How many RPMs am I going to lose?

[KJHill]

Okay so I've got 35" tires now with an AOD and at 60 MPH i'm right at about 2K RPMs. I'm thinking I'd like to move up to 37" tires this spring but I'm worried about losing my overdrive going down the highway. It already wants to kick me out of overdrive if there is any incline or if I accelerate the slightest. So what RPM would you say I'd see with 37"s going 60 MPH? I've got 4.56 gears. I'm sure there is a mathematical equation for this, but I'm just lazy. Thanks! ;D

 

[triracer67]

http://www.anycalculator.com/gear_ratio_calculator.html

Try this?

 

[Cooter_76]

Here's a really comprehensive calculator: http://www.grimmjeeper.com/gears.html

 

[DirtDonk]

here are the ones I use: https://www.yukongear.com/calc_rpm.aspx
Looks like approx. 200 rpm lower and yes, if it's hunting for gears now, it's going to really be bad after putting on larger and heavier tires with more rolling resistance. It's not just about the rpm, but the sheer weight and resistance increase too.

But to know for sure down to the best accuracy, you need to actually measure the two tires on the vehicle. Measure from the hub to the ground and double that for your tire's "rolling diameter" which is what the speedo sees.
You can go by the tire charts that give the "revolutions per mile" or whatever they use, but I don't trust those any farther than I trust the stated inches.

Paul

 

[Broncobowsher]

It's just not the loss of RPM, there is the loss of driving force. And the added resistance. Even if the tire is the same tread pattern it will raise the Bronco and increase wind drag. The tire might even have less rolling resistance, but that won't counter the added wind drag. Wind drag is a much higher force than rolling resistance at speed.

Now the loss of driving force. This is often overlooked. Even if the engine has a flat torque curve and makes the same level of torque at bit RPMs, it will have less driving force. For 100 ft-lb of torque on the axle, a 35" tire (assuming actual size) will have 68 pounds of thrust. For the same 100 ft-lb of shaft torque a 37" tire will be down to 64 pounds of thrust.

So while a 37" tire is only a little taller than a 35" tire, all the factors are going against you. Lower engine speed, which at your level is generally a lower torque output, more drag, and less driving force.

If you thought it was hunting in and out of OD too much before, it will be MUCH worse. Adding more gear, even with the 35s, will help. With 37s, add a lot more gear. The AOD having a really tall overdrive isn't in your favor. The ratio is 2/3 or .667 The 4.56 gears feel like 3.04 in OD, and that is just too tall of a gear for that tall of a tire.

 

[garberz]

You need to consider unsprung weight and rotational weight as well. I went to 5.13’s when I went up to 37’s.

Mark

 

[nvrstuk]

Lots of good info posted up for the OP.

Wind drag is SO much more of a factor than anybody seems to realize for coeffiecient of drag requiring HP (more skinny pedal pressure) to overcome.

I used to keep extremely detailed mpg numbers in a mileage log for about 150,000 miles... all this time (same Bronco) my Bronco went from "stockish" to close to what it is today. (I just fill it up now but do accurate mpg checks when running the sand dunes, snow wheeling and usually a couple 150+ mile trip each year)...not as diligent now.

Anyway, tire size made a difference of course for power and mileage but SO was installing a 3 1/2" suspension lift and later a 2" body lift (influenced mpg as much as going up to larger dia tires). Like Broncobowsher said, many more factors than what most people ever imagine influencing power, gear selection, driveability, mpg, etc, etc...

 

[blubuckaroo]

This issue isn't just for Broncos.
I see a lot of pickup buyers going to 3/4 and 1 ton because their 1/2 ton didn't do the job. But if they hadn't put huge tires on the 1/2 ton, they might have been fine.
I guess it's the price of vanity.

 

[nvrstuk]

X2 blubuckaroo!!

 

[661buster1963]

Spot on Bluebuck,
My brother in law put giant tires, not just tall but really wide. Toyota 4 anger pushing it down the road manual transmission and didn’t change gears or anything. I guess what is clear to some folks isn’t very clear to everyone. I tried to explain simple concept of keeping an engine in its ‘sweet spot’ of rpm for torque and tall tires mean deep gear changes. Wide tires not helping either. He said his buddy did it to a dodge diesel 1 ton. So I tried explain how the diesel makes shit tons of torque, and solid engineers for auto companies make pretty intelligent calls that novice folks want tall tires and my brother never goes off road and drives windy mountain roads and will likely see some brake fade issues.

 

[KJHill]

Thank you for all the great posts! This is the kind of info I really need to consider before deciding what to do. I agree that if I really want to go up to 37" tires I should drop down to 5.13 gears to keep the engine & trans happy. I guess I'll stick with the existing size for now. Thanks again everyone!

 

[garberz]

I’ve always considered 37’s are the max dia. when running a Dana44. Because of the gearing required for optimum rpm’s. 5.13’s make for a small pinion gear, and lower gears with an even larger tire just seems too much of a weak link if you’re actually gonna use it. Running a HP44 will give you a little more strength with the R&P because it runs on the drive side.

Mark