I recently bought a '95 F150 XLT with the 5.8 that the prev owner used to tow a boat. Was on a lot so I cannot verify, but was told he swapped the rear. I went by a radar sign and it indicated 53MPH while the gauge (bouncing around quite a bit) seemed to be about 60 so I figure it prob has a 4.10 rear now.

The truck has two main purposes, haul stuff like mulch, dirt, bulky stuff locally and to use to run to the airport each week (mostly in the winter) so that my Mercedes doesn't have to sit outside. The latter is my concern as its revving more than I want on the Interstate.

The truck has the stock 235s on 15s. How much of a RPM drop can I expect from something taller and what is recommended for a truck without a lift? I was thinking 31x10.5 but maybe something bigger?

Thanks

 

if you have the 235/85 and drop to 31 you will gain but if you go to say a 33 whitch is almost equivalent you will so a slight drop but not much

 

I have a 95 F150 4X4 5.8 Auto 3.55 I will check my RPMs here tomorrow. My truck has 157,000 miles and the front coils are sagged out some, and I have 31X10.50X15's and have no issues with rubs. I'd say you could fit a 32 without much trouble at all. Unless your off-roading it, with some twist thrown, you should be good with 32's. I'll check back tomorrow, long as I don't forget,

 

the 235/75s you have on there now come out to about 29" tall. so if you were to go with a 32" tire, that 10% taller which means your engine will be revving 10% lower. so if you're currently revving 2500 at a given speed, the bigger tires will put you at 2250 at that same speed.

 

I went from 235/75 x 15 to 31 inch tires and I have a factory 4.10 and the RPMs dropped 150 RPMs @ 60MPH.

It is now taching 2150RPM @ 60MPH in overdrive.

 

sounds about right. its simple math. i just used a 3" increase in tire size cause it was more convenient to show the math.

if you want bigger gains, you could consider swapping out the axle to something with taller gears. again, simple math will get you to your answer of RPM differences. you can usually find a used rearend for 2-300 and swap it yourself in an afternoon assuming you have average skills, a decent set of tools, and aren't afraid of moving heavy objects.
with an axle swap, you can find gear ratios as far as about 2.5 - which would be too far for you. sounds like you're doing some light hauling, so somewhere around 3.00 to 3.25 would probably be the ideal ratio for you

 

The stock tire size for your truck is 31x10.5x15, my 94 Bronco had that and my 96 F150 has the same size, so I doubt if your truck is any different.

I worked at discount tire for a year, and every truck this body style that came in had the 31 as a stock tire size.

 

The speedometer on these trucks is only affected by tire size.

I have 31 10.50 and its just about the perfect size. The stock tires really are a bit on the small side.

Stock size would have been 235 75 15. You can probably see this on the decal on the door jamb. Some Broncos and 4wds got a metric size that is close to the same as a 31 10.50.

You can reprogram the PSOM for the tire size.

 

x2. The PO must have changed it at some time or you have an issue. If the speedo is bouncing, look and see if the speed sensor in the diff is ok. If it is take the diff cover off and check the tone ring. If the teeth are damaged there is your issue. After that, if its off, the speedo needs to be re-calibrated.

 

I might go the route of the taller rear vs a lot of money for new rubber. I will check the rear for the speedo pickup also. Looking like around 2000 RPM when I am on the highway cruising about where I want to be. 150 rpm doesn't sound like much but it prob would make a noticeable difference