I've got a 1993 E350. 7.3 IDI (non-turbo), three speed automatic, 4.10 rear end ratio (two wheel drive), 225/75X16 tires. Right now, it's turning a little over 3000 rpm at 60 mph. I'd like to get the rpms below 2000 at 60. Does the 7.3 have enough horse power and torque (185HP @ 3000rpm/360lb-ft @ 1400rpm) to gracefully turn a 3.08 gear and 33 inch tires?
Ideal tire size/gear ratio?
- Thread starter HarryT
- Start date
This site has a good gear/tire/mph calculator.
http://www.wallaceracing.com/gear-speed.php
http://www.wallaceracing.com/gear-speed.php
RocKrawler
Supporting Sponsor
225/75X16 tires are 29.28" tall (unloaded) and with a 4.10 gear ratio you should be at about 3095 RPM at 65mph without an overdrive transmission. With a 3.08 gear and the same size tire your RPM at 65mph would drop to about 2312 RPM. Increasing the tire to 33" and running the 3.08 ratio, your RPM at 65mph would drop to 2032 RPM. While the numbers look good, and a diesel does better at low RPM compared to gas motors, I agree with Chris that installing the tires first would be your best bet and go from there. A 33" tire with your current 4.10 ratio will drop RPM to 2720 and give you a feeling of what the lower gears will feel like. Its likely you will notice the difference and after living with it for a while you might decide to stick there and save the extra money if you're more pleased with the change, and then put the money into a traction device or other upgrade.
Desert Dan
Explorer
Try 285 x 75 x 16 to see how that does.
Since you are 2x4 you can start with buying 2 rear tires?
Since you are 2x4 you can start with buying 2 rear tires?
mike.marcacci
Adventurer
@nely, that's an awesome calculator – I'm curious about this too, but mostly to avoid overloading my drivetrain. I'm looking at running 285/75R/17 now (I was previously looking at 35"), and according to the calculator they would have almost identical ratios with a 4.10 as my current 3.73 give with the 245/75R/16 stock tires.
I'm OK with the van's current feel in both city and highway so I guess the real question is how much less life will I get out of the van if I keep the 3.73 and compensate for the larger tires with a chip/more power? That's a LOT more torque on the transmission, etc...
I'm OK with the van's current feel in both city and highway so I guess the real question is how much less life will I get out of the van if I keep the 3.73 and compensate for the larger tires with a chip/more power? That's a LOT more torque on the transmission, etc...
Mickey Bitsko
Adventurer
Make sure it has a adequate sized aux trans cooler for your type of driving ie: towiing w/temp gauge wouldn't hurt.
Unfortunately torque for the idi starts to fall off almost as soon as you start moving. Peak torque (340-ish) happens at about 1400 rpm, whereas your hp starts off in the basement and continues to climb throughout the rpm range, peaking at about 3300 rpm with a whopping 180-ish. Of course these numbers are for a stock, properly running idi. HP at peak torque is about 100. Torque at peak HP is about 285.
If it were me I'd shoot for a cruising rpm where those two numbers cross. In this case it looks to be around 2200 rpm where you'd be 140hp and 330tq.
If it were me I'd shoot for a cruising rpm where those two numbers cross. In this case it looks to be around 2200 rpm where you'd be 140hp and 330tq.
Mwilliamshs
Explorer
This^. I've logged nearly 200k miles in a 7.3 IDI truck and can tell you 2,200-2,300 rpm is perfect for highway cruise speed when loaded. It's a little high when running empty but ya just slow down a bit. Ironically, you can drive a little faster when lightly loaded and get the same or slightly better mpg as when driving unloaded. I run 235/85-16s (about 31.5"), 4.10s, and a zf5 (.76 OD) to get cruise speed up to 70mph. OE was 215/85-16s (basically 30 inches tall) and 2,200 was just under 65 mph. Swapping up to 235s made a big difference because you could drive 60 and get 18-19 mpg or 70 and get 12-14 before but now you can drive 70 and get 17-18. 225/75-16s are even shorter than 215/85-16s so you're giving up some travel speed there.
If I were set on running 33" tires and wanted to cruise at 70 mph (speed limit in these parts) I'd want at least 3.55 or better 3.73s with a c6. Your usage of the van will make a big difference here. If your van is light or doesn't tow or go off road you can use longer gears but keep in mind it's more work (and heat) for your transmission at anything below highway cruise speed. I view a van (especially a campervan) as always loaded and would want a real good transmission cooler and 3.73s minimum with 33s and a c6.
You can always buy the 33s and run them with your existing gears then make an educated guess as to how much "too much" gearing you have.