408WPN 38 Posted October 30, 2023 Hey Guy's Howz it.. Touching base to query some excessive RPM on New buy I'm in the final stages of. 87 XF Fairmont Ghia 408c, race car spec's on the new engine build, running a strong C4 trans & 8 3/4" 3.25 ratio diff but doing 4000 rpm at 100k. That kinda sounds like it sucks, what you reackon Share this post Link to post Share on other sites
SPArKy_Dave 9,295 Posted October 30, 2023 Had an F350 once that did that... 4000rpm in 4th @100 Â Share this post Link to post Share on other sites
deankxf 20,904 Posted October 30, 2023 56 minutes ago, 408WPN said: Hey Guy's Howz it.. Touching base to query some excessive RPM on New buy I'm in the final stages of. 87 XF Fairmont Ghia 408c, race car spec's on the new engine build, running a strong C4 trans & 8 3/4" 3.25 ratio diff but doing 4000 rpm at 100k. That kinda sounds like it sucks, what you reackon  sounds like you have a 6cyl tacho ? and possibly 3.5 ratio diff 4000 definitely doesn't sound right (unless it has a massive hi stall converter also ?) even 4.11 diff ratio would only be about 3400rpm in my calculations 1 SPArKy_Dave reacted to this Share this post Link to post Share on other sites
bear351c 10,402 Posted October 30, 2023 Nah, ............. something ain't right. As Dean said, prob uncalibrated tacho. My XB has 3.50 gears, does around 2800 from memory. Easiest way to tell is download an App to your phone with a HUD (head-up display) uses GPS to give a fairly accurate speed. Could follow a mate in another car at 100 km/h. If you get the speedo recalibrated, take a reading at 60, 80, 100, 120 with your mate on cruise control (or the App) on a straight flat road. Give the numbers to the instrument guy. Share this post Link to post Share on other sites
Thom 6,707 Posted November 1, 2023 If it is pulling 4krpm @100kph are you sure it's changing out of second into top? Could be in need of a kickdown or vacuum modulator adjustment Share this post Link to post Share on other sites
