Bret Linden

Hello,

Here's my situation....

I bought the Hypertech Max energy for (among other reasons) to calibrate my speedometer and odometer. I do not have the stock tires. The ones I'm running are roughly 1.5" greater in diameter. I didn't put these tires on. I am the second owner of the truck. The door jamb says I "should" be running P265/60R18, and I'm actually running on P275/45R22

According to the tomtom, my speedo was previously roughly 2 mph slow. I blamed the larger tires for this discrepancy. Now that I calibrated with the Max Energy (well, I could only come within .02", 31.73" vs. 31.75"), my speedo is about 2-3mph fast (again, according to the tomtom).

Now, what I'm thinking is perhaps the previous owner changed rear gear ratios on me. I have no way of contacting him to ask. I know this is a long shot anyway, so I don't want to disassemble the entire rear end just to look.

My question is is there an easy way to see what gear ratio is actually installed and not just what the computer thinks is there?

Bret Linden

My bad...30.52" diameter (stock) vs. 31.73" diameter actual. Not an inch and a half, about an inch and a quarter

FoX2012

Get the rear up in the air and rotate the rear tires one full revolution and count how many times the drive shaft spins

Bret Linden

That sounds great. But wouldn't this only work if I had the stock tires/wheels still on?

azharley

It will work with any size wheel tire. One revolution of the tire is still only one revolution when the truck is in the air.

shortride

iTrader: (1)

It easiest to see when you put a chalk mark on the outer tire and on the driveshaft.

NASSTY

Did you program in the actual diameter or did you allow for some tire squat? I've heard of some people subtracting 2-3% from the actual diameter for squat.

Bret Linden

I programmed what the Nitto folks said was the diameter for a fully inflated tire. Once I check the gear ratio, if it is still off I think I'll take this idea.

Thanks for all the replies. I usually search before asking, but to be honest this time yesterday the thought of something so simple and easy as this method existing seemed so remote I didn't bother. Lesson learned I guess.

I'm really glad I found this site. And, god help me...a lifelong GM guy coming from a GM family...I'm glad I'm driving a Ford! First On Race Day!

JVPPC187

If it is the stock gears that came with the truck, then just go to the website below and punch in your VIN number.

http://researchmaniacs.com/VIN-Numbe...cker/Ford.html

That'll bring up the original window sticker that should state what kind of gears came with your truck.

Bret Linden

Wow, that's a great link. Thanks.

Both that sticker and my door jamb (axle code 19) told me that my stock ratio should be 3.55.

Due to rainy weather, and my not wanting to crawl on the ground in puddles of water, I didn't jack it up today. I did, however, use a formula I saw online. http://www.crankshaftcoalition.com/w...ar_ratio_check
I used the "first method".

Went out for a drive today. At 2000 rpm even, my Tomtom said 74 mph and my speedo said 75. So I calculated both numbers. For the tomtom number I came up with 3.91, with the speedo number came up with 3.97. So, either my math is off, or the previous owner did indeed install different gears. Here's what gets me, though...

My stock gearing should be 3.55. The next two up (down?) from that are 3.73 and 4.1.