I just upgraded my tire and wheel package from stock (275/55R20) to (295/70/R17). I know my speedometer and shift points will be off, and I believe the Hypertech spedo programmer will fix that. But how will my stocl 3.73 rear end be affected? My diameter has gone from 31.9 (stock) to 33.3, width 10.83 to 11.61, and circumference from 100.2 to 104.4. I know its not a huge increase, but not sure if I need to "re-gear". Any help would be great.

 

You've effectively made yer gearing 'taller' by the same percentage you increased the diameter/circumference.

Hence - despite yer corrections fer speedo/odo/shift points, yer truck will be a tad less responsive. The weight you've added is also a factor - both static and (more importantly) rotational inertia. Add in increased rolling resistance, aero effects as well.

Did you weigh the existing rolling stock, and the replacements? You should have.

Only you can gauge whether or not this aggregate impact is acceptable to you from a drive-ability perspective. And whether a regear is worth it (~$2,000 fer both ends total - give or take). Likely not = unless you tow very heavy & often.

And - it's the reason folks buy fully-featured custom programmers and not waste money on simplistic recalibrators which add no power.

Lastly - nothing will change the fact that yer mileage WILL take a hit.

 

The tires are heavier than the stock Perelli's and the new rims are lighter. Not sure what the total difference is. The spedo will be off only slightly 20 MPH stock to 20.9, 30 MPH to 31.3, 40 MPH to 41.8, a bit more off at higher speed, 70 MPH to 73.1. Hence the reason I will be changing the spedo calibration. I have always been hesistant to do a full tune, as I don't want to mess with the stock settings. I don't think I am going to mess with the gearing at all.

 

Whaler, the math is pretty simple. You've increased your tire diameter (or radius) by just under 4.4%. The "go force" the taller tires deliver to the road at a given engine speed is reduced by that same 4.4%. To recover the effective gear ratio you had before, you'd need to increase the gear ratio by the same percentage: 3.73 x 1.044 = 3.89. You aren't going to find that gear ratio anywhere that I know of so you'd be looking at 4.10 gearing if you decided to regear.

The increased rotational inertia of the heavier wheels add to the total power loss, so I'd estimate you might see up to 5% less total power, but the narrower tread provides less rolling resistance, so maybe that's a wash and the power loss is 4.4% also.

Additionally, your speedometer error should be 4.4%, which gives you the 73.1 mph when your speedometer says 70. (The axle will make fewer revs per mile, so the truck will indicate a slower speed). However, F150 speedos read about 2-3% fast due to the errors involved in converting a digital signal from the PCM to the analog needle, so your new tires will probably only give you an observed error of maybe -1 mph at that speed. I don't think it's anything to worry about, personally.

Your odometer WILL be off by 4.4% though. So you'll see 95.6 miles on the clock when you've actually traveled 100 miles. Keep that in mind when you figure your gas mileage. But, as MGD said, you WILL see a drop in economy due to the engine having to operate at a higher load.

- Jack

 

MGD

 

I was confused too - obviously! I got hung up on the aspect ratio and read "55" to "70" as a switch to a narrower tire.

Ah well, it's nice to be kept humble.

So, to revise my earlier post, you have a wider tread, so there is MORE rolling resistance as well as increased air resistance and more rotational inertia. All of these will contribute to a loss of power and reduction in gas mileage over and above the 4.4% that the change in diameter causes.

With that in mind, I'll revise my estimate of power loss back to somewhere between 5.0-5.5%.

Thanks, MGD!

- Jack

 

No worries, Sir Jack.

These events are ever so exceedingly rare, that I cherish them

And - thankee fer cipherin' th' mayth fer Ol' Bubber - he be feelin' a-might poorly wif da vapors an' coon't cot wif it, no how.

Wellsir, back to me self-medicatin'

MGD

***EDIT*** - OP - just noticed that you already have an open thread on this topic - AND you already kinda received a partial answer in it (such as it is and what there is of it). Why'd you feel the need to start another - you could (and should) have just asked any additional Q's in yer existing thread.

Perhaps ask (PM) Sir Jack to merge ye tow threads fer good housekeepin' and to avoid cornfusion down the road. Plus - it's a Forum Decorum kinda thang, y'know?

This'un hyur: https://www.f150online.com/forums/20...l-upgrade.html

Thankee - Ol' Bubber