large tire engine lag
Postmaster
Joined: Nov 2001
Posts: 4,742
Likes: 8
From: OK
large tire engine lag
You're right about taking overall gearing into account. Using your RPM/speed/truck info and where ER = effective gear ratio and all percentages in relation to OEM numbers, here's what I came up with:
35" tires & 3.73 gears:
3rd = (3.73 - 9% tires) x 1 = 3.39 ER == 2465 RPM @ 65mph
OD = 3.39ER x 0.71 = 2.41 ER == 1750 RPM @ 65mph in OD
35" tires & 4.30 gears:
3rd = (3.73 (+15% gears - 9% tires)) x 1 = 3.95 ER == 2849 RPM @ 65mph
OD = 3.95ER x 0.71 = 2.80 ER == 2023 RPM @ 65mph in OD
Realizing that there is a problem with excessive down-shifting from OD with 3.73 gears and our new 35" tires, we have a few options:
1) keep what we have and just run high RPMs in 3rd gear
2) buy "lower" rear gears and run at medium RPMs in OD
3) increase engine power and run at lower RPMs in OD w/3.73 gears
The question is "how much" of any of these solutions is enough to end the excessive down-shifts. The best answer is up to the individual as each solution has both positives and negatives.
35" tires & 3.73 gears:
3rd = (3.73 - 9% tires) x 1 = 3.39 ER == 2465 RPM @ 65mph
OD = 3.39ER x 0.71 = 2.41 ER == 1750 RPM @ 65mph in OD
35" tires & 4.30 gears:
3rd = (3.73 (+15% gears - 9% tires)) x 1 = 3.95 ER == 2849 RPM @ 65mph
OD = 3.95ER x 0.71 = 2.80 ER == 2023 RPM @ 65mph in OD
Realizing that there is a problem with excessive down-shifting from OD with 3.73 gears and our new 35" tires, we have a few options:
1) keep what we have and just run high RPMs in 3rd gear
2) buy "lower" rear gears and run at medium RPMs in OD
3) increase engine power and run at lower RPMs in OD w/3.73 gears
The question is "how much" of any of these solutions is enough to end the excessive down-shifts. The best answer is up to the individual as each solution has both positives and negatives.
Junior User
Joined: Feb 2002
Posts: 63
Likes: 0
From: Arcata USA
large tire engine lag
NoMo,
Thanks for the 'numbers'. It is always great, when math and the real world comes out the same, validating each other.
For now, I use option #1, down shifting into 3rd gear when needed for towing, and not having the tranny 'hunting'. Besides it keeps the RPM up, during this limited time.
I'd sure like option #3 and add a Kenne Bell Supercharger. Right now the truck's warranty is an issue, and having to run 92 octane. If regular is $1.81 (Eureka area of Northern California, "Home of the Renner Petroleum Distributor Gas Monopoly"), I can't even think to look at the price of 92 octane...
BTW... NoMo, nice combination of F-trucks, they sure are good-looking...
Mark Peterson
'02 F250 CC, SB, V10 (sure wish a SC for supercharger was in front of that V10)
Thanks for the 'numbers'. It is always great, when math and the real world comes out the same, validating each other.
For now, I use option #1, down shifting into 3rd gear when needed for towing, and not having the tranny 'hunting'. Besides it keeps the RPM up, during this limited time.
I'd sure like option #3 and add a Kenne Bell Supercharger. Right now the truck's warranty is an issue, and having to run 92 octane. If regular is $1.81 (Eureka area of Northern California, "Home of the Renner Petroleum Distributor Gas Monopoly"), I can't even think to look at the price of 92 octane...
BTW... NoMo, nice combination of F-trucks, they sure are good-looking...
Mark Peterson
'02 F250 CC, SB, V10 (sure wish a SC for supercharger was in front of that V10)
Elder User
Joined: May 2002
Posts: 500
Likes: 0
From: Valley of the Su, Arizona
large tire engine lag
Very interesting info.
I just completed my first towing road trip with my new 35" tires. I wasn't pleased with the mileage (around 6 to 7.5) to say the least.
I was pulling a 27' RV and yes, I like my speed, so I tried to stay in the 65-75 MPH range.
There were some hills where I had to go down to 2nd to hold at 55-60 with RPMs around the 4000+ range. I could almost watch the fuel gage drop!!!
The only time the SD would remain in OD was going down grade, and a good down grade at that, otherwise I was in 3rd or 4th (whatever it would pick) which was basically running with OD off.
I don't have any other mods other than a moded air intake with an K&N.
The truck did do a lot of shifting, but the tranny never got hot - on the F350, I get the tranny temp gauge.
I would say that 35" tires and 3.73 gears are NOT a good towing combo. I'd consider going to 4.30 or 4.56 in the rear, but will that do anything negative to mileage on those mostly non-towing days or will the 35" tires kinda negate it?
-superV10
2002 F350 CC SB V10 3.73 Lariat 315/75R16 2.5" Leveling K&N
I just completed my first towing road trip with my new 35" tires. I wasn't pleased with the mileage (around 6 to 7.5) to say the least.
I was pulling a 27' RV and yes, I like my speed, so I tried to stay in the 65-75 MPH range.
