I currently have my stock gears in my 88 F350 4x4. I am putting 37s and 6 inch lifton it this year and am considering some 4:56 gears. Would I still need to get a new speedometer gear or will they account for it? Will the 4:56 gears increase or decrease my top speed? I have 4:10 gears currently. Thanks

 

On older trucks like this vehicle speed is derived at the transmission/T-case output so both tire size and axle gearing affect speedo accuracy.

What is the stock tire size as listed on the drivers door pillar decal?
Do you want to preserve existing pulling power with this tire change or would you be willing to live with a little less power to gain a little better highway fuel milage?

 

Lower gears (higher numerically) make your truck go slower at a given engine speed (just like shifting the trans to a lower gear). It's just ratios, so compared to 4.10 gears, 4.56 gears will be 4.10/4.56 = 0.899 times (or 89.9%) as fast.

Taller tires make your truck go faster at a given engine speed. Again, it's just ratios, so assuming your stock tires are 31.7" tall (235/85-16), your 37s will be 37/31.7 = 1.167 times (or 116.7%) as fast (if you don't have 31.7" tires do the math with the actual size).

To get the net effect, multiply the results together. So 0.899 x 1.167 = 1.049 times (or 104.9%). You'll be going about 5% faster than your speedo will read. So when the speedo reads 55 mph you'll be going 55 x 1.049 = 57.7 mph.

Do you need to get a new speedo gear? You need to decide if the difference is significant enough to matter.

For anyone with a newer truck doing the math, as Conanski implied, the speedo will not be thrown off by changing axle gears on trucks with the VSS on the axle. Changing tire size will still change your speedo reading. The axle gears obviously still affect the engine speed and vehicle performance, but not the speedo.

And to be a little picky, you aren't putting 4:56 gears in. That would be read as "four to fifty six" and would be the equivalent of a 0.07:1 EXTREME overdrive. You are putting 4.56:1 (read "four point five six to one") gears in. That means that for every 1 turn of the axle the driveshaft turns 4.56 times.