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**hatzie** · 12-19-2021, 11:52 AM

The actual diameter at the tread is what makes the biggest difference to things. 245/75R16 or 30.6" tires came on my truck. I'm running 265/75R17 or 31.6" diameter now. There are a bunch of different possible stock tire sizes listed in the ABS computer. 265/75R17 are not among them but there are some that are the same diameter so I re-coded the ABS to a tire that matches the diameter of the new ones.

1" in diameter doesn't sound like much but it drops your final drive ratio noticeably.
I played with my RPM and gearing spreadsheet to come up with some numbers to illustrate what's happening.
1" larger tire drops your effective gearing
4.10:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2145 going 65mph.
4.10:1 axle gears, .73:1 OD, 265/75R17 tires sets your engine RPMs at 2002 going 65mph.
I then fooled around playing what if with the axle ratio in the spreadsheet to see what lower axle ratio would get the 245/75R16 tires to run at 65 at nearly the same engine RPMs as the larger tires with 4.10 gears.
Here's what I got...
3.83:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2004 going 65mph. 2RPMs is pretty close.

Add somewhere around another 1/4"-3/8" on the diameter and you essentially turn 4.10 axle gears into 3.73 gears without cracking open the diffs. That's a pretty serious difference.

I'd check the tire size in the ABS computer.

Your ABS motors or pumps may be fouled up but I would guess it's a combo of the larger tires without recoding the ABS computer and an air pocket you still haven't gotten out through the bleeders.

I usually do an ABS bleed under pressure then I crack open the bleeders and draw off a pint of fluid from each corner.
Since the system is full of fresh fluid... if it looks clean and it's not aerated you can re-cycle it back into the pressure pot.
Command another ABS bleed under pressure and draw off another pint at each corner.
Rinse and repeat til you've cycled a gallon or so through each bleeder.
I bet you'll get a small slug of air or ten out one of the rear bleeders.

12-19-2021, 11:52 AM	#5
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,916	Re: '01 2500 HD spongy brake pedal The actual diameter at the tread is what makes the biggest difference to things. 245/75R16 or 30.6" tires came on my truck. I'm running 265/75R17 or 31.6" diameter now. There are a bunch of different possible stock tire sizes listed in the ABS computer. 265/75R17 are not among them but there are some that are the same diameter so I re-coded the ABS to a tire that matches the diameter of the new ones. 1" in diameter doesn't sound like much but it drops your final drive ratio noticeably. I played with my RPM and gearing spreadsheet to come up with some numbers to illustrate what's happening. 1" larger tire drops your effective gearing 4.10:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2145 going 65mph. 4.10:1 axle gears, .73:1 OD, 265/75R17 tires sets your engine RPMs at 2002 going 65mph. I then fooled around playing what if with the axle ratio in the spreadsheet to see what lower axle ratio would get the 245/75R16 tires to run at 65 at nearly the same engine RPMs as the larger tires with 4.10 gears. Here's what I got... 3.83:1 axle gears, .73:1 OD, 245/75R16 tires sets your engine RPMs at 2004 going 65mph. 2RPMs is pretty close. Add somewhere around another 1/4"-3/8" on the diameter and you essentially turn 4.10 axle gears into 3.73 gears without cracking open the diffs. That's a pretty serious difference. I'd check the tire size in the ABS computer. Your ABS motors or pumps may be fouled up but I would guess it's a combo of the larger tires without recoding the ABS computer and an air pocket you still haven't gotten out through the bleeders. I usually do an ABS bleed under pressure then I crack open the bleeders and draw off a pint of fluid from each corner. Since the system is full of fresh fluid... if it looks clean and it's not aerated you can re-cycle it back into the pressure pot. Command another ABS bleed under pressure and draw off another pint at each corner. Rinse and repeat til you've cycled a gallon or so through each bleeder. I bet you'll get a small slug of air or ten out one of the rear bleeders. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 12-19-2021 at 12:56 PM.