Help me understand tire size vs gear ratio

[The Jester Race]

Iv been reading about tire sizes and gear ratios and I am thoroughly confused. Lets say you have 28" tires and 3.55 gears, if you run 33" tires with the same gear ratio, how will this change effect top speed, fuel mileage, acceleration, or anything else? Now what if you kept the 28" tires and changed the ratio instead, to 4.56, how will that effect things?

thanks

-Jester

 

[Wicked_Sludge]

you can think of gears and tires as levers.

when you go from a 28" tire to a 33" tire, your increasing the length of the lever, which gives the road more leverage against the rotating axle (harder for the axle to spin the tire).

numerically lower gears have the same effect. a 3.55:1 gear ratio gives the axles a larger lever to work on the driveline than, say, a 4.10:1 ratio...making it require more force for the driveline to turn the axles.

the trade off in gearing and tires is cruising RPM. if you ran a very high gear ratio like 4.56 with a tiny 28" tire, you would have amazing acceleration, but you'd be spinning the engine at a million revolutions per minute just to go 55MPH down the highway. likewise, if you ran a 33" tire with 3.55's, you would have very poor acceleration, but you would have a very low highway cruise RPM. ideally, you want to match your tire size and gear ratio for decent low-speed acceleration, but also a sane RPM at highway speeds.

heres a chart showing the relationship of tire size, gear ratios, and engine RPM:
http://www.therangerstation.com/tech_library/gearandtirechart.html

 

[The Jester Race]

Thanks Sludge, I think I got it now.

-Jester

 

[07rangersport]

you can think of gears and tires as levers.

when you go from a 28" tire to a 33" tire, your increasing the length of the lever, which gives the road more leverage against the rotating axle (harder for the axle to spin the tire).

numerically lower gears have the same effect. a 3.55:1 gear ratio gives the axles a larger lever to work on the driveline than, say, a 4.10:1 ratio...making it require more force for the driveline to turn the axles.

the trade off in gearing and tires is cruising RPM. if you ran a very high gear ratio like 4.56 with a tiny 28" tire, you would have amazing acceleration, but you'd be spinning the engine at a million revolutions per minute just to go 55MPH down the highway. likewise, if you ran a 33" tire with 3.55's, you would have very poor acceleration, but you would have a very low highway cruise RPM. ideally, you want to match your tire size and gear ratio for decent low-speed acceleration, but also a sane RPM at highway speeds.

heres a chart showing the relationship of tire size, gear ratios, and engine RPM:
http://www.therangerstation.com/tech_library/gearandtirechart.html

excellent explanation