Learn about gear ratios
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Junior User
Joined: Dec 1999
Posts: 58
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From: Hubbard USA
Learn about gear ratios
Could someone give me some basic info on gear ratios, like what the main use of each is, how they differ in operation.
My F250 has a 4.10 rear. I'm not planning ogn changing anything,
just want to learn more about my truck.
Thanks.
My F250 has a 4.10 rear. I'm not planning ogn changing anything,
just want to learn more about my truck.
Thanks.
Postmaster
Joined: Dec 2002
Posts: 3,959
Likes: 9
From: USA
Assuming the truck has stock size tires (because tire diameter is part of the gearing), a 4.10 ratio is good for towing and off roading. This is considered a 'low' ratio. A 'high' ratio, like 3.55, is good for highway mileage but doesn't give as much towing power. 3.73 is a good in-between ratio that gives a mix of power and economy.
As tire diameter increases, the ratio needs to be lowered, meaning go up numerically. The same truck with 35" tires would need 4.56 gears to get the same performance as a truck with stock tires and 4.10 gears.
One function of gearing is the lower ratio, the higher the engine speed at any vehicle speed. So a truck with 4.10 gears spins higher rpms at say 60 mph than a truck with 3.55's, assuming they both have the same size tires and the same transmission gearing.
When you have a truck with an overdrive transmission, you can use a lower axle ratio because the overdrive gear lowers the engine rpms at cruising speed. That way you can have the best of both worlds- the power of low ratio gears and the economy of high ratio gears, all in the same truck.
As tire diameter increases, the ratio needs to be lowered, meaning go up numerically. The same truck with 35" tires would need 4.56 gears to get the same performance as a truck with stock tires and 4.10 gears.
One function of gearing is the lower ratio, the higher the engine speed at any vehicle speed. So a truck with 4.10 gears spins higher rpms at say 60 mph than a truck with 3.55's, assuming they both have the same size tires and the same transmission gearing.
When you have a truck with an overdrive transmission, you can use a lower axle ratio because the overdrive gear lowers the engine rpms at cruising speed. That way you can have the best of both worlds- the power of low ratio gears and the economy of high ratio gears, all in the same truck.
Last edited by SoCalDesertRider; Jan 30, 2005 at 07:35 PM.
Post Fiend
Joined: Jul 2003
Posts: 11,945
Likes: 3
From: Spokane, WA
Any time you increase your tire size withouth changing your gearing you will lower your overall RPM. A 4.10 ratio means your driveshaft has to turn 4.1 times to make your tires turn once. 3.55 turns the driveshaft. 3.55 times for the wheels to turn once, etc. Your drive gear (not overdrive) will turn your driveshaft at the same rate your engine turns, usually 1:1. So if you're cruising down the freeway at 2000 RPM with overdrive turned off you are turning your driveshaft 2000 times a minute. Divide that by 4.1 and you will see how many times your tires spin per minute. 
If you have a larger tire it doesn't have to turn as fast to move you the same distance so your RPM drops.
If you have a larger tire it doesn't have to turn as fast to move you the same distance so your RPM drops.
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