Wheel size effect on transmission

[Belsh]

My truck has 20'' wheels with 275/40 tyres, by my rough calculation 1 revolution travels approx 80''. As the trucks came with 15'' wheels and 75?? rubber that would equate to around 70''. Does this put extra strain on the transmission and does it matter? I realise it will throw the speedo out

[72SB]

if you have a high gear ratio rear end, the 20's will add stress to the trans, especially if you stomp on pedal or tow anything. "Normal" driving should be OK

Speedo can be corrected by swapping in a new driven gear with the correct tooth count for your tire size, rear gear and drive gear

[Belsh]

Originally Posted by 72SB

if you have a high gear ratio rear end, the 20's will add stress to the trans, especially if you stomp on pedal or tow anything. "Normal" driving should be OK

Speedo can be corrected by swapping in a new driven gear with the correct tooth count for your tire size, rear gear and drive gear

I have no idea of the gear ratio on the rear end due to the fact Ive not long ago purchased it, its something I'll get into further down the line so thanks for the info.

Do you know where I would find a table to calculate the driven gear for the speedo?

Cheers

[pjmoreland]

Originally Posted by Belsh

Do you know where I would find a table to calculate the driven gear for the speedo?

Here's one:

https://www.tciauto.com/speedometer-gear-calculator

[72SB]

Originally Posted by Belsh

I have no idea of the gear ratio on the rear end due to the fact Ive not long ago purchased it, its something I'll get into further down the line so thanks for the info.

Do you know where I would find a table to calculate the driven gear for the speedo?

Cheers

Your SPID label would show what the truck "came" with.....assuming sometime in its life the gears were not changed

You can just jack up rear so tires are off ground, truck in neutral, engine off, rotate PS tire forward. If DS tires moves same direction, you have posi, if not peg leg. On posi truck rotate PS tire 1 revolution and count how many times the DS rotates. 3 3/4 turns would be 3:73. A little over 3 would be 3:08. On peg leg truck rotate PS tire 2 times and count how many times DS rotates.

To calculate the "driven" gear, you need to know how many teeth the "drive" gear has. Basically 1 tooth in driven gear = 5mph +/- depending on adding or subtracting a tooth from driven gear. So if speedo is 10mph off at 50mph than you need a 2 tooth change, +/- depending on if speedo is reading high or low

['68OrangeSunshine]

Your gear ratio may be listed on the SPID. If not, standard ratios for half-tons [C/10, C/1500] are 3.73:1 for manual transmission, and 3.08:1 for automatics.

[geezer#99]

Your tires are 28.6 inches high.
Stock 15’s were likely 28 inches tall.
Darn near the same height.
No stress on anything.

[Steeveedee]

Originally Posted by geezer#99

Your tires are 28.6 inches high.
Stock 15’s were likely 28 inches tall.
Darn near the same height.
No stress on anything.

Yup.

['68OrangeSunshine]

Mr Belsh,
It would benefit all, if you could enlighten us as to the Make, Year, Model, GVW, Engine size, Transmission, and any other pertinent details...