Gear Ratio and Tire Size

[brentleedragon]

First, How can I tell what gear ratio I have on my truck?

Idk if there was a stock for 79 F-150 302 C6...and I can't remember how to check at all.

Second, Tire size how much is gear ratio and tire size combination going to affect higher RPMS?

The reason I ask is I know when I used to run 235/75 R15 that I could climb hills perfectly fine at 65+

About three years ago I got a set of tires that are off the top of my head are 255/70/15. And I haven't honestly driven the truck as much as I used to recently but I do remember when I got them put on and I notice it now that at 65+ the truck seems to not really want to be above that. So up hills my truck kind of pushes itself back down to 55mph. I have a tach on the truck and it seems to be about 2300/2500rpms at 65mph. I will definitely double check the rpms tomorrow when I am driving. But the most point is could my tire size be affecting this a friend of mine pointed out that it could be. I figured i'd ask you guys.

I need new tires and I would like to go to 31x10.5 r15 but I don't want to change ratio or anything so if anything i'll just go back to 235/75 r15. Or is it more likely I have another problem.

 

[fatherdoug]

Your 2WD 79 f-150 with a 302 motor is probably? going to have a 3.00 axle ratio. If the rear didderential is undisturbed, which is rare, there should be a tag on the back of the differential which states the gear ratio.

Increasing tire size to a 31" tire(stock tire was 29") will shave about 0.2 off the axle ratio, which would make the ratio a 2.80. It will also make your speedo read about 5 mph low. If the speedo says 60 mph, you are actually doing 65 mph. At lower speeds the difference will diminish, and vice versa.

There are some really good references under the manuals section of this forum that Duncan(DNFXLI) has scanned. Some of those reference materials show axle ratios available for various engine combinations.

 

[john112deere]

Quick back-of-an-envelope conversion suggests your new tires are only about 0.2" greater diameter than your previous ones. (235/75-15 = 28.9", 255/70-15 = 29.1")

Moving up to a 31" will make a difference, but I'm kinda wondering if your current concern is unrelated to tires.


*The other way to determine your rear-end ratio involves jacking up a rear wheel, and counting how many times the driveshaft rotates for each wheel rotation. I'm sure there's writeups on here that explain it better than I could.

 

[Alvin in AZ]

The two top tags are both off of 3.25 Ford9inch axles.
Second one down is off the '75 I ordered, it's got Limited slip.

Alvin in AZ

 

[Beach66Bum]

Hi Alvin, you can use the calculator at the top right hand corner of the home page and throw in all the combinations you might want to try. It's great information.

 

[Mil1ion]

The original tire size was L70-R15

P235 = 235mm across the tread

75 = 75% of 235 mm = 176.25 mm sidewall

R15 = Radial tire with 15" tire opening

I think you will find an L70-15 tire diameter was 30.86 "


Only ONE exact way to verify the ratio= pull and count the gears and then do the math

I suspect a 13 Spline pinion gear and a 39 spline ring gear.