high rpm's
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Senior User
Joined: Mar 2004
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From: southwestern,pa
high rpm's
hello all, on my '87 f-150 with 5.0 4spd transmission 31x10.50r15 tires,
at 65mph it's turning 3000 rpm's is this normal or not. every thing is basically
stock on the truck,the tires were changed and the exhaust was changed also.
was thinking of changing the transmission to either a 5spd or automatic.
any help on this would be appreciated.
at 65mph it's turning 3000 rpm's is this normal or not. every thing is basically
stock on the truck,the tires were changed and the exhaust was changed also.
was thinking of changing the transmission to either a 5spd or automatic.
any help on this would be appreciated.
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Joined: Dec 2002
Posts: 3,959
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From: USA
3000rpm at 65mph with 31" tires with a manual tranny in direct gear (no overdrive) calculates out to about a 4.30 axle ratio. That doesn't sound believeable in a 1/2 ton with 31" tires.
Your speedo is probably off because of the tire size change, so you're probably going faster than the 65mph your speedo is showing. The difference between 29" stock tires and 31" tires is about 7%, so your actual speed is probably around 70mph when your speedo is showing 65.
3000rpms at 70mph with 31" tires in direct gear calculates to about a 3.95 gear ratio. Closest available ratio would be 3.91, but that's an odd ratio to change to. Most people change to 3.73 or 4.10.
I'm thinking either your clutch is slipping alot or the tach and speedo readings you reported were incorrect 'cause the calculations don't come up with an answer that makes much sense. Were you guestimating the engine speed?
Your speedo is probably off because of the tire size change, so you're probably going faster than the 65mph your speedo is showing. The difference between 29" stock tires and 31" tires is about 7%, so your actual speed is probably around 70mph when your speedo is showing 65.
3000rpms at 70mph with 31" tires in direct gear calculates to about a 3.95 gear ratio. Closest available ratio would be 3.91, but that's an odd ratio to change to. Most people change to 3.73 or 4.10.
I'm thinking either your clutch is slipping alot or the tach and speedo readings you reported were incorrect 'cause the calculations don't come up with an answer that makes much sense. Were you guestimating the engine speed?
Senior User
Joined: Jun 2004
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That's exactly where I was running when I had exactly the same size tires.
I think they want you to turn a higher rpm with a Windsor motor. At least with a 5.0L.
I have 33's now so it's a whole new ball game. Not sure what ratio I have now but I will be needing to change it.
I think they want you to turn a higher rpm with a Windsor motor. At least with a 5.0L.
I have 33's now so it's a whole new ball game. Not sure what ratio I have now but I will be needing to change it.
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