There were some hills where I had to go down to 2nd to hold at 55-60 with RPMs around the 4000+ range. I could almost watch the fuel gage drop!!!
The only time the SD would remain in OD was going down grade, and a good down grade at that, otherwise I was in 3rd or 4th (whatever it would pick) which was basically running with OD off.
I don't have any other mods other than a moded air intake with an K&N.
The truck did do a lot of shifting, but the tranny never got hot - on the F350, I get the tranny temp gauge.
I would say that 35" tires and 3.73 gears are NOT a good towing combo. I'd consider going to 4.30 or 4.56 in the rear, but will that do anything negative to mileage on those mostly non-towing days or will the 35" tires kinda negate it?
-superV10
2002 F350 CC SB V10 3.73 Lariat 315/75R16 2.5" Leveling K&N
Posting Guru
Joined: Jan 2002
Posts: 1,194
Likes: 1
From: Frankfort, IL. USA
large tire engine lag
>I would say that 35" tires and 3.73 gears are NOT a good
>towing combo. I'd consider going to 4.30 or 4.56 in the
>rear, but will that do anything negative to mileage on those
>mostly non-towing days or will the 35" tires kinda negate
>it?
>
>-superV10
>
>2002 F350 CC SB V10 3.73 Lariat 315/75R16 2.5" Leveling K&N
Just a reminder... You will have to change the gears in the front as well as the rear!
>towing combo. I'd consider going to 4.30 or 4.56 in the
>rear, but will that do anything negative to mileage on those
>mostly non-towing days or will the 35" tires kinda negate
>it?
>
>-superV10
>
>2002 F350 CC SB V10 3.73 Lariat 315/75R16 2.5" Leveling K&N
Just a reminder... You will have to change the gears in the front as well as the rear!
New User
Joined: Dec 2000
Posts: 14
Likes: 0
large tire engine lag
Buffy, could you explain to me how or why recalibrating the speedo would actually make the truck run better. I just went to 285/75's on my truck and noticed a slight lag and didn't think I would need to change the speedo for a matter of a 2-3 mph at 65. But if it will bring back some of that power loss, I will have it done.
Thanks, Scott
Oh, and a hello to everyone here in the SD forum, I'm not new to FTE, but haven't posted in here till now.
Thanks, Scott
Oh, and a hello to everyone here in the SD forum, I'm not new to FTE, but haven't posted in here till now.
Posting Guru
Joined: Jan 2002
Posts: 1,194
Likes: 1
From: Frankfort, IL. USA
large tire engine lag
>Buffy, could you explain to me how or why recalibrating the
>speedo would actually make the truck run better. I just went
>to 285/75's on my truck and noticed a slight lag and didn't
>think I would need to change the speedo for a matter of a
>2-3 mph at 65. But if it will bring back some of that power
>loss, I will have it done.
I don't see how recalibrating the speedometer could have any effect on performance!
>speedo would actually make the truck run better. I just went
>to 285/75's on my truck and noticed a slight lag and didn't
>think I would need to change the speedo for a matter of a
>2-3 mph at 65. But if it will bring back some of that power
>loss, I will have it done.
I don't see how recalibrating the speedometer could have any effect on performance!
New User
Joined: Dec 2000
Posts: 14
Likes: 0
large tire engine lag
>When I went from the stock tires to the 255/85's the truck
>was very doggy until I recalibrated the speedometer. Then
>it ran much better.
I would have to agree Mr. Rottendog, But this is what Buffy stated in an above post. I just wanted to make sure I wasn't missing anything
Scott
>was very doggy until I recalibrated the speedometer. Then
>it ran much better.
I would have to agree Mr. Rottendog, But this is what Buffy stated in an above post. I just wanted to make sure I wasn't missing anything
Scott
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Senior User
Joined: Mar 2002
Posts: 158
Likes: 0
From: mid michigan
large tire engine lag
I've gone through all this also. You definitly need to go to the dealer. The computer reads your speed as tire revolutions per mile. I had 265's stock which had rpm at 655. My 315's should rotate 580 times per mile. The closest setting for my computer was 605 rpms. When this got changed I also asked if the shifts could be firmer. The tech said no problem. For about $35 I got the speedo within 1 mph @ 60 and nice firm shifts. The truck respondes better knowing the more accurate speed.
Some of this is stuff I got from this forum, thanks.
Steve
99 SD, V10, EC, SB, LARIOT.
315-75-16 ProComp XTerrains M/T ClassicII 16x10
Fabtech 3.5 lift kit
More to come I'm sure.
Some of this is stuff I got from this forum, thanks.
Steve
99 SD, V10, EC, SB, LARIOT.
315-75-16 ProComp XTerrains M/T ClassicII 16x10
Fabtech 3.5 lift kit
More to come I'm sure.
Senior User
Joined: Aug 2000
Posts: 102
Likes: 0
large tire engine lag
I guess I should have been more specific with my earlier remark. My truck was doggy because the transmission was shifting too early after I put on the taller tires. I used a TruSpeed ratio converter to modify the signal from the rear differential. It corrects the speedometer, as well as corrects the shift points. Sorry for the vague statement.
