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i have a 78 f150 4x4 with a 400m with a 2 inch suspension lift and a 3 inch body lift. it has the ford 9 inch in the rear. what is the biggest tires i could go with without changing gear rations or lifting it more. i want to go with 37x12.50 super swamper tsl. thanks in advance
37's will fit but I would ditch the body lift and go with 35's. If your running stock gears (prob 3.50's) the truck will be a dog unless you regear. Next you prob have a Dana 44 up front, running anything over a 35 is asking for trouble if you wheel it hard. The only good thing is the low rpm's you will be turning on the highway. Even with 35's you will feel a big hit on low end power but with the 400 it will work for mild wheeling.
You are limited by the D44 up front. The recommendations are that for wheeling do not exceed 33". If you use your truck for mostly highway use (65+%) then 35" tires can be used. Keep in mind that the D44 bearings will need to be serviced more do to the weight increase by going with 35" tires.
The FORD 9" can successfully go with 36" tires max for mostly highway use.
If you are wanting 37" tires, go with D60's front and rear. Pick a gear like 4.10's to keep you going down the road.
Conclusion: I would recommend 33" tires if you do not want to change out gear ratio's. Your 400 (stock?) small block would work to hard if you did not change out gears for the 35" tires. Plus your mileage would take a hard nose dive! JMHO...
If all your doing is mud youll be set with 37's. You dont need a super low gear ratio like you would if you were crawling it. The stock gears will give you good wheel speed just so long as you dont get such an aggesive tire that your motor cant turn them in four low. As far as the axle strength is concerned youll have to take it easy when your out slingin. Since your going with such a large tire I wouldnt suggest a swamper style tire, a good MT or AT would do best. Just make sure you keep a couple of spare axle shafts with you in case you do lose an axle in the mud.