I'm thinking about going from my 18" Lightning wheels to 20" Harley wheels. Will I have to worry about my speedo being off? I'm not sure of the size tires I have on the Lightning wheels right now off the top of my head but I know they are dead on as far as being accurate (GPS tested). I'm wondering if I upgrade to Harley wheels if I should have to worry and if it will help to go with a specific size of tire.

Thanks for any advice.

 

It's the height of the tire that matters, not wheel size. If you go to taller or shorter 20 inch tire, your speedo will be off. How much depends on the size difference.

 

OK, maybe I should have worded that differently. When I upgraded from my factory 16" wheels to the 18" Lightnings the tires I got matched my stock wheel height so close that my speedometer is dead on. Any idea what size tires I would need to go with to keep my speedo accurate with the 20" Harley wheels?

Also, say that it is off. Any idea how to correct it? On my Mustang you would go with a different speedo gear. On newer Mustangs it's electronic. I have no clue what I would do with my 2002 F-150 to get the speedo accurate.

 

Your speedo is electronic also. It can be corrected to limited tire heights by the dealer. As for a tire size, you didn't say what size 18 inch tire you have. You could always go to tire manufacturers websites and look at the specs on different sizes.

 

2002 S'Crew,
Assuming you began with 255/70R16's, you had a 30" diameter tire. If you had a 235/70R16, you had a ~29" diameter tire. With your 18" L wheels, I believe the stock tire that came on them was a 295/45R18 (?), which comes to a ~28.5" diameter. If you started with the 235's, and used the stock Lightning tires, then it was probably close enough that you saw no difference. If you started with the 255 tires, then you should have noticed that you were going slower than indicated on the speedo. Not much maybe, but some. If you don't want to recalibrate your speedo, you basically need to stay with as close to the original diameter tire as possible. Obviously, as the wheel diameter increases, the tire height has to decrease to compensate. I've done my own 'what if' for my truck if I installed 20's, and what I came up with (my truck came with 255/70R16's = 30") was a 255/50R20. That comes to almost a perfect 30" diameter. I know this has probably been posted many times before, but here's the way to calculate the diameter of any metric tire:

if the tire size is given "A"/"B" R "C" (on a 255/70R16 tire, A=255, B=70, and C=16), then:

divide "A" by 25.4, then
multiply that by "B", then
divide that by 100, then
multiply that by 2, then
add that to "C"

so 255/25.4 = 10.04
10.04 x 70 = 702.76
702.76/100 = 7.03
7.03 x 2 = 14.06
14.06 + 16 = 30.06

I don't mean to over-simplify if I did. I think the stock tire size for the 20" Harley wheel is a 275/45R20 (?), which gives you 29.7" diameter. It more comes down to what you can afford. Twenties aren't cheap to start with. You seem to pay more as the second number ("B") goes down, so a 45 series tire will cost more than a 50 series tire with all other parameters equal. If you're determined to go with a 20" wheel and you don't want your overall diameter to increase accordingly, then you're looking at low profile tires, which means $$ no matter how you slice it. Hope I've helped some. Good luck.

Fordman3

 

I installed 20" harley takeoffs onto my 2001 F-150 I had no problems and they were the same as my factory 17" wheels. it looks nice though having my '03 2 tone H-D and my '01 lariat with Harley wheels sitting in the driveway. I want to paint the '01 stepside black and put on a H-D front bumper to really mess people up.

STOFFER