401 and 360 Engine RPM

[Thomas792]

So I've seen a few posts lately talking about vehicle speeds and others about gearing swaps. There appears to be a wide range of engine RPMs that members are looking for. Some talk about doing 2000 rpm @ 65 mph and some say 3000 rmp @ 65 mph. I am at the point where I want to start purchasing the necessary parts to go to 35" tires and I want to make sure the gearing puts me at the right rpm @ 65 mph. Currently I have 28" and 3.31 gears which is about 2550-2600 rpm @ 65 mph. I've done the math and cannot decide on 3.73 ro 4.10. What RPM am I shooting for?

http://www.grimmjeeper.com/gears.html

[Thomas792]

Thanks for the link to the gear calculator. I've seen that many times before. I've done the calculations many times and due to recent threads am now confused on what speed my engine should be turning at 65 mph. Is it 2000, 2200, 2400 as I have seen mentioned or something else?

[addicted]

For the best fuel economy a lot of people including myself find that as close to 2000 RPM as possible is the sweet spot for fuel economy due to it being right in the middle of most engines horsepower and torque bands.

[Thomas792]

So with a warmed over 360 or 401 I will be fine at 2000 rpm. Will I have enough power to pass someone if necessary on the highway or am I looking for more rpm?
According to the formulas I am at 2675 rpm at 65 with 3.31 gears and 28" tires. If I want to change to 35" tires and retain the same engine rpm then I should switch to 4.10 gears. But if I want to change to 35" tires and spin 2000 rpm on the engine then I should keep the 3.31 gears.
Maybe I should split the difference and shoot for 3.73 gears.
The more I think about this the more confused I become.

[cu4whln]

Based on my experience ... 35" tires ? I think the 4.10 will be a much better pick.

Old tech V8, just won't have the "grunt" of a modern Fuel injected engine. I think you will want it to be around 2400 @ 65 mph. Remember, 35" tires weigh a LOT more.. moving and stopping efforts are going to be a LOT higher. You still want it to be fun to drive, right?

Just my opinion.. I have driven a lot of Jeeps on 35" tires btw.

Good Luck !

[68glad]

How bout getting the tires & run them and see how you like it. Actual tire diam. are all over the board. A 35X12.50 isn't really 35" tall which will affect rpm alot. Who knows, you may end up liking the 3.31's.

[Kris]

Im SOA on 35's with 4.56 gears.
Its running around 3300 at 70mph.
Averages 10.5mpg at 70-75 with a spare tire and gas can on the roof. It does a little better without them.

When I was on 33's with 3.31 gears and no SOA, Id get 11mpg at 70mph/2300rpm.

These engines dont mind spinning. The 1mpg was more from the increased resistance with the height and tires than the engine rpm.
Before I did the SOA/gears/tires, I tried running on the fwy in second gear to get the rpm's up in the 3000-3500 area. At the time I thought it was going to come apart.
They really dont mind though, and it does much better off the line.

k.

[Chris P.]

My J10 has 31 in. tires with 3.54 gears. According to the gear calculator I run about 2700 rpms at 70 mph. I have decent pickup and acceleration. I often do interstate runs at 65-70mph and get on averge 13 mpg.

Like others have said, I would go with 4.10. This will put you around 2600 rpm at 65 mph. I think this would be a good compromise between power and mileage.

[Thomas792]

68glad - I always figure that the tire is one inch less than advertised size when running the formulas as a starting point so my numbers might not be "exact".

I had a CJ7 with a rebuilt I6, Howell EFI kit, and Borla Header/Cat Back system. With 33" Dick Cepek mud tires and 3.73 gears it moved pretty well.

In my Wagoneer I currently have 3.31 and 28" tires. If I'm not cautious, the tires spin quite easily around corners and I really like the acceleration. 4.10 puts me at nearly the same rpm with 35" tires so that sounds really good. But 3.73 gears puts me at 2400 rpm @ 65 mph. What a decision to make...

I'm just glad that I realized I don't need an OD transmission. I was really dreading the thought of swapping in a "built" 700R4. I'll just stay with the 401 and TH400.

[will e]

Does the TH400 have a locking converter? If not, you will get some slip so add a couple of hundred RPM to your calcs.

As a general rule if you are at a good range at 60 or 65 then to pass you drop into 2nd. In my mustang I run 3.50 gears with 235 70R14 tires. It HUMS at 70 mph but I don't need to drop it into second to sprint up to 100.

[RottenDog]

With a stock 401 rebuild, TH400 with 4.10s and 35s, I was spinning 3000 rpms at 65 mph.

With the 700R4, cruising at 2200 rpms at 65.

[Chevelleguy]

That doesn't sound right. Those are 4.56/33" numbers. Maybe your tach is reading high.

[FloridaFSJ]

Currently have 38.5" tires with 4.56 gears. I read on here from a guy named Brutus so I swapped the 44" tires for these 38.5s and got 4.56 gears. He was dead on, it is like a bat outta hell with this combo all the way up to 85 (private road). Oh and I get 10-11mpg

I can change lanes at 70 with the throttle alone.

[midmichfsj]

stock 360/ 727 with shift kit/ 3.73 gears/ 35" tires I run around 2600 RPM @ 65. If you plan on leaving the motor stock and don't see alot of highway I would run 4.10's. Mine is a liitle under powered for running around town.

[Thomas792]

Thanks for all the great replies. I currently have an 88 GW with a "slightly" built 360 that is my daily driver. In my parent's barn in Michigan I have a 74 J10 with a built up 401 (Z code VIN and $3000 into the motor). From the calculations I and words of others I think 3.73 and 35" tires will be fine for the J10 but 4.10 and 35" tires would be best for the GW.

RottenDog - are you sure your numbers are right? I'm not getting that via the various calculators as Chevelleguy mentioned.

[Headhoncho]

I ran 36's on my Honcho (6cyl t176) with the 3.31's and it was a dog. I would go with the 4.10's.


JR

[RottenDog]

Took my J10 on the freeway today. 65 mph and 2200 rpms.

VDO tach. Can it be off? I don't know. I am just happy I do not hear any high rpm wind anymore at 65.

410 gears for sure. Checked to confirm.