Tires Too Big?
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Tires Too Big?
I have a '96 I6 with 33x9.5x15 tires and I'm just wondering if or how much power I am losing with these bigger tires compared to stock 235's. My truck is completely stock other than the tires. Thanks.
Moderator
Joined: Dec 2007
Posts: 7,987
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From: Boise, Idaho
What gearing you have makes a big difference.
The 235s are 29" in diameter.
Going from a 29" tire to a 33" tire really throws off your gearing. (By about 14%)
This means your engine has to work harder to get your vehicle moving (much like if you started out in 2nd gear instead of 1st. Same idea.)
It also means that at the same speeds you used to drive at, your RPMs are now lower, and you may be bogging the engine.
The EFI 4.9's peak torque is at 2000 RPMs and lets say, for example, if you used to be at 65mph at 1800 RPMs, you'll now be at 65mph at 1600 RPMs. That means instead of 200 RPMs below you're peak torque, you're now 400. So, your engine has less strength, and has to work harder to maintain the same speed.
This is why people regear their trucks when they put on larger tires.
If you currently have a 3.55 rear end, a 14% increase would bring it to 4.05 (closest to that is 4.11s). That'll put your engine's RPM range right back to where it used to be and you'll have most your power back (minus the extra power it simply takes to get heavier tires rolling).
The 235s are 29" in diameter.
Going from a 29" tire to a 33" tire really throws off your gearing. (By about 14%)
This means your engine has to work harder to get your vehicle moving (much like if you started out in 2nd gear instead of 1st. Same idea.)
It also means that at the same speeds you used to drive at, your RPMs are now lower, and you may be bogging the engine.
The EFI 4.9's peak torque is at 2000 RPMs and lets say, for example, if you used to be at 65mph at 1800 RPMs, you'll now be at 65mph at 1600 RPMs. That means instead of 200 RPMs below you're peak torque, you're now 400. So, your engine has less strength, and has to work harder to maintain the same speed.
This is why people regear their trucks when they put on larger tires.
If you currently have a 3.55 rear end, a 14% increase would bring it to 4.05 (closest to that is 4.11s). That'll put your engine's RPM range right back to where it used to be and you'll have most your power back (minus the extra power it simply takes to get heavier tires rolling).
Elder User
Joined: Dec 2003
Posts: 660
Likes: 3
From: Northern California
What gearing you have makes a big difference.
The 235s are 29" in diameter.
Going from a 29" tire to a 33" tire really throws off your gearing. (By about 14%)
This means your engine has to work harder to get your vehicle moving (much like if you started out in 2nd gear instead of 1st. Same idea.)
It also means that at the same speeds you used to drive at, your RPMs are now lower, and you may be bogging the engine.
The EFI 4.9's peak torque is at 2000 RPMs and lets say, for example, if you used to be at 65mph at 1800 RPMs, you'll now be at 65mph at 1600 RPMs. That means instead of 200 RPMs below you're peak torque, you're now 400. So, your engine has less strength, and has to work harder to maintain the same speed.
This is why people regear their trucks when they put on larger tires.
If you currently have a 3.55 rear end, a 14% increase would bring it to 4.05 (closest to that is 4.11s). That'll put your engine's RPM range right back to where it used to be and you'll have most your power back (minus the extra power it simply takes to get heavier tires rolling).
The 235s are 29" in diameter.
Going from a 29" tire to a 33" tire really throws off your gearing. (By about 14%)
This means your engine has to work harder to get your vehicle moving (much like if you started out in 2nd gear instead of 1st. Same idea.)
It also means that at the same speeds you used to drive at, your RPMs are now lower, and you may be bogging the engine.
The EFI 4.9's peak torque is at 2000 RPMs and lets say, for example, if you used to be at 65mph at 1800 RPMs, you'll now be at 65mph at 1600 RPMs. That means instead of 200 RPMs below you're peak torque, you're now 400. So, your engine has less strength, and has to work harder to maintain the same speed.
This is why people regear their trucks when they put on larger tires.
If you currently have a 3.55 rear end, a 14% increase would bring it to 4.05 (closest to that is 4.11s). That'll put your engine's RPM range right back to where it used to be and you'll have most your power back (minus the extra power it simply takes to get heavier tires rolling).
Not only all of the above.......
You probably need to recalibrate your speedometer to the tire size you're running.
All of your engine management (ie. fuel injection and ignition system) done in the PCM, is, or at least partly, based upon speed and rpms.
Using larger tires without a "speedometer recalibration", I'd say, is messin' with your trucks performance big time.
Just some thoughts.
Good luck,
Bob
Moderator
Joined: Dec 2007
Posts: 7,987
Likes: 104
From: Boise, Idaho
Not only all of the above.......
You probably need to recalibrate your speedometer to the tire size you're running.
All of your engine management (ie. fuel injection and ignition system) done in the PCM, is, or at least partly, based upon speed and rpms.
Using larger tires without a "speedometer recalibration", I'd say, is messin' with your trucks performance big time.
Just some thoughts.
Good luck,
Bob
You probably need to recalibrate your speedometer to the tire size you're running.
All of your engine management (ie. fuel injection and ignition system) done in the PCM, is, or at least partly, based upon speed and rpms.
Using larger tires without a "speedometer recalibration", I'd say, is messin' with your trucks performance big time.
Just some thoughts.
Good luck,
Bob
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Moderator
Joined: Dec 2007
Posts: 7,987
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From: Boise, Idaho
You can do it yourself.
HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles
Just be sure to do it right. I think you can only do it a certain amount of times before the system locks.
HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles
Just be sure to do it right. I think you can only do it a certain amount of times before the system locks.
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 5
From: Almost Heaven
You probably have 3.08's. Ford didn't put a ton of 3.55's in the 300 powered trucks.
Taller tires rob a lot more power than people realize. I have a 4.11 gear with 31 inch tall tires and miss the power the truck had with the shorter 27 inch tires!I couldn't imagine 33 inch tires with 3.08 or 3.55.
Taller tires rob a lot more power than people realize. I have a 4.11 gear with 31 inch tall tires and miss the power the truck had with the shorter 27 inch tires!I couldn't imagine 33 inch tires with 3.08 or 3.55.
Postmaster
Joined: Feb 2003
Posts: 3,441
Likes: 5
From: Muskegon, MI (home)
You probably have 3.08's. Ford didn't put a ton of 3.55's in the 300 powered trucks.
Taller tires rob a lot more power than people realize. I have a 4.11 gear with 31 inch tall tires and miss the power the truck had with the shorter 27 inch tires!I couldn't imagine 33 inch tires with 3.08 or 3.55.
Taller tires rob a lot more power than people realize. I have a 4.11 gear with 31 inch tall tires and miss the power the truck had with the shorter 27 inch tires!I couldn't imagine 33 inch tires with 3.08 or 3.55.
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 5
From: Almost Heaven
I bet you even got better fuel mileage with the 4.10's over the 3.08 gear with those tires!