your expectations: what will you use the truck for and what trade-offs are you willing to make. And everyone will answer those questions differently, so you need to find your answers. Mine may or may not fit for you , and the same for others who might think differently from me.

That said, I had a '95 F-150 with a 351, E4OD and 3.55s and I went from the stock tires to 33s, the same as you are looking at. I was very happy with the performance / mileage tradeoff with that setup. It couldn't reliably pull overdrive at 50 - 55 mph, so on winding, hilly two-lanes I had to take it out of overdrive to keep it from hunting. I was OK with that. And it couldn't pull a load up a hill at 60 - 65 in OD, but I was OK with 3rd for that as well. At a steady 65 - 70 mph cruise it could haul my slide-in pop-up camper, towing my Jeep (about a 5000 - 6000 lb load), only shifting out of overdrive on bigger hills. And I was happy with the off-the-line performance as well.

It certainly wasn't the same as it had been with the stock 29" tires, and while I was satisfied with the performance, others may not have been. But for me, especially considering an estimated $2000 to have someone regear my axles, it was an easy call.

 

I agree with Nothing Special. It is completely up to the individual. I have a 94 F150, 5.8 and 3.55's and went from the stock tires to essentially a 31" tire and I hate the power loss. Re-gearing is definitely in my future.

 

You'll be fine with 31x10.50's. Check out my signature. I have the same engine, trans, and gearing you do riding on 31x10.50's and my truck ain't hurtin for power or acceleration by no means and tows full-size Chevy's just fine.
Hell I could do a burnout on pavement if I wanted to... not that I tried it once or anything...

 

I see you live in Washington. So mountain driving is probably in your plans.

You will have to drive it and see. As others said, everyone is different

If it was all flatland you would probably be ok.

 

Agreed with the others. it is what YOU expect out of it. i would never drive a small block with anything higher than 4.10 and stock tires.

 

With an F150 33" tires requires 4.10 gears and a speedo recalibration IMO, but at 31" you can get away without the gear change.

 

Here's a handy calculator I was just playing with that lets you "what if" all you want...

Calculators

 

Thank you for that calculator.

I've also applied my inner-geek-ness to Excel and made a conversion calculator to compare RPM's across the board in all gears based on tire size, R&P, speed & tranny gearing. From the stock 235/75's, the 31's would drop my RPM's on average 119 RPM's but the 33's would drop 242 RPM's.

I think to be safe, I will just level the truck with new coils and run 31's.

Thank you all for your help.

 

i guess tranny might make a difference but my truck was a 5spd and i went from 31's to 35's on stock gearing

 

i have a 90 with the 4.9 and went to 31". really didnt notice any difference at all. i do have about a 4 mph difference on my speedo. but as for power and acceleration, well it feels the same. i do remember a friend that had a 90 bronco with the 5.0 V8 and went to 33" tires, i drove it once to two a boat home, seem to not have much power but then i was towing a 18' jet boat with a 460 engine and a lot of it was uphill.

 

I made the exact same change from stock size to 31's with a 351 and 3.55 gears years ago on my old truck. Right after I made the change, I thought about changing gears to adjust for the tires. After driving for a while, I forgot all about it.

My current truck has the same engine, but 3.08 gears and 32" tires. This is borderline acceptable if you run on flat ground and never ever drive loaded, It really does depend on what you can tollerate.

I would suggest getting the tires you want, driving the truck for a while, and deciding if it's good enough for what you want to do. If it's not, don't bother going from a 3.55 to a 3.73, because you are spending a lot of money for a small change. Go straight to a 4.10. If a 3.55 is unacceptable, 3.73 will be borderline.

 

agree, if you dont like it then go with the 4:10