37" tires...

After a recent round trip in the M1008 that involved some interstate time, I'm thinking of throwing a set of 37's on. I like the 35's I currently have on the truck, but I felt like I was going to get ran over while going 56-57mph on the highway.

I'm not going to swap out the T400, so it's either 37's or regear the differentials if I do it. I currently have a factory style 4 leaf set up in the front that nets 2.5-3in of lift. The brake lines currently have plenty of length for the current set up.... I'm thinking of installing an EZ Inch in the front. How did anyone install an EZ Inch on the passenger side of the Dana 60? Here's a few pics of how the shop set up my springs on the front.

LT - increasing tire diameter from 35 to 37 is not going to provide the increased highway speed you desire. I am currently runing 37” BFG’s on my M1028 with TH400 and 4.56 and am fearful to get over 50 mph due to excessive engine RPMs.

Regarding the the thought of changing axle ratios, I would not recommend it, and here’s why - I have been building and running 4WD trucks since 1968, a total of 50 years. I have lost track of how many 4WD trucks I have owned and modified, and from my experience, it is almost impossible to set up the new ring gear and pinion to factory specs where it will last on long trips. Ask me how I know.

Replacing the TH400 with a 700R4 is the way to go. I am preparing to do just that on my truck. I want to preserve the low gearing of the 4.56, and the 700R4 will provide a 4th final ratio of .70 which is 30% better than the Th400’s ratio of 1.00. This will allow for highway speeds at safe engine RPMs, while still retaining the rock crawling ability of the factory 4.56.

Hope ths helps.

I will not attempt to do a gear swap myself.... I'll gladly pay the extra to someone that specializes in ring n pinions to do it.

I have 4:10's with 33in tires on my 79Kw20 and I can run 63mph at 2500rpm's on the highway. The only issue with that truck at those rpm's is watching the 406 small block drain the gas tank in a rapid manner.....

LT67 said:

I will not attempt to do a gear swap myself.... I'll gladly pay the extra to someone that specializes in ring n pinions to do it.

I have 4:10's with 33in tires on my 79Kw20 and I can run 63mph at 2500rpm's on the highway. The only issue with that truck at those rpm's is watching the 406 small block drain the gas tank in a rapid manner.....

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OK sport. Do as you wish. I was only trying to help. Your example provided above indicates a 2500 RPM on a 406 GAS engine. Our original 6.2 diesels feel the best around 2000-2200 RPM. Around 2800 is pushing the limits of the engine design. I will respectfully end my participation in this thread and watch what happens.

Id have to agree with @sharecropper

I had a 69 f250 that had 4.10s when I bought it and I swapped to 4.88s in anticipation of larger tires. Yes I was going for the opposite that you’re going for I believe. I wanted to retain low end for Offroad situations for my application. In any case, Highway speeds were reduced for the 4.88s with same tires but not by as much as I expected and a different transmission like 700r4, 4L80E or one of those add on overdrive deals would give a much bigger difference for Highway mpg, noise from engine speed, and probably drive better having engine in a more comfortable rpm range.

My personal future goal is a 4L80 but not sure if that will happen. It’s the the overdrive version of the turbo 400 as I understand but requires a computer and other modifications.




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My personal goal is an NV4500... I already have the pedal set out of a hydroboost one ton and I have the hydraulic clutch cylinder as well. One day...

60mph on a set of 37's is not going to strain a 6.2 beyond its limits....

LT67 said:

60mph on a set of 37's is not going to strain a 6.2 beyond its limits....

Click to expand...

I've ran a many of miles at 64mph and 2800rpm.

And here's the specs from GM:

6.2L GM/Detroit Diesel Specs

I currently have 38" swampers on my 4:56 geared M1009 (one ton axles) and i can do 60mph fine but it still feels weird going any faster than that. I agree that either a OD swap or an axle re-gear is a better way to get highway speed.

I've ran three different CUCV's on tire sizes including stock 32's, 35's, 36's, 37's, and 38's. The "comfortable" speed gains from the larger tires is fairly marginal. But, I have driven CUCV's at highway speeds (60-65mph) on 500+ mile stretches without any known mechanical damage (with the larger tires).

I'm not as attached to my 6.2 as some are. I will run it till it dies (seems like it will be a while) and then replace with whatever is handy at the time. For more comfort at road speed I would keep the tire size smallish and do gears / OD stuff. I wheel my truck, so the tires are for ground clearance, not road driving (but I do keep it street legal).

Just get another truck to run "Highway Speed", don't think these trucks were made for highway speeds. Gladly run my truck under sixty-five all day long. Wanna go faster, Mercedes 300 SD turbo.....just saying...my truck looks better going slow anyway....just saying...lol

I typically run 62-65 MPH on my 37's in my M1031 (converted to an M1028A1). If I keep it at 60 MPH, I average 17 MPG. Not bad. I still want a five speed manual transmission though...

The difference in speed between 35's and 37's is about 5% at any given rpm. It's a linear function.

There's always more to being comfortable at X mph than just rpm...