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New Process/Dodge NP540 gear ratios

10745 Views 1 Reply 2 Participants Last post by  ImperialCrown
Does anybody know what gear ratios were used in the NP540 5-speed manual? I'm looking at an old (1966) school bus with one in it. Also how to decode the plate to determine the axle ratio.

I'm interested mainly in a gear ratio best for highway speed. Another one (a 1976) I'm checking out has an Allison 4-speed automatic and its top-gear ratio (according to what I read elsewhere on the internet) is 1:1 with an axle ratio of 6.83 (love it when an old car or truck has its factory spec sheet!) Its owner says it can go 50-55 loaded down with people.

The 1976 has a 360. Don't remember what the '66 has but I'm speculating a 361.

I don't need one that can cruise at 75... I don't have to take Interstates wherever we go. However the interstate we use most often goes through Chicagoland and the times we've been through there, top speed was irrelevant.

Any info is helpful. Thx.
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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
17,289 Posts
I have a Motor's Truck repair manual from 1976 that lists the NP-540, 541 and 542 transmissions. All similar in service procedures.
The school buses were designated S400, S500, S550 and S600. The 318 was listed as the V8 in them. It may be a Spicer, Eaton, Timken or Chrysler rear axle.
Any code plate or ID tag information would be useful. Different school bus coach builders may have put the plates in different places. I remember the Thomas bus ID plate located above the windshield.
